![Joel Fernandes (Google)](/assets/img/avatar_default.png)
Android uses ashmem for sharing memory regions. We are looking forward to migrating all usecases of ashmem to memfd so that we can possibly remove the ashmem driver in the future from staging while also benefiting from using memfd and contributing to it. Note staging drivers are also not ABI and generally can be removed at anytime. One of the main usecases Android has is the ability to create a region and mmap it as writeable, then add protection against making any "future" writes while keeping the existing already mmap'ed writeable-region active. This allows us to implement a usecase where receivers of the shared memory buffer can get a read-only view, while the sender continues to write to the buffer. See CursorWindow documentation in Android for more details: https://developer.android.com/reference/android/database/CursorWindow This usecase cannot be implemented with the existing F_SEAL_WRITE seal. To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal which prevents any future mmap and write syscalls from succeeding while keeping the existing mmap active. A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week where we don't need to modify core VFS structures to get the same behavior of the seal. This solves several side-effects pointed by Andy. self-tests are provided in later patch to verify the expected semantics. [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/ Thanks a lot to Andy for suggestions to improve code. Link: http://lkml.kernel.org/r/20190112203816.85534-2-joel@joelfernandes.org Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Acked-by: John Stultz <john.stultz@linaro.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Minchan Kim <minchan@kernel.org> Cc: Jann Horn <jannh@google.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andy Lutomirski <luto@kernel.org> Cc: Hugh Dickins <hughd@google.com> Cc: J. Bruce Fields <bfields@fieldses.org> Cc: Jeff Layton <jlayton@kernel.org> Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Shuah Khan <shuah@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
328 lines
7.8 KiB
C
328 lines
7.8 KiB
C
/*
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* memfd_create system call and file sealing support
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*
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* Code was originally included in shmem.c, and broken out to facilitate
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* use by hugetlbfs as well as tmpfs.
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*
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* This file is released under the GPL.
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*/
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#include <linux/fs.h>
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#include <linux/vfs.h>
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#include <linux/pagemap.h>
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#include <linux/file.h>
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#include <linux/mm.h>
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#include <linux/sched/signal.h>
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#include <linux/khugepaged.h>
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#include <linux/syscalls.h>
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#include <linux/hugetlb.h>
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#include <linux/shmem_fs.h>
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#include <linux/memfd.h>
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#include <uapi/linux/memfd.h>
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/*
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* We need a tag: a new tag would expand every xa_node by 8 bytes,
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* so reuse a tag which we firmly believe is never set or cleared on tmpfs
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* or hugetlbfs because they are memory only filesystems.
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*/
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#define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
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#define LAST_SCAN 4 /* about 150ms max */
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static void memfd_tag_pins(struct xa_state *xas)
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{
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struct page *page;
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unsigned int tagged = 0;
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lru_add_drain();
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xas_lock_irq(xas);
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xas_for_each(xas, page, ULONG_MAX) {
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if (xa_is_value(page))
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continue;
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if (page_count(page) - page_mapcount(page) > 1)
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xas_set_mark(xas, MEMFD_TAG_PINNED);
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if (++tagged % XA_CHECK_SCHED)
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continue;
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xas_pause(xas);
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xas_unlock_irq(xas);
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cond_resched();
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xas_lock_irq(xas);
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}
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xas_unlock_irq(xas);
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}
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/*
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* Setting SEAL_WRITE requires us to verify there's no pending writer. However,
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* via get_user_pages(), drivers might have some pending I/O without any active
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* user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
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* and see whether it has an elevated ref-count. If so, we tag them and wait for
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* them to be dropped.
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* The caller must guarantee that no new user will acquire writable references
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* to those pages to avoid races.
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*/
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static int memfd_wait_for_pins(struct address_space *mapping)
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{
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XA_STATE(xas, &mapping->i_pages, 0);
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struct page *page;
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int error, scan;
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memfd_tag_pins(&xas);
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error = 0;
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for (scan = 0; scan <= LAST_SCAN; scan++) {
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unsigned int tagged = 0;
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if (!xas_marked(&xas, MEMFD_TAG_PINNED))
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break;
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if (!scan)
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lru_add_drain_all();
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else if (schedule_timeout_killable((HZ << scan) / 200))
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scan = LAST_SCAN;
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xas_set(&xas, 0);
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xas_lock_irq(&xas);
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xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
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bool clear = true;
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if (xa_is_value(page))
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continue;
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if (page_count(page) - page_mapcount(page) != 1) {
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/*
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* On the last scan, we clean up all those tags
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* we inserted; but make a note that we still
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* found pages pinned.
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*/
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if (scan == LAST_SCAN)
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error = -EBUSY;
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else
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clear = false;
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}
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if (clear)
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xas_clear_mark(&xas, MEMFD_TAG_PINNED);
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if (++tagged % XA_CHECK_SCHED)
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continue;
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xas_pause(&xas);
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xas_unlock_irq(&xas);
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cond_resched();
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xas_lock_irq(&xas);
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}
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xas_unlock_irq(&xas);
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}
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return error;
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}
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static unsigned int *memfd_file_seals_ptr(struct file *file)
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{
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if (shmem_file(file))
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return &SHMEM_I(file_inode(file))->seals;
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#ifdef CONFIG_HUGETLBFS
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if (is_file_hugepages(file))
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return &HUGETLBFS_I(file_inode(file))->seals;
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#endif
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return NULL;
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}
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#define F_ALL_SEALS (F_SEAL_SEAL | \
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F_SEAL_SHRINK | \
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F_SEAL_GROW | \
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F_SEAL_WRITE | \
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F_SEAL_FUTURE_WRITE)
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static int memfd_add_seals(struct file *file, unsigned int seals)
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{
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struct inode *inode = file_inode(file);
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unsigned int *file_seals;
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int error;
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/*
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* SEALING
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* Sealing allows multiple parties to share a tmpfs or hugetlbfs file
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* but restrict access to a specific subset of file operations. Seals
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* can only be added, but never removed. This way, mutually untrusted
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* parties can share common memory regions with a well-defined policy.
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* A malicious peer can thus never perform unwanted operations on a
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* shared object.
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*
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* Seals are only supported on special tmpfs or hugetlbfs files and
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* always affect the whole underlying inode. Once a seal is set, it
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* may prevent some kinds of access to the file. Currently, the
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* following seals are defined:
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* SEAL_SEAL: Prevent further seals from being set on this file
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* SEAL_SHRINK: Prevent the file from shrinking
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* SEAL_GROW: Prevent the file from growing
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* SEAL_WRITE: Prevent write access to the file
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*
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* As we don't require any trust relationship between two parties, we
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* must prevent seals from being removed. Therefore, sealing a file
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* only adds a given set of seals to the file, it never touches
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* existing seals. Furthermore, the "setting seals"-operation can be
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* sealed itself, which basically prevents any further seal from being
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* added.
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*
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* Semantics of sealing are only defined on volatile files. Only
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* anonymous tmpfs and hugetlbfs files support sealing. More
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* importantly, seals are never written to disk. Therefore, there's
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* no plan to support it on other file types.
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*/
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if (!(file->f_mode & FMODE_WRITE))
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return -EPERM;
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if (seals & ~(unsigned int)F_ALL_SEALS)
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return -EINVAL;
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inode_lock(inode);
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file_seals = memfd_file_seals_ptr(file);
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if (!file_seals) {
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error = -EINVAL;
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goto unlock;
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}
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if (*file_seals & F_SEAL_SEAL) {
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error = -EPERM;
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goto unlock;
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}
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if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
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error = mapping_deny_writable(file->f_mapping);
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if (error)
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goto unlock;
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error = memfd_wait_for_pins(file->f_mapping);
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if (error) {
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mapping_allow_writable(file->f_mapping);
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goto unlock;
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}
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}
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*file_seals |= seals;
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error = 0;
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unlock:
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inode_unlock(inode);
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return error;
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}
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static int memfd_get_seals(struct file *file)
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{
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unsigned int *seals = memfd_file_seals_ptr(file);
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return seals ? *seals : -EINVAL;
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}
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long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
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{
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long error;
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switch (cmd) {
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case F_ADD_SEALS:
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/* disallow upper 32bit */
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if (arg > UINT_MAX)
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return -EINVAL;
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error = memfd_add_seals(file, arg);
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break;
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case F_GET_SEALS:
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error = memfd_get_seals(file);
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break;
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default:
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error = -EINVAL;
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break;
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}
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return error;
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}
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#define MFD_NAME_PREFIX "memfd:"
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#define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
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#define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
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#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
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SYSCALL_DEFINE2(memfd_create,
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const char __user *, uname,
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unsigned int, flags)
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{
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unsigned int *file_seals;
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struct file *file;
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int fd, error;
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char *name;
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long len;
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if (!(flags & MFD_HUGETLB)) {
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if (flags & ~(unsigned int)MFD_ALL_FLAGS)
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return -EINVAL;
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} else {
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/* Allow huge page size encoding in flags. */
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if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
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(MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
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return -EINVAL;
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}
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/* length includes terminating zero */
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len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
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if (len <= 0)
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return -EFAULT;
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if (len > MFD_NAME_MAX_LEN + 1)
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return -EINVAL;
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name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
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if (!name)
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return -ENOMEM;
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strcpy(name, MFD_NAME_PREFIX);
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if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
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error = -EFAULT;
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goto err_name;
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}
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/* terminating-zero may have changed after strnlen_user() returned */
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if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
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error = -EFAULT;
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goto err_name;
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}
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fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
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if (fd < 0) {
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error = fd;
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goto err_name;
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}
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if (flags & MFD_HUGETLB) {
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struct user_struct *user = NULL;
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file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
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HUGETLB_ANONHUGE_INODE,
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(flags >> MFD_HUGE_SHIFT) &
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MFD_HUGE_MASK);
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} else
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file = shmem_file_setup(name, 0, VM_NORESERVE);
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if (IS_ERR(file)) {
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error = PTR_ERR(file);
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goto err_fd;
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}
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file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
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file->f_flags |= O_LARGEFILE;
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if (flags & MFD_ALLOW_SEALING) {
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file_seals = memfd_file_seals_ptr(file);
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*file_seals &= ~F_SEAL_SEAL;
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}
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fd_install(fd, file);
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kfree(name);
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return fd;
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err_fd:
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put_unused_fd(fd);
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err_name:
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kfree(name);
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return error;
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}
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