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mm: fix potential data race in SyS_swapon 

While running KernelThreadSanitizer (ktsan) on upstream kernel with
trinity, we got a few reports from SyS_swapon, here is one of them:

Read of size 8 by thread T307 (K7621):
 [<     inlined    >] SyS_swapon+0x3c0/0x1850 SYSC_swapon mm/swapfile.c:2395
 [<ffffffff812242c0>] SyS_swapon+0x3c0/0x1850 mm/swapfile.c:2345
 [<ffffffff81e97c8a>] ia32_do_call+0x1b/0x25

Looks like the swap_lock should be taken when iterating through the
swap_info array on lines 2392 - 2401: q->swap_file may be reset to
NULL by another thread before it is dereferenced for f_mapping.

But why is that iteration needed at all?  Doesn't the claim_swapfile()
which follows do all that is needed to check for a duplicate entry -
FMODE_EXCL on a bdev, testing IS_SWAPFILE under i_mutex on a regfile?

Well, not quite: bd_may_claim() allows the same "holder" to claim the
bdev again, so we do need to use a different holder than "sys_swapon";
and we should not replace appropriate -EBUSY by inappropriate -EINVAL.

Index i was reused in a cpu loop further down: renamed cpu there.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
hifive-unleashed-5.1
Hugh Dickins 2015-08-17 17:34:27 -07:00 committed by Al Viro
parent 061f98e959
commit 6f179af88f
1 changed files with 7 additions and 18 deletions
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25
mm/swapfile.c
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@ -2143,11 +2143,10 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
if (S_ISBLK(inode->i_mode)) {
p->bdev = bdgrab(I_BDEV(inode));
error = blkdev_get(p->bdev,
FMODE_READ | FMODE_WRITE | FMODE_EXCL,
sys_swapon);
FMODE_READ | FMODE_WRITE | FMODE_EXCL, p);
if (error < 0) {
p->bdev = NULL;
return -EINVAL;
return error;
}
p->old_block_size = block_size(p->bdev);
error = set_blocksize(p->bdev, PAGE_SIZE);
@ -2348,7 +2347,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
struct filename *name;
struct file *swap_file = NULL;
struct address_space *mapping;
int i;
int prio;
int error;
union swap_header *swap_header;
@ -2388,19 +2386,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
p->swap_file = swap_file;
mapping = swap_file->f_mapping;
for (i = 0; i < nr_swapfiles; i++) {
struct swap_info_struct *q = swap_info[i];
if (q == p || !q->swap_file)
continue;
if (mapping == q->swap_file->f_mapping) {
error = -EBUSY;
goto bad_swap;
}
}
inode = mapping->host;
/* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
error = claim_swapfile(p, inode);
if (unlikely(error))
@ -2433,6 +2420,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
goto bad_swap;
}
if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
int cpu;
p->flags |= SWP_SOLIDSTATE;
/*
* select a random position to start with to help wear leveling
@ -2451,9 +2440,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
error = -ENOMEM;
goto bad_swap;
}
for_each_possible_cpu(i) {
for_each_possible_cpu(cpu) {
struct percpu_cluster *cluster;
cluster = per_cpu_ptr(p->percpu_cluster, i);
cluster = per_cpu_ptr(p->percpu_cluster, cpu);
cluster_set_null(&cluster->index);
}
}