proc: make check_mem_permission() return an mm_struct on success
This change allows us to take advantage of access_remote_vm(), which in turn eliminates a security issue with the mem_write() implementation. The previous implementation of mem_write() was insecure since the target task could exec a setuid-root binary between the permission check and the actual write. Holding a reference to the target mm_struct eliminates this vulnerability. Signed-off-by: Stephen Wilson <wilsons@start.ca> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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18f661bcf8
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8b0db9db19
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@ -191,14 +191,20 @@ static int proc_root_link(struct inode *inode, struct path *path)
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return result;
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}
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static int __check_mem_permission(struct task_struct *task)
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static struct mm_struct *__check_mem_permission(struct task_struct *task)
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{
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struct mm_struct *mm;
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mm = get_task_mm(task);
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if (!mm)
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return ERR_PTR(-EINVAL);
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/*
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* A task can always look at itself, in case it chooses
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* to use system calls instead of load instructions.
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*/
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if (task == current)
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return 0;
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return mm;
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/*
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* If current is actively ptrace'ing, and would also be
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@ -210,20 +216,23 @@ static int __check_mem_permission(struct task_struct *task)
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match = (tracehook_tracer_task(task) == current);
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rcu_read_unlock();
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if (match && ptrace_may_access(task, PTRACE_MODE_ATTACH))
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return 0;
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return mm;
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}
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/*
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* Noone else is allowed.
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*/
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return -EPERM;
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mmput(mm);
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return ERR_PTR(-EPERM);
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}
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/*
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* Return zero if current may access user memory in @task, -error if not.
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* If current may access user memory in @task return a reference to the
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* corresponding mm, otherwise ERR_PTR.
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*/
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static int check_mem_permission(struct task_struct *task)
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static struct mm_struct *check_mem_permission(struct task_struct *task)
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{
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struct mm_struct *mm;
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int err;
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/*
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@ -232,12 +241,12 @@ static int check_mem_permission(struct task_struct *task)
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*/
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err = mutex_lock_killable(&task->signal->cred_guard_mutex);
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if (err)
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return err;
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return ERR_PTR(err);
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err = __check_mem_permission(task);
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mm = __check_mem_permission(task);
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mutex_unlock(&task->signal->cred_guard_mutex);
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return err;
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return mm;
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}
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struct mm_struct *mm_for_maps(struct task_struct *task)
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@ -795,18 +804,14 @@ static ssize_t mem_read(struct file * file, char __user * buf,
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if (!task)
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goto out_no_task;
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if (check_mem_permission(task))
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goto out;
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ret = -ENOMEM;
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page = (char *)__get_free_page(GFP_TEMPORARY);
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if (!page)
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goto out;
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ret = 0;
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mm = get_task_mm(task);
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if (!mm)
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mm = check_mem_permission(task);
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ret = PTR_ERR(mm);
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if (IS_ERR(mm))
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goto out_free;
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ret = -EIO;
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@ -820,8 +825,8 @@ static ssize_t mem_read(struct file * file, char __user * buf,
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int this_len, retval;
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this_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
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retval = access_process_vm(task, src, page, this_len, 0);
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if (!retval || check_mem_permission(task)) {
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retval = access_remote_vm(mm, src, page, this_len, 0);
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if (!retval) {
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if (!ret)
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ret = -EIO;
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break;
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@ -860,22 +865,25 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
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char *page;
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struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);
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unsigned long dst = *ppos;
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struct mm_struct *mm;
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copied = -ESRCH;
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if (!task)
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goto out_no_task;
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if (check_mem_permission(task))
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goto out;
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mm = check_mem_permission(task);
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copied = PTR_ERR(mm);
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if (IS_ERR(mm))
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goto out_task;
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copied = -EIO;
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if (file->private_data != (void *)((long)current->self_exec_id))
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goto out;
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goto out_mm;
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copied = -ENOMEM;
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page = (char *)__get_free_page(GFP_TEMPORARY);
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if (!page)
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goto out;
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goto out_mm;
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copied = 0;
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while (count > 0) {
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@ -886,7 +894,7 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
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copied = -EFAULT;
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break;
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}
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retval = access_process_vm(task, dst, page, this_len, 1);
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retval = access_remote_vm(mm, dst, page, this_len, 1);
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if (!retval) {
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if (!copied)
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copied = -EIO;
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@ -899,7 +907,9 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
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}
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*ppos = dst;
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free_page((unsigned long) page);
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out:
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out_mm:
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mmput(mm);
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out_task:
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put_task_struct(task);
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out_no_task:
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return copied;
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