[06/13] userfaultfd: release page in error path to avoid BUG_ON

Commit Message

Andrew Morton May 15, 2021, 12:27 a.m. UTC

From: Axel Rasmussen <[e-mail]>
Subject: userfaultfd: release page in error path to avoid BUG_ON

Consider the following sequence of events:

1. Userspace issues a UFFD ioctl, which ends up calling into
   shmem_mfill_atomic_pte(). We successfully account the blocks, we
   shmem_alloc_page(), but then the copy_from_user() fails. We return
   -ENOENT. We don't release the page we allocated.
2. Our caller detects this error code, tries the copy_from_user() after
   dropping the mmap_lock, and retries, calling back into
   shmem_mfill_atomic_pte().
3. Meanwhile, let's say another process filled up the tmpfs being used.
4. So shmem_mfill_atomic_pte() fails to account blocks this time, and
   immediately returns - without releasing the page.

This triggers a BUG_ON in our caller, which asserts that the page
should always be consumed, unless -ENOENT is returned.

To fix this, detect if we have such a "dangling" page when accounting
fails, and if so, release it before returning.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
Signed-off-by: Axel Rasmussen <[e-mail]>
Reported-by: Hugh Dickins <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/shmem.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Patch

--- a/mm/shmem.c~userfaultfd-release-page-in-error-path-to-avoid-bug_on
+++ a/mm/shmem.c
@@ -2361,8 +2361,18 @@  static int shmem_mfill_atomic_pte(struct
 	pgoff_t offset, max_off;
 
 	ret = -ENOMEM;
-	if (!shmem_inode_acct_block(inode, 1))
+	if (!shmem_inode_acct_block(inode, 1)) {
+		/*
+		 * We may have got a page, returned -ENOENT triggering a retry,
+		 * and now we find ourselves with -ENOMEM. Release the page, to
+		 * avoid a BUG_ON in our caller.
+		 */
+		if (unlikely(*pagep)) {
+			put_page(*pagep);
+			*pagep = NULL;
+		}
 		goto out;
+	}
 
 	if (!*pagep) {
 		page = shmem_alloc_page(gfp, info, pgoff);