* + userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch added to -mm tree
@ 2021-05-10 5:33 akpm
0 siblings, 0 replies; only message in thread
From: akpm @ 2021-05-10 5:33 UTC (permalink / raw)
To: axelrasmussen, hughd, mm-commits, peterx, stable
The patch titled
Subject: userfaultfd: release page in error path to avoid BUG_ON
has been added to the -mm tree. Its filename is
userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch
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------------------------------------------------------
From: Axel Rasmussen <axelrasmussen@google.com>
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/20210428230858.348400-1-axelrasmussen@google.com
Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reported-by: Hugh Dickins <hughd@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/shmem.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
--- 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);
_
Patches currently in -mm which might be from axelrasmussen@google.com are
userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch
userfaultfd-hugetlbfs-avoid-including-userfaultfd_kh-in-hugetlbh.patch
userfaultfd-shmem-combine-shmem_mcopy_atomicmfill_zeropage_pte.patch
userfaultfd-shmem-support-minor-fault-registration-for-shmem.patch
userfaultfd-shmem-support-uffdio_continue-for-shmem.patch
userfaultfd-shmem-advertise-shmem-minor-fault-support.patch
userfaultfd-shmem-modify-shmem_mfill_atomic_pte-to-use-install_pte.patch
userfaultfd-selftests-use-memfd_create-for-shmem-test-type.patch
userfaultfd-selftests-create-alias-mappings-in-the-shmem-test.patch
userfaultfd-selftests-reinitialize-test-context-in-each-test.patch
userfaultfd-selftests-exercise-minor-fault-handling-shmem-support.patch
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2021-05-10 5:33 + userfaultfd-release-page-in-error-path-to-avoid-bug_on.patch added to -mm tree akpm
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