[PATCH] ext4: stop retrying saturated xattr cache entries

[Matthias Goergens <matthias.goergens@gmail.com>]

ext4_xattr_block_set() retries when a cache entry selected for reuse
has a saturated reference count after taking the buffer lock. The retry
returns to the mbcache lookup without making that entry ineligible, so
it can select the same unusable entry indefinitely. A task spinning
there can hold the parent directory's i_rwsem and leave concurrent
rmdir callers blocked.

Normally a reusable entry has a reference count below
EXT4_XATTR_REFCOUNT_MAX because the count and MBE_REUSABLE_B are
updated under the same buffer lock. A corrupted filesystem can violate
that invariant. The syzbot reproducer reports allocator and xattr
corruption before triggering this retry loop.

Check the untrusted on-disk count before incrementing it, avoiding
overflow, and clear MBE_REUSABLE_B when it is already saturated. The
next lookup then skips the entry that was just proven unusable. This
mirrors the normal transition at EXT4_XATTR_REFCOUNT_MAX; the release
path marks the entry reusable again on the exact 1024-to-1023
transition.

Using the same QEMU harness and guest parameters, current unpatched
Linux hung in 6 of 8 420-second trials with the do_rmdir signature;
representative NMI backtraces caught the owner spinning in
ext4_xattr_block_set(). The patched kernel completed 28 of 28 trials
without a hung-task report; the final twelve trials exercised the
reviewed overflow-safe form of the change. syzbot's patch testing also
completed without reproducing the hang.

Reported-and-tested-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e68dbebd9617a9250e8d
Fixes: 65f8b80053a1 ("ext4: fix race when reusing xattr blocks")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
 fs/ext4/xattr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 982a1f831e228..3c1bb547b2f54 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -2075,12 +2075,13 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
 				 * stable so we can check the additional
 				 * reference fits.
 				 */
-				ref = le32_to_cpu(BHDR(new_bh)->h_refcount) + 1;
-				if (ref > EXT4_XATTR_REFCOUNT_MAX) {
+				ref = le32_to_cpu(BHDR(new_bh)->h_refcount);
+				if (ref >= EXT4_XATTR_REFCOUNT_MAX) {
 					/*
 					 * Undo everything and check mbcache
 					 * again.
 					 */
+					clear_bit(MBE_REUSABLE_B, &ce->e_flags);
 					unlock_buffer(new_bh);
 					dquot_free_block(inode,
 							 EXT4_C2B(EXT4_SB(sb),
@@ -2091,6 +2092,7 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
 					new_bh = NULL;
 					goto inserted;
 				}
+				ref++;
 				BHDR(new_bh)->h_refcount = cpu_to_le32(ref);
 				if (ref == EXT4_XATTR_REFCOUNT_MAX)
 					clear_bit(MBE_REUSABLE_B, &ce->e_flags);
-- 
2.55.0