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 --- 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