CVE-2026-92499

Summary

In the Linux kernel, the following vulnerability has been resolved:

ext4: validate readdir offset before accessing dirent

A corrupted directory can trigger the following KASAN report when ext4_readdir() resumes from an invalid position:

BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x5ef/0x820 Read of size 2 at addr ffff88810a646000 by task repro_linear/509

Call Trace: <TASK> dump_stack_lvl+0x53/0x70 print_report+0xd0/0x630 kasan_report+0xce/0x100 __ext4_check_dir_entry+0x5ef/0x820 ext4_readdir+0xcde/0x2b70 iterate_dir+0x1a1/0x520 __x64_sys_getdents64+0x12b/0x220 do_syscall_64+0xf9/0x540 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK>

KASAN reports use-after-free because the out-of-bounds access lands in an adjacent freed page. The directory buffer itself is still referenced.

ext4_dir_llseek() invalidates the directory cookie so that ext4_readdir() rescans directory entries from the start of the block. The rescan checks only the lower bound of rec_len before advancing. A corrupted rec_len can therefore place the offset where the block has insufficient space for a complete directory entry. The rescan itself may dereference that truncated entry, or the main loop may pass it to __ext4_check_dir_entry(). The latter reads de->rec_len before validating the range. For example:

block offset 0 4092 4096 |—- de1.rec_len = 4092 —–|—-| de2.inode | de2.rec_len ^ OOB, reported as UAF

de2 starts at offset 4092 in this 4 KiB block. Its four-byte inode fits in the block, but its rec_len starts at offset 4096 and crosses the boundary.

The minimum safe length is inode-dependent. Encrypted and casefolded directory entries need eight additional hash bytes, while a valid metadata checksum tail is only 12 bytes.

Cache the metadata checksum feature state and derive the minimum directory entry length from the on-disk format. Use it to bound both the rescan and the offset passed to the main loop. Report an offset in a truncated block tail and skip the remainder of the block, while continuing to accept an offset exactly at the block boundary.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/64d445d40e5ea4c4d4d88880db753b370cb69161
• https://git.kernel.org/stable/c/b4c728577933753180e3e97f08424d5bcaff705e
• https://git.kernel.org/stable/c/bc4b7b0414c33b2c8898eb04386df0d21a13dad8

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