CVE-2026-63808

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

exfat: fix potential use-after-free in exfat_find_dir_entry()

In exfat_find_dir_entry(), the buffer_head obtained from exfat_get_dentry() is released with brelse(bh) before the fall-through TYPE_EXTEND branch reads the directory entry through ep (which points into bh->b_data):

brelse(bh);
if (entry_type == TYPE_EXTEND) {
	...
	len = exfat_extract_uni_name(ep, entry_uniname);
	...
}

After brelse() drops our reference, nothing guarantees that the underlying page backing bh->b_data remains valid for the subsequent exfat_extract_uni_name() read. This is the same pattern fixed in commit fc961522ddbd (exfat: Fix potential use after free in exfat_load_upcase_table()).

Move brelse(bh) so it runs after ep is no longer dereferenced on each branch.

Confirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y

• CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image (long filename with same-hash collisions forcing the TYPE_EXTEND path). With a debug-only invalidate_bdev() inserted between brelse(bh) and the ep read to make the stale-deref window deterministic, the unpatched kernel faults:

  BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0 BUG: unable to handle page fault for address: ffff88801a5fa0c2 Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0

With this patch applied, the same instrumented harness completes cleanly under the same sanitizer stack. I have not reproduced a crash on an uninstrumented kernel under ordinary reclaim; the instrumented A/B establishes the lifetime violation and that the patch closes it, not an unaided triggerability claim.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/06c4e1e9967d332ac33ba38b7819851089ff9359
• https://git.kernel.org/stable/c/3f5f8ee9917cc2b9076ac533492d8a200edcabb8
• https://git.kernel.org/stable/c/4d101016d5e587f820b3ae2d5bb6770d86342649
• https://git.kernel.org/stable/c/708b97e792945d3e4653939fd3405d71a61ad065
• https://git.kernel.org/stable/c/8e0abc17fbd7e305802e84fe98b4950d50f9c433
• https://git.kernel.org/stable/c/adfacfbaeae2cb760f492357cc36b41f84ef7f86
• https://git.kernel.org/stable/c/e48f413c2815787b8cade2795e194e3c4cd782ef
• https://git.kernel.org/stable/c/e6f1a11cfb808441a43ffae9b476cc135732cd27