CVE-2026-63808

Summary

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.

Affected Software

Weaknesses

References

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

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