CVE-2026-63808 — exfat: fix potential use-after-free in exfat_find_dir_entry()

Affected: Linux

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 clean…

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CVE-2026-63808 — exfat: fix potential use-after-free in exfat_find_dir_entry()