From 3f5f8ee9917cc2b9076ac533492d8a200edcabb8 Mon Sep 17 00:00:00 2001 From: Michael Bommarito Date: Wed, 22 Apr 2026 11:58:44 -0400 Subject: [PATCH] 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. Fixes: ca06197382bd ("exfat: add directory operations") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-7 Signed-off-by: Michael Bommarito Signed-off-by: Namjae Jeon --- fs/exfat/dir.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/exfat/dir.c b/fs/exfat/dir.c index ac008ccaa97d..561fd2349218 100644 --- a/fs/exfat/dir.c +++ b/fs/exfat/dir.c @@ -1027,12 +1027,12 @@ int exfat_find_dir_entry(struct super_block *sb, struct exfat_inode_info *ei, continue; } - brelse(bh); if (entry_type == TYPE_EXTEND) { unsigned short entry_uniname[16], unichar; if (step != DIRENT_STEP_NAME || name_len >= MAX_NAME_LENGTH) { + brelse(bh); step = DIRENT_STEP_FILE; continue; } @@ -1043,6 +1043,7 @@ int exfat_find_dir_entry(struct super_block *sb, struct exfat_inode_info *ei, uniname += EXFAT_FILE_NAME_LEN; len = exfat_extract_uni_name(ep, entry_uniname); + brelse(bh); name_len += len; unichar = *(uniname+len); @@ -1061,6 +1062,7 @@ int exfat_find_dir_entry(struct super_block *sb, struct exfat_inode_info *ei, continue; } + brelse(bh); if (entry_type & (TYPE_CRITICAL_SEC | TYPE_BENIGN_SEC)) { if (step == DIRENT_STEP_SECD) {