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

commit 3f5f8ee991 upstream.

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 fc961522dd ("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: ca06197382 ("exfat: add directory operations")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/exfat/dir.c

@@ -1081,12 +1081,12 @@ rewind:
 				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;
 				}
@@ -1097,6 +1097,7 @@ rewind:
 					uniname += EXFAT_FILE_NAME_LEN;
 
 				len = exfat_extract_uni_name(ep, entry_uniname);
+				brelse(bh);
 				name_len += len;
 
 				unichar = *(uniname+len);
@@ -1115,6 +1116,7 @@ rewind:
 				continue;
 			}
 
+			brelse(bh);
 			if (entry_type &
 					(TYPE_CRITICAL_SEC | TYPE_BENIGN_SEC)) {
 				if (step == DIRENT_STEP_SECD) {