[PATCH] exfat: bound uniname advance in exfat_find_dir_entry()

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

In exfat_find_dir_entry(), each TYPE_EXTEND (file name) entry advances the
output pointer by a fixed amount while the loop guard only tracks the
accumulated name length:

	if (++order == 2)
		uniname = p_uniname->name;
	else
		uniname += EXFAT_FILE_NAME_LEN;
	len = exfat_extract_uni_name(ep, entry_uniname);
	name_len += len;
	unichar = *(uniname+len);
	*(uniname+len) = 0x0;

uniname grows by EXFAT_FILE_NAME_LEN (15) per name entry, but name_len
grows only by the actual extracted length, which is shorter when a name
fragment contains an early NUL.  The only guard is
`name_len >= MAX_NAME_LENGTH`, so a crafted directory with many short
name fragments lets uniname run far past the
p_uniname->name[MAX_NAME_LENGTH + 3] buffer while name_len stays small,
causing an out-of-bounds read and write at *(uniname+len).

The sibling extractor exfat_get_uniname_from_ext_entry() already stops
on a short fragment (the lockstep `len != EXFAT_FILE_NAME_LEN` guard
added in commit d42334578eba ("exfat: check if filename entries exceeds
max filename length")); exfat_find_dir_entry() never got the
equivalent.  Bound the access to the p_uniname->name buffer.

Fixes: ca06197382bd ("exfat: add directory operations")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
---
 fs/exfat/dir.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/exfat/dir.c b/fs/exfat/dir.c
index ac008ccaa97d..d4769a0f9978 100644
--- a/fs/exfat/dir.c
+++ b/fs/exfat/dir.c
@@ -1043,6 +1043,11 @@ 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);
+				if (uniname + len >
+				    p_uniname->name + MAX_NAME_LENGTH) {
+					step = DIRENT_STEP_FILE;
+					continue;
+				}
 				name_len += len;
 
 				unichar = *(uniname+len);

---
base-commit: 8e65320d91cdc3b241d4b94855c88459b91abf66
change-id: 20260611-b4-disp-c0e7d249-c1db1bca7353

Best regards,
-- 
Bryam Vargas <hexlabsecurity@xxxxxxxxx>