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exfat: bound uniname advance in exfat_find_dir_entry()

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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 d42334578e ("exfat: check if filename entries exceeds
max filename length")); exfat_find_dir_entry() never got the
equivalent.  Track the per-entry write offset as a count and reject a
fragment once the offset, or the offset plus the extracted length, would
exceed MAX_NAME_LENGTH, before forming the output pointer.

Fixes: ca06197382 ("exfat: add directory operations")
Cc: stable@vger.kernel.org
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
This commit is contained in:
Bryam Vargas 2026-06-12 14:29:06 -05:00 committed by Namjae Jeon
parent 03a43677ca
commit 3a1230e7b0
1 changed files with 8 additions and 5 deletions
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13
fs/exfat/dir.c
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@ -1069,6 +1069,7 @@ int exfat_find_dir_entry(struct super_block *sb, struct exfat_inode_info *ei,
if (entry_type == TYPE_EXTEND) {
unsigned short entry_uniname[16], unichar;
unsigned int offset;
if (step != DIRENT_STEP_NAME ||
name_len >= MAX_NAME_LENGTH) {
@ -1077,13 +1078,15 @@ int exfat_find_dir_entry(struct super_block *sb, struct exfat_inode_info *ei,
continue;
}
if (++order == 2)
uniname = p_uniname->name;
else
uniname += EXFAT_FILE_NAME_LEN;
offset = (++order - 2) * EXFAT_FILE_NAME_LEN;
len = exfat_extract_uni_name(ep, entry_uniname);
brelse(bh);
if (offset > MAX_NAME_LENGTH ||
len > MAX_NAME_LENGTH - offset) {
step = DIRENT_STEP_FILE;
continue;
}
uniname = p_uniname->name + offset;
name_len += len;
unichar = *(uniname+len);