isofs: fix KMSAN uninit-value bug in do_isofs_readdir()

In do_isofs_readdir() when assigning the variable
"struct iso_directory_record *de" the b_data field of the buffer_head is accessed and an offset is added to it, the size of b_data is 2048 and the offset size is 2047, meaning
"de = (struct iso_directory_record *) (bh->b_data + offset);" yields the final byte of the 2048 sized b_data block.

The first byte of the directory record (de_len) is then read and found to be 31, meaning the directory record size is 31 bytes long. The directory record is defined by the structure:

struct iso_directory_record {
__u8 length; // 1 byte
__u8 ext_attr_length; // 1 byte
__u8 extent[8]; // 8 bytes
__u8 size[8]; // 8 bytes
__u8 date[7]; // 7 bytes
__u8 flags; // 1 byte
__u8 file_unit_size; // 1 byte
__u8 interleave; // 1 byte
__u8 volume_sequence_number[4]; // 4 bytes
__u8 name_len; // 1 byte
char name[]; // variable size
} __attribute__((packed));

The fixed portion of this structure occupies 33 bytes. Therefore, a valid directory record must be at least 33 bytes long (even without considering the variable-length name field). Since de_len is only 31, it is insufficient to contain the complete fixed header.

The code later hits the following sanity check that compares de_len against the sum of de->name_len and sizeof(struct iso_directory_record):

	if (de_len < de->name_len[0] + sizeof(struct iso_directory_record)) {
		...
	}

Since the fixed portion of the structure is
33 bytes (up to and including name_len member),
a valid record should have de_len of at least 33 bytes; here, however, de_len is too short, and the field de->name_len (located at offset 32) is accessed even though it lies beyond the available 31 bytes.

This access on the corrupted isofs data triggers a KASAN uninitialized memory warning. The fix would be to first verify that de_len is at least sizeof(struct iso_directory_record) before accessing any fields like de->name_len.

Reported-by syzbot <[email protected]>

Tested-by syzbot <[email protected]>

Closes https://syzkaller.appspot.com/bug?extid=812641c6c3d7586a1613

Fixes 2deb1acc653c ("isofs: fix access to unallocated memory when reading corrupted filesystem")

Signed-off-by Qasim Ijaz <[email protected]>

---
 fs/isofs/dir.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

fs/isofs/dir.c +2 -1

index eb2f8273e6f1..366ac8b95330 100644
@@ -147,7 +147,8 @@ static int do_isofs_readdir(struct inode *inode, struct file *file,
 			de = tmpde;
 		}
 		/* Basic sanity check, whether name doesn't exceed dir entry */
-		if (de_len < de->name_len[0] +
+		if (de_len < sizeof(struct iso_directory_record) ||
+		    de_len < de->name_len[0] +
 					sizeof(struct iso_directory_record)) {
 			printk(KERN_NOTICE "iso9660: Corrupted directory entry"
 			       " in block %lu of inode %lu\n", block,

Signature · 1 line

2.39.5