fs/minix: reject too-large maximum file size

If the minix filesystem tries to map a very large logical block number to
its on-disk location, block_to_path() can return offsets that are too
large, causing out-of-bounds memory accesses when accessing indirect index
blocks.  This should be prevented by the check against the maximum file
size, but this doesn't work because the maximum file size is read directly
from the on-disk superblock and isn't validated itself.

Fix this by validating the maximum file size at mount time.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+c7d9ec7a1a7272dd71b3@syzkaller.appspotmail.com
Reported-by: syzbot+3b7b03a0c28948054fb5@syzkaller.appspotmail.com
Reported-by: syzbot+6e056ee473568865f3e6@syzkaller.appspotmail.com
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Qiujun Huang <anenbupt@gmail.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200628060846.682158-4-ebiggers@kernel.orgSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/minix/inode.c

@@ -150,6 +150,23 @@ static int minix_remount (struct super_block * sb, int * flags, char * data)
 	return 0;
 }
 
+static bool minix_check_superblock(struct minix_sb_info *sbi)
+{
+	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
+		return false;
+
+	/*
+	 * s_max_size must not exceed the block mapping limitation.  This check
+	 * is only needed for V1 filesystems, since V2/V3 support an extra level
+	 * of indirect blocks which places the limit well above U32_MAX.
+	 */
+	if (sbi->s_version == MINIX_V1 &&
+	    sbi->s_max_size > (7 + 512 + 512*512) * BLOCK_SIZE)
+		return false;
+
+	return true;
+}
+
 static int minix_fill_super(struct super_block *s, void *data, int silent)
 {
 	struct buffer_head *bh;
@@ -228,11 +245,12 @@ static int minix_fill_super(struct super_block *s, void *data, int silent)
 	} else
 		goto out_no_fs;
 
+	if (!minix_check_superblock(sbi))
+		goto out_illegal_sb;
+
 	/*
 	 * Allocate the buffer map to keep the superblock small.
 	 */
-	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
-		goto out_illegal_sb;
 	i = (sbi->s_imap_blocks + sbi->s_zmap_blocks) * sizeof(bh);
 	map = kzalloc(i, GFP_KERNEL);
 	if (!map)