linux-kernel - [PATCH] Squashfs: check return result of sb_min

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Message-Id: <20250409024747.876480-1-phillip@squashfs.org.uk>
Date: Wed,  9 Apr 2025 03:47:47 +0100
From: Phillip Lougher <phillip@...ashfs.org.uk>
To: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Cc: Phillip Lougher <phillip@...ashfs.org.uk>,
	syzbot+65761fc25a137b9c8c6e@...kaller.appspotmail.com
Subject: [PATCH] Squashfs: check return result of sb_min_blocksize

Syzkaller reports an "UBSAN: shift-out-of-bounds in squashfs_bio_read" bug.

Syzkaller forks multiple processes which after mounting the Squashfs
filesystem, issues an ioctl("/dev/loop0", LOOP_SET_BLOCK_SIZE, 0x8000).
Now if this ioctl occurs at the same time another process is in the
process of mounting a Squashfs filesystem on /dev/loop0, the failure
occurs.   When this happens the following code in squashfs_fill_super()
fails.

----
msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
msblk->devblksize_log2 = ffz(~msblk->devblksize);
----

sb_min_blocksize() returns 0, which means msblk->devblksize is set to 0.

As a result, ffz(~msblk->devblksize) returns 64, and msblk->devblksize_log2
is set to 64.

This subsequently causes the

UBSAN: shift-out-of-bounds in fs/squashfs/block.c:195:36
shift exponent 64 is too large for 64-bit type 'u64' (aka
'unsigned long long')

This commit adds a check for a 0 return by sb_min_blocksize().

Reported-by: syzbot+65761fc25a137b9c8c6e@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/67f0dd7a.050a0220.0a13.0230.GAE@google.com/
Fixes: 0aa666190509 ("Squashfs: super block operations")
Signed-off-by: Phillip Lougher <phillip@...ashfs.org.uk>
---
 fs/squashfs/super.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index 67c55fe32ce8..d8256b23ad9a 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -202,6 +202,11 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	msblk->panic_on_errors = (opts->errors == Opt_errors_panic);
 
 	msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
+	if (!msblk->devblksize) {
+		errorf(fc, "squashfs: unable to set blocksize\n");
+		return -EINVAL;
+	}
+
 	msblk->devblksize_log2 = ffz(~msblk->devblksize);
 
 	mutex_init(&msblk->meta_index_mutex);
-- 
2.39.2