Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in vfs_readlink

[Kees Cook <kees@kernel.org>]

On Wed, Feb 05, 2025 at 07:21:04PM +0100, Jan Kara wrote:
> On Tue 04-02-25 01:46:28, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    69b8923f5003 Merge tag 'for-linus-6.14-ofs4' of git://git...
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=1258aeb0580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=57ab43c279fa614d
> > dashboard link: https://syzkaller.appspot.com/bug?extid=48a99e426f29859818c0
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15825724580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1658aeb0580000
> > 
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/ea84ac864e92/disk-69b8923f.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/6a465997b4e0/vmlinux-69b8923f.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/d72b67b2bd15/bzImage-69b8923f.xz
> > mounted in repro: https://storage.googleapis.com/syzbot-assets/7c2919610764/mount_0.gz
> > 
> > The issue was bisected to:
> > 
> > commit bae80473f7b0b25772619e7692019b1549d4a82c
> > Author: Mateusz Guzik <mjguzik@gmail.com>
> > Date:   Wed Nov 20 11:20:35 2024 +0000
> > 
> >     ext4: use inode_set_cached_link()
> > 
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1248c3df980000
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=1148c3df980000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1648c3df980000
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
> > Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
> 
> Please check attached patch:
> 
> #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR

> From df00b84402fb67d94a9eb6b86633092983cb388c Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@suse.cz>
> Date: Wed, 5 Feb 2025 19:02:35 +0100
> Subject: [PATCH] ext4: Verify fast symlink length
> 
> Verify fast symlink length stored in inode->i_size matches the string
> stored in the inode to avoid surprises from corrupted filesystems.
> 
> Reported-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
> Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
> Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/inode.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 7c54ae5fcbd4..fbda5a67f7f9 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5007,8 +5007,16 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
>  			inode->i_op = &ext4_encrypted_symlink_inode_operations;
>  		} else if (ext4_inode_is_fast_symlink(inode)) {
>  			inode->i_op = &ext4_fast_symlink_inode_operations;
> -			nd_terminate_link(ei->i_data, inode->i_size,
> -				sizeof(ei->i_data) - 1);
> +			if (inode->i_size == 0 ||
> +			    inode->i_size >= EXT4_N_BLOCKS * 4 ||

This took me a while to understand. ei->i_data is u32[15]. It looks like
EXT4_N_BLOCKS is also 15. I feel like it would be much more readable to
have the above check be:

			    inode->i_size >= sizeof(ei->i_data) ||

instead of using a literal "4" for sizeof(u32) as "4", and having a
EXT4_N_BLOCKS standing in for the literal "15" in i_data.
sizeof(ei->i_data) is precisely what is being checked, so why not use
it?

And while at it, the definition of i_data could be adjusted to to use
EXT4_N_BLOCKS, e.g.:

struct ext4_inode_info {
        __le32  i_data[EXT4_N_BLOCKS];     /* unconverted */

?

> +			    strnlen((char *)ei->i_data, inode->i_size + 1) !=
> +								inode->i_size) {
> +				ext4_error_inode(inode, function, line, 0,
> +					"invalid fast symlink length %llu",
> +					 (unsigned long long)inode->i_size);
> +				ret = -EFSCORRUPTED;
> +				goto bad_inode;
> +			}

But regardless, yes, the math checks out, and looks correct to me.

>  			inode_set_cached_link(inode, (char *)ei->i_data,
>  					      inode->i_size);
>  		} else {
> -- 
> 2.43.0
> 


-Kees

-- 
Kees Cook