Re: [PATCH] ext4: Verify fast symlink length

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Feb 06, 2025 at 10:24:19AM -0500, Theodore Ts'o wrote:
> On Thu, Feb 06, 2025 at 10:44:55AM +0100, Jan Kara wrote:
> > 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
> > Tested-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..64e280fed911 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 >= sizeof(ei->i_data) ||
> > +			    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;
> > +			}
> >  			inode_set_cached_link(inode, (char *)ei->i_data,
> >  					      inode->i_size);
> 
> 
> I don't think this will do the right thing if the fast symlink is
> encrypted.  See ext4_encrypted_get_link() in fs/ext4/symlink.c in the
> kernel sources, and also look at how e2fsck_pass1_check_symlink()
> handles checking the size of an encrypted, fast symlink.
> 

Encrypted symlinks are handled separately just a couple lines above.

- Eric