linux-ext4 - Re: [PATCH] ext4: Verify fast symlink length

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Message-ID: <24k77s6puqfnzyj3h4yagfyxln777ejohc56xpfll3n7yjziap@lkyoqk725rx6>
Date: Fri, 7 Feb 2025 13:54:13 +0100
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, 
	syzbot+48a99e426f29859818c0@...kaller.appspotmail.com, "Darrick J. Wong" <djwong@...nel.org>
Subject: Re: [PATCH] ext4: Verify fast symlink length

On Thu 06-02-25 10:24:19, Theodore Ts'o wrote:
> On Thu, Feb 06, 2025 at 10:44:55AM +0100, Jan Kara wrote:
> > 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.

Thanks for having a look but as Eric writes, there's:

                if (IS_ENCRYPTED(inode)) {
                        inode->i_op = &ext4_encrypted_symlink_inode_operations;
		} else if (ext4_inode_is_fast_symlink(inode)) {

just above the context of this hunk so I *think* encrypted symlinks cannot
get into this code?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR