Re: [PATCH] fs: block cross-uid sticky symlinks

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Kees Cook (kees.cook@canonical.com):
> A long-standing class of security issues is the symlink-based
> time-of-check-time-of-use race, most commonly seen in world-writable
> directories like /tmp. The common method of exploitation of this flaw
> is to cross privilege boundaries when following a given symlink (i.e. a
> root process follows a symlink belonging to another user).  For a likely
> incomplete list of hundreds of examples across the years, please see:
> http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

That is quite a list :)

> +int cap_inode_follow_link(struct dentry *dentry,
> +			  struct nameidata *nameidata)
> +{
> +	const struct inode *parent = dentry->d_parent->d_inode;
> +	const struct inode *inode = dentry->d_inode;
> +	const struct cred *cred = current_cred();
> +
> +	if (weak_sticky_symlinks)
> +		return 0;
> +
> +	if (S_ISLNK(inode->i_mode) &&

Q: is the S_ISLNK() check actually needed?

In either case:

Acked-by: Serge E. Hallyn <serue@us.ibm.com>

thanks,
-serge