On Sun, 2010-05-30 at 20:04 -0700, Kees Cook wrote: > 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 > > The solution is to permit symlinks to only be followed when outside a sticky > world-writable directory, or when the uid of the symlink and follower match, > or when the directory owner matches the symlink's owner. > > Some pointers to the history of earlier discussion that I could find: > > 1996 Aug, Zygo Blaxell > http://marc.info/?l=bugtraq&m=87602167419830&w=2 > 1996 Oct, Andrew Tridgell > http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html > 1997 Dec, Albert D Cahalan > http://lkml.org/lkml/1997/12/16/4 > 2005 Feb, Lorenzo Hernández García-Hierro > http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html > > Past objections and rebuttals could be summarized as: > > - Violates POSIX. > - POSIX didn't consider this situation and it's not useful to follow > a broken specification at the cost of security. > - Might break unknown applications that use this feature. > - Applications that break because of the change are easy to spot and > fix. Applications that are vulnerable to symlink ToCToU by not having > the change aren't. > - Applications should just use mkstemp() or O_CREATE|O_EXCL. > - True, but applications are not perfect, and new software is written > all the time that makes these mistakes; blocking this flaw at the > kernel is a single solution to the entire class of vulnerability. > > This patch is based on the patch in Openwall and grsecurity. I have > added a sysctl to toggle the behavior back to the old logic via > /proc/sys/fs/weak-sticky-symlinks, documentation, and a ratelimited > warning. > > v2: > - dropped redundant S_ISLNK check. > - moved sysctl extern into security.h. > - asked to include CC to linux-fsdevel. We need to call this function in the SELinux case. So you'll need a patch like the one attached (not even compiled but I think it is right) diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 5c9f25b..d6ebee2 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -2668,8 +2668,13 @@ static int selinux_inode_readlink(struct dentry *dentry) static int selinux_inode_follow_link(struct dentry *dentry, struct nameidata *nameidata) { + int rc; const struct cred *cred = current_cred(); + rc = cap_inode_follow_link(dentry, nameidata); + if (rc) + return rc; + return dentry_has_perm(cred, NULL, dentry, FILE__READ); } > +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 ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) && > + parent->i_uid != inode->i_uid && > + cred->fsuid != inode->i_uid) { > + printk_ratelimited(KERN_NOTICE "non-matching-uid symlink " > + "following attempted in sticky-directory by " > + "%s (fsuid %d)\n", current->comm, cred->fsuid); > + return -EACCES; > + } > + return 0; > +} What stops us from racing between the assignment of parent and it's first use with a rename on our object and rmdir on the old parent? I'm wondering if we need to be doing this test holding dentry->d_lock (which is what protects dentry->d_parent if I recall correctly) Certainly doesn't fix all of the raciness, but I think it would close the opps part. Maybe someone who knows the VFS better can tell me if I am misguided. -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
