On Sat, 7 Jan 2012 10:55:48 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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 Hern__ndez Garc__a-Hierro > http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html > 2010 May, Kees Cook > https://lkml.org/lkml/2010/5/30/144 > > 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. Additionally, no applications have yet been found > that rely on this behavior. > - 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 should live in the core VFS. > - This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135) > - This should live in an LSM. > - This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188) > > This patch is based on the patch in Openwall and grsecurity, along with > suggestions from Al Viro. I have added a sysctl to enable the protected > behavior, documentation, and an audit notification. Looks reasonable to me. It's a viropatch. I shall merge it into 3.4-rc1 if nothing happens to prevent that. > ... > > +config PROTECTED_STICKY_SYMLINKS > + bool "Evaluate vulnerable symlink conditions" > + default y > + help > + 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 malicious symlink belonging to another user). > + > + Enabling this adds the logic to examine these dangerous symlink > + conditions. Whether or not the dangerous symlink situations are > + allowed is controlled by PROTECTED_STICKY_SYMLINKS_ENABLED. > + > +config PROTECTED_STICKY_SYMLINKS_ENABLED > + depends on PROTECTED_STICKY_SYMLINKS > + bool "Disallow symlink following in sticky world-writable dirs" > + default y > + help > + Solve ToCToU symlink race vulnerablities by permitting symlinks > + to be followed only when outside a sticky world-writable directory, > + or when the uid of the symlink and follower match, or when the > + directory and symlink owners match. > + > + When PROC_SYSCTL is enabled, this setting can also be controlled > + via /proc/sys/kernel/protected_sticky_symlinks. I think I disagree with this. If the person compiling the kernel includes the feature in his kernel via the time-honoured process of "wtf is that thing? Yeah, whatev", it gets turned on by default. This could easily result in weird failures which would take a *long* time for an unsuspecting person to debug. Would it not be kinder to our users to start this out as turned-off-at-runtime unless the kernel configurer has deliberately gone in and enabled it? > --- a/kernel/sysctl.c > +++ b/kernel/sysctl.c > @@ -109,6 +109,9 @@ extern int sysctl_nr_trim_pages; > #ifdef CONFIG_BLOCK > extern int blk_iopoll_enabled; > #endif > +#ifdef CONFIG_PROTECTED_STICKY_SYMLINKS > +extern int sysctl_protected_sticky_symlinks; > +#endif > Grumble. Yes, it's a site of much badness. Let's not worsen things. From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Subject: fs-symlink-restrictions-on-sticky-directories-fix move sysctl_protected_sticky_symlinks declaration into .h Cc: Kees Cook <keescook@xxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- a/kernel/sysctl.c~fs-symlink-restrictions-on-sticky-directories-fix +++ a/kernel/sysctl.c @@ -109,9 +109,6 @@ extern int sysctl_nr_trim_pages; #ifdef CONFIG_BLOCK extern int blk_iopoll_enabled; #endif -#ifdef CONFIG_PROTECTED_STICKY_SYMLINKS -extern int sysctl_protected_sticky_symlinks; -#endif /* Constants used for minimum and maximum */ #ifdef CONFIG_LOCKUP_DETECTOR --- a/include/linux/fs.h~fs-symlink-restrictions-on-sticky-directories-fix +++ a/include/linux/fs.h @@ -422,6 +422,7 @@ extern unsigned long get_max_files(void) extern int sysctl_nr_open; extern struct inodes_stat_t inodes_stat; extern int leases_enable, lease_break_time; +extern int sysctl_protected_sticky_symlinks; struct buffer_head; typedef int (get_block_t)(struct inode *inode, sector_t iblock, _ -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
