[PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution

[Christian Brauner]

On Fri, Oct 05, 2018 at 02:26:11AM +1000, Aleksa Sarai wrote:
> On 2018-09-29, Jann Horn <jannh at google.com> wrote:
> > You attempt to open "C/../../etc/passwd" under the root "/A/B".
> > Something else concurrently moves /A/B/C to /A/C. This can result in
> > the following:
> > 
> > 1. You start the path walk and reach /A/B/C.
> > 2. The other process moves /A/B/C to /A/C. Your path walk is now at /A/C.
> > 3. Your path walk follows the first ".." up into /A. This is outside
> > the process root, but you never actually encountered the process root,
> > so you don't notice.
> > 4. Your path walk follows the second ".." up to /. Again, this is
> > outside the process root, but you don't notice.
> > 5. Your path walk walks down to /etc/passwd, and the open completes
> > successfully. You now have an fd pointing outside your chroot.
> 
> I've been playing with this and I have the following patch, which
> according to my testing protects against attacks where ".." skips over
> nd->root. It abuses __d_path to figure out if nd->path can be resolved
> from nd->root (obviously a proper version of this patch would refactor
> __d_path so it could be used like this -- and would not return
> -EMULTIHOP).
> 
> I've also attached my reproducer. With it, I was seeing fairly constant
> breakouts before this patch and after it I didn't see a single breakout
> after running it overnight. Obviously this is not conclusive, but I'm
> hoping that it can show what my idea for protecting against ".." was.
> 
> Does this patch make sense? Or is there something wrong with it that I'm
> not seeing?

Interesting.
Apart from the abuse of __d_path() :) the question I'd have is whether
this just minimizes the race window or if you can provide a sound
argument that this actually can't happen anymore with this patch.

> 
> --8<-------------------------------------------------------------------
> 
> There is a fairly easy-to-exploit race condition with chroot(2) (and
> thus by extension AT_THIS_ROOT and AT_BENEATH) where a rename(2) of a
> path can be used to "skip over" nd->root and thus escape to the
> filesystem above nd->root.
> 
>   thread1 [attacker]:
>     for (;;)
>       renameat2(AT_FDCWD, "/a/b/c", AT_FDCWD, "/a/d", RENAME_EXCHANGE);
>   thread2 [victim]:
>     for (;;)
>       openat(dirb, "b/c/../../etc/shadow", O_THISROOT);
> 
> With fairly significant regularity, thread2 will resolve to
> "/etc/shadow" rather than "/a/b/etc/shadow". With this patch, such cases
> will be detected during ".." resolution (which is the weak point of
> chroot(2) -- since walking *into* a subdirectory tautologically cannot
> result in you walking *outside* nd->root).
> 
> The use of __d_path here might seem suspect, however we don't mind if a
> path is moved from within the chroot to outside the chroot and we
> incorrectly decide it is safe (because at that point we are still within
> the set of files which were accessible at the beginning of resolution).
> However, we can fail resolution on the next path component if it remains
> outside of the root. A path which has always been outside nd->root
> during resolution will never be resolveable from nd->root and thus will
> always be blocked.
> 
> DO NOT MERGE: Currently this code returns -EMULTIHOP in this case,
> 	      purely as a debugging measure (so that you can see that
> 	      the protection actually does something). Obviously in the
> 	      proper patch this will return -EXDEV.
> 
> Signed-off-by: Aleksa Sarai <cyphar at cyphar.com>
> ---
>  fs/namei.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 6f995e6de6b1..c8349693d47b 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -53,8 +53,8 @@
>   * The new code replaces the old recursive symlink resolution with
>   * an iterative one (in case of non-nested symlink chains).  It does
>   * this with calls to <fs>_follow_link().
> - * As a side effect, dir_namei(), _namei() and follow_link() are now 
> - * replaced with a single function lookup_dentry() that can handle all 
> + * As a side effect, dir_namei(), _namei() and follow_link() are now
> + * replaced with a single function lookup_dentry() that can handle all
>   * the special cases of the former code.
>   *
>   * With the new dcache, the pathname is stored at each inode, at least as
> @@ -1375,6 +1375,20 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  				return -EXDEV;
>  			break;
>  		}
> +		if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT))) {
> +			char *pathbuf, *pathptr;
> +
> +			pathbuf = kmalloc(PATH_MAX, GFP_ATOMIC);
> +			if (!pathbuf)
> +				return -ECHILD;
> +			pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX);
> +			kfree(pathbuf);
> +			if (IS_ERR_OR_NULL(pathptr)) {
> +				if (!pathptr)
> +					pathptr = ERR_PTR(-EMULTIHOP);
> +				return PTR_ERR(pathptr);
> +			}
> +		}
>  		if (nd->path.dentry != nd->path.mnt->mnt_root) {
>  			struct dentry *old = nd->path.dentry;
>  			struct dentry *parent = old->d_parent;
> @@ -1510,6 +1524,20 @@ static int follow_dotdot(struct nameidata *nd)
>  				return -EXDEV;
>  			break;
>  		}
> +		if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT))) {
> +			char *pathbuf, *pathptr;
> +
> +			pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
> +			if (!pathbuf)
> +				return -ENOMEM;
> +			pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX);
> +			kfree(pathbuf);
> +			if (IS_ERR_OR_NULL(pathptr)) {
> +				if (!pathptr)
> +					pathptr = ERR_PTR(-EMULTIHOP);
> +				return PTR_ERR(pathptr);
> +			}
> +		}
>  		if (nd->path.dentry != nd->path.mnt->mnt_root) {
>  			int ret = path_parent_directory(&nd->path);
>  			if (ret)
> -- 
> 2.19.0
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>