[PATCH bpf-next 2/4] landlock: Use path_parent()

[Mickaël Salaün]

On Sat, May 31, 2025 at 02:51:22PM +0100, Tingmao Wang wrote:
> On 5/28/25 23:26, Song Liu wrote:
> > Use path_parent() to walk a path up to its parent.
> > 
> > While path_parent() has an extra check with path_connected() than existing
> > code, there is no functional changes intended for landlock.
> > 
> > Signed-off-by: Song Liu <song at kernel.org>
> > ---
> >  security/landlock/fs.c | 34 +++++++++++++++++-----------------
> >  1 file changed, 17 insertions(+), 17 deletions(-)
> > 
> > diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> > index 6fee7c20f64d..32a24758ad6e 100644
> > --- a/security/landlock/fs.c
> > +++ b/security/landlock/fs.c
> > @@ -837,7 +837,6 @@ static bool is_access_to_paths_allowed(
> >  	 * restriction.
> >  	 */
> >  	while (true) {
> > -		struct dentry *parent_dentry;
> >  		const struct landlock_rule *rule;
> >  
> >  		/*
> > @@ -896,19 +895,17 @@ static bool is_access_to_paths_allowed(
> >  		if (allowed_parent1 && allowed_parent2)
> >  			break;
> >  jump_up:
> > -		if (walker_path.dentry == walker_path.mnt->mnt_root) {
> > -			if (follow_up(&walker_path)) {
> > -				/* Ignores hidden mount points. */
> > -				goto jump_up;
> > -			} else {
> > -				/*
> > -				 * Stops at the real root.  Denies access
> > -				 * because not all layers have granted access.
> > -				 */
> > -				break;
> > -			}
> > -		}
> > -		if (unlikely(IS_ROOT(walker_path.dentry))) {
> > +		switch (path_parent(&walker_path)) {
> > +		case PATH_PARENT_CHANGED_MOUNT:
> > +			/* Ignores hidden mount points. */
> > +			goto jump_up;
> > +		case PATH_PARENT_REAL_ROOT:
> > +			/*
> > +			 * Stops at the real root.  Denies access
> > +			 * because not all layers have granted access.
> > +			 */
> > +			goto walk_done;
> > +		case PATH_PARENT_DISCONNECTED_ROOT:
> >  			/*
> >  			 * Stops at disconnected root directories.  Only allows
> >  			 * access to internal filesystems (e.g. nsfs, which is
> 
> I was looking at the existing handling of disconnected root in Landlock
> and I realized that the comment here confused me a bit:
> 
> /*
>  * Stops at disconnected root directories.  Only allows
>  * access to internal filesystems (e.g. nsfs, which is
>  * reachable through /proc/<pid>/ns/<namespace>).
>  */
> 
> In the original code, this was under a
> 
>     if (unlikely(IS_ROOT(walker_path.dentry)))
> 
> which means that it only stops walking if we found out we're disconnected
> after reaching a filesystem boundary.  However if before we got to this
> point, we have already collected enough rules to allow access, access
> would be allowed, even if we're currently disconnected.  Demo:
> 
> / # cd /
> / # cp /linux/samples/landlock/sandboxer .
> / # mkdir a b
> / # mkdir a/foo
> / # echo baz > a/foo/bar
> / # mount --bind a b
> / # LL_FS_RO=/ LL_FS_RW=/ ./sandboxer bash
> Executing the sandboxed command...
> / # cd /b/foo
> /b/foo # cat bar
> baz
> /b/foo # mv /a/foo /foo
> /b/foo # cd ..     # <- We're now disconnected
> bash: cd: ..: No such file or directory
> /b/foo # cat bar
> baz                # <- but landlock still lets us read the file
> 
> However, I think this patch will change this behavior due to the use of
> path_connected
> 
> root at 10a8fff999ce:/# mkdir a b
> root at 10a8fff999ce:/# mkdir a/foo
> root at 10a8fff999ce:/# echo baz > a/foo/bar
> root at 10a8fff999ce:/# mount --bind a b
> root at 10a8fff999ce:/# LL_FS_RO=/ LL_FS_RW=/ ./sandboxer bash
> Executing the sandboxed command...
> bash: cannot set terminal process group (191): Inappropriate ioctl for device
> bash: no job control in this shell
> root at 10a8fff999ce:/# cd /b/foo
> root at 10a8fff999ce:/b/foo# cat bar
> baz
> root at 10a8fff999ce:/b/foo# mv /a/foo /foo
> root at 10a8fff999ce:/b/foo# cd ..
> bash: cd: ..: No such file or directory
> root at 10a8fff999ce:/b/foo# cat bar
> cat: bar: Permission denied

This is a good test case, we should add a test for that.

> 
> I'm not sure if the original behavior was intentional, but since this
> technically counts as a functional changes, just pointing this out.

This is indeed an issue.

> 
> Also I'm slightly worried about the performance overhead of doing
> path_connected for every hop in the iteration (but ultimately it's
> Mickaël's call).

Yes, we need to check with a benchmark.  We might want to keep the
walker_path.dentry == walker_path.mnt->mnt_root check inlined.

> At least for Landlock, I think if we want to block all
> access to disconnected files, as long as we eventually realize we have
> been disconnected (by doing the "if dentry == path.mnt" check once when we
> reach root), and in that case deny access, we should be good.
> 
> 
> > @@ -918,12 +915,15 @@ static bool is_access_to_paths_allowed(
> >  				allowed_parent1 = true;
> >  				allowed_parent2 = true;
> >  			}
> > +			goto walk_done;
> > +		case PATH_PARENT_SAME_MOUNT:
> >  			break;
> > +		default:
> > +			WARN_ON_ONCE(1);
> > +			goto walk_done;
> >  		}
> > -		parent_dentry = dget_parent(walker_path.dentry);
> > -		dput(walker_path.dentry);
> > -		walker_path.dentry = parent_dentry;
> >  	}
> > +walk_done:
> >  	path_put(&walker_path);
> >  
> >  	if (!allowed_parent1) {
> 
>