Re: [PATCH v6] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Christian Brauner]

On Thu, Aug 03, 2023 at 12:09:33PM -0400, Jeff Layton wrote:
> On Thu, 2023-08-03 at 15:27 +0200, Christian Brauner wrote:
> > On Wed, Aug 02, 2023 at 03:34:27PM -0400, Jeff Layton wrote:
> > > On Wed, 2023-08-02 at 14:16 -0400, Paul Moore wrote:
> > > > On Aug  2, 2023 Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> > > > > 
> > > > > When NFS superblocks are created by automounting, their LSM parameters
> > > > > aren't set in the fs_context struct prior to sget_fc() being called,
> > > > > leading to failure to match existing superblocks.
> > > > > 
> > > > > Fix this by adding a new LSM hook to load fc->security for submount
> > > > > creation when alloc_fs_context() is creating the fs_context for it.
> > > > > 
> > > > > However, this uncovers a further bug: nfs_get_root() initialises the
> > > > > superblock security manually by calling security_sb_set_mnt_opts() or
> > > > > security_sb_clone_mnt_opts() - but then vfs_get_tree() calls
> > > > > security_sb_set_mnt_opts(), which can lead to SELinux, at least,
> > > > > complaining.
> > > > > 
> > > > > Fix that by adding a flag to the fs_context that suppresses the
> > > > > security_sb_set_mnt_opts() call in vfs_get_tree().  This can be set by NFS
> > > > > when it sets the LSM context on the new superblock.
> > > > > 
> > > > > The first bug leads to messages like the following appearing in dmesg:
> > > > > 
> > > > > 	NFS: Cache volume key already in use (nfs,4.2,2,108,106a8c0,1,,,,100000,100000,2ee,3a98,1d4c,3a98,1)
> > > > > 
> > > > > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > > Fixes: 9bc61ab18b1d ("vfs: Introduce fs_context, switch vfs_kern_mount() to it.")
> > > > > Fixes: 779df6a5480f ("NFS: Ensure security label is set for root inode)
> > > > > Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > > Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > > Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > > > > Acked-by: "Christian Brauner (Microsoft)" <brauner@xxxxxxxxxx>
> > > > > Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1
> > > > > Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2
> > > > > Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v3
> > > > > Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v4
> > > > > Link: https://lore.kernel.org/r/217595.1662033775@xxxxxxxxxxxxxxxxxxxxxx/ # v5
> > > > > ---
> > > > > This patch was originally sent by David several months ago, but it
> > > > > never got merged. I'm resending to resurrect the discussion. Can we
> > > > > get this fixed?
> > > > 
> > > > Sorry, I sorta lost track of this after the ROOTCONTEXT_MNT discussion
> > > > back in v3.  Looking at it a bit closer now I have one nitpicky
> > > > request and one larger concern (see below).
> > > > 
> > > > > diff --git a/fs/super.c b/fs/super.c
> > > > > index e781226e2880..13adf43e2e5d 100644
> > > > > --- a/fs/super.c
> > > > > +++ b/fs/super.c
> > > > > @@ -1541,10 +1541,12 @@ int vfs_get_tree(struct fs_context *fc)
> > > > >  	smp_wmb();
> > > > >  	sb->s_flags |= SB_BORN;
> > > > >  
> > > > > -	error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> > > > > -	if (unlikely(error)) {
> > > > > -		fc_drop_locked(fc);
> > > > > -		return error;
> > > > > +	if (!(fc->lsm_set)) {
> > > > > +		error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> > > > > +		if (unlikely(error)) {
> > > > > +			fc_drop_locked(fc);
> > > > > +			return error;
> > > > > +		}
> > > > >  	}
> > > > 
> > > > I generally dislike core kernel code which makes LSM calls conditional
> > > > on some kernel state maintained outside the LSM.  Sometimes it has to
> > > > be done as there is no other good options, but I would like us to try
> > > > and avoid it if possible.  The commit description mentioned that this
> > > > was put here to avoid a SELinux complaint, can you provide an example
> > > > of the complain?  Does it complain about a double/invalid mount, e.g.
> > > > "SELinux: mount invalid.  Same superblock, different security ..."?
> > > > 
> > > 
> > > The problem I had was not so much SELinux warnings, but rather that in a
> > > situation where I would expect to share superblocks between two
> > > filesystems, it didn't.
> > > 
> > > Basically if you do something like this:
> > > 
> > > # mount nfsserver:/export/foo /mnt/foo -o context=system_u:object_r:root_t:s0
> > > # mount nfsserver:/export/bar /mnt/bar -o context=system_u:object_r:root_t:s0
> > > 
> > > ...when "foo" and "bar" are directories on the same filesystem on the
> > > server, you should get two vfsmounts that share a superblock. That's
> > > what you get if selinux is disabled, but not when it's enabled (even
> > > when it's in permissive mode).
> > > 
> > > The problems that David hit with the automounter have a similar root
> > > cause though, I believe.
> > > 
> > > > I'd like to understand why the sb_set_mnt_opts() call fails when it
> > > > comes after the fs_context_init() call.  I'm particulary curious to
> > > > know if the failure is due to conflicting SELinux state in the
> > > > fs_context, or if it is simply an issue of sb_set_mnt_opts() not
> > > > properly handling existing values.  Perhaps I'm being overly naive,
> > > > but I'm hopeful that we can address both of these within the SELinux
> > > > code itself.
> > > > 
> > > 
> > > The problem I hit was that nfs_compare_super is called with a fs_context
> > > that has a NULL ->security pointer. That caused it to call
> > > selinux_sb_mnt_opts_compat with mnt_opts set to NULL, and at that point
> > > it returns 1 and decides not to share sb's.
> > 
> > I tried to follow this because I'm really still quite puzzled by this
> > whole thing. Two consecutive mounts that should share the superblock
> > don't share the superblock. But behavior differs between nfs3 and nfs4
> > due to how automounting works.
> > 
> > Afaict, the callchain you're looking at in this scenario is:
> > 
> > (1) nfs3
> > 
> > (1.1) mount 127.0.0.1:/export/foo /mnt/foo -o context=system_u:object_r:root_t:s0,nfsvers=3
> >       vfs_get_tree(fc_foo)
> >       -> fs_contex_operations->get_tree::nfs_get_tree(fc_foo)
> >             -> ctx->nfs_mod->rpc_ops->try_get_tree::nfs_try_get_tree(fc_foo)
> >                -> nfs_get_tree_common(fc_foo)
> >                   -> sb_foo = sget_fc(fc_foo, nfs_compare_super, ...)
> > 
> > (1.2) mount 127.0.0.1:/export/bar /mnt/bar -o context=system_u:object_r:root_t:s0,nfsvers=3
> >       vfs_get_tree(fc_bar)
> >       -> fs_contex_operations->get_tree::nfs_get_tree(fc_bar)
> >             -> ctx->nfs_mod->rpc_ops->try_get_tree::nfs_try_get_tree(fc_bar)
> >                -> nfs_get_tree_common(fc_bar)
> >                   -> sb_foo = sget_fc(fc_bar, nfs_compare_super, ...)
> >                      -> nfs_compare_super(sb_foo, fc_bar)
> >                         -> selinux_sb_mnt_opts_compat(sb_foo, fc_bar->security)
> > 
> > And fc_bar->security is non-NULL and compatible with sb_foo's current
> > security settings. Fine.
> > 
> > (2) nfs4
> > 
> > But for nfs4 we're looking at a vastly more complicated callchain at
> > least looking at this from a local nfs:
> > 
> > (2.1) mount 127.0.0.1:/export/foo /mnt/foo -o context=system_u:object_r:root_t:s0
> >       vfs_get_tree(fc_foo)
> >       -> fs_contex_operations->get_tree::nfs_get_tree(fc_foo)
> >          -> if (!ctx->internal) branch is taken
> >             -> ctx->nfs_mod->rpc_ops->try_get_tree::nfs4_try_get_tree(fc_foo)
> >                -> do_nfs4_mount(fc_foo)
> >                   -> fc_dup_foo = vfs_dup_fs_context(fc_foo)
> >                     -> security_fs_context_dup(fc_dup_foo, fc_foo)
> >                        {
> >                                 fc_dup_foo->security = kmemdup(fc_foo->security)
> >                        }
> >                        new_fs_context->internal = true
> >                   -> foo_mnt = fc_mount(fc_dup_foo)
> >                     -> vfs_get_tree(fc_dup_foo)
> >                        -> if (!ctx->internal) branch is _not_ taken
> >                           -> nfs_get_tree_common(fc_dup_foo)
> >                                  sb_foo = sget_fc(fc, nfs_compare_super, ...)
> >                   -> mount_subtree()
> >                      -> vfs_path_lookup(..., "/export/foo", LOOKUP_AUTOMOUNT)
> >                         -> nfs_d_automount("export")
> >                            -> fc_sub_foo = fs_context_for_submount()
> >                               {
> >                                       fc_sub_bar->security = NULL
> 
> 
> Should the above be:
> 
> 					fc_sub_foo->security = NULL;

Yes, typo for whatever reason.

> 
> ?
> 
> If so, then with this patch, the above would no longer be NULL. We'd
> inherit the security context info from the reference dentry passed to
> fs_context_for_submount().
> 
> >                               {
> >                            -> nfs4_submount(fc_sub_foo)
> >                               -> nfs4_do_submount(fc_sub_foo)
> >                                  -> vfs_get_tree(fc_sub_foo)
> >                                     -> nfs_get_tree_common(fc_sub_foo)
> >                                        -> sb_foo_2 = sget_fc(fc_sub_foo, nfs_compare_super, ...)
> >                         -> nfs_d_automount("foo")
> >                            -> fc_sub_foo = fs_context_for_submount()
> >                               {
> >                                       fc_sub_bar->security = NULL
> 
> Ditto here -- that should be fc_sub_foo , correct?

Yes, same. Was just a typo.

> >                               {
> >                            -> nfs4_submount(fc_sub_foo)
> >                               -> nfs4_do_submount(fc_sub_foo)
> >                                  -> vfs_get_tree(fc_sub_foo)
> >                                     -> nfs_get_tree_common(fc_sub_foo)
> >              |--------------------------> sb_foo_3 = sget_fc(fc_sub_foo, nfs_compare_super, ...)
> >              |
> > As far as I can see you're already allocating 3 separate superblocks of
> > which two are discarded and only one survives. Afaict, the one that
> > survives is _| the last one. Under the assumption that I'm correct,
> > where does the third superblock get it's selinux context from given that
> > fc->security isn't even set during submount?
> > 
> 
> That's the problem this patch is intended to fix. It allows child mounts
> to properly inherit security options from a parent dentry.

Yeah, I'm aware. Your patch will ensure that the last superblock is
found again. But you're always going to allocate addititional
superblocks afaict. That's at least what I can gather from the logic.
Say you have:

/export/a/b/c/d/e/foo   *(rw,insecure,no_subtree_check,no_root_squash)
/export/a/b/c/d/e/bar   *(rw,insecure,no_subtree_check,no_root_squash)

you allocate 8 superblocks (it's always path components +1) of which you
immediately discard 7 after you finished. That's easily reproducible
with selinux completely disabled. I'm just astonished.

> 
> > And where is the context=%s output generated for mountinfo?
> > 
> 
> security_sb_show_options / selinux_sb_show_options
> 
> > Is this a correct callchain?
> > 
> 
> I think it looks about right, but I didn't verify the details to the
> degree you have.
> 
> > > 
> > > Filling out fc->security with this new operation seems to fix that, but
> > > if you see a better way to do this, then I'm certainly open to the idea.
> > > 
> > > > In a worst case situation, we could always implement a flag *inside*
> > > > the SELinux code, similar to what has been done with 'lsm_set' here.
> > > > 
> > > 
> > > I'm fine with a different solution, if you see a better one. You'll have
> > 
> > Independent of the modification in fs_context_for_submount() you might want to
> > think about something like:
> > 
> > static const struct fs_context_operations nfs4_fs_context_ops = {
> >       .free           = nfs4_free,
> >       .parse_param    = nfs4_parse_param,
> >       .get_tree       = nfs4_get_tree,
> > };
> > 
> > static const struct fs_context_operations nfs4_fs_submount_ops = {
> >       .free           = nfs4_free_submount,
> >       .parse_param    = nfs4_parse_param_submount,
> >       .get_tree       = nfs4_get_tree_submount,
> > };
> > 
> > static int nfs4_init_fs_context_submount(struct fs_context *fc)
> > {
> >         return 0;
> > }
> > 
> > static int nfs4_fs_context_get_tree(struct fs_context *fc)
> > {
> >         if (fc->purpose == FS_CONTEXT_FOR_SUBMOUNT)
> >                 fc->ops = &nfs4_fs_submount_ops;
> >         else
> >                 fc->ops = &nfs4_fs_context_ops;
> >         .
> >         .
> >         .
> > }
> > 
> > which will make the callchain probably a lot to follow instead of wafting
> > through the same nested functions over and over. But just a thought.
> 
> Sounds reasonable. I'd rather do that sort of cleanup afterward though,
> to make this patch easier to eventually backport.

Yeah, sure.