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.