[PATCH 10/29] selinux: implement set acl hook

[Christian Brauner <brauner@kernel.org>]

The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

So far posix acls were passed as a void blob to the security and
integrity modules. Some of them like evm then proceed to interpret the
void pointer and convert it into the kernel internal struct posix acl
representation to perform their integrity checking magic. This is
obviously pretty problematic as that requires knowledge that only the
vfs is guaranteed to have and has lead to various bugs. Add a proper
security hook for setting posix acls and pass down the posix acls in
their appropriate vfs format instead of hacking it through a void
pointer stored in the uapi format.

I spent considerate time in the security module infrastructure and
audited all codepaths. SELinux has no restrictions based on the posix
acl values passed through it. The capability hook doesn't need to be
called either because it only has restrictions on security.* xattrs. So
this all becomes a very simple hook for SELinux.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
---
 security/selinux/hooks.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 79573504783b..bbc0ce3bde35 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3239,6 +3239,13 @@ static int selinux_inode_setxattr(struct user_namespace *mnt_userns,
 			    &ad);
 }
 
+static int selinux_inode_set_acl(struct user_namespace *mnt_userns,
+				 struct dentry *dentry, const char *acl_name,
+				 struct posix_acl *kacl)
+{
+	return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
+}
+
 static void selinux_inode_post_setxattr(struct dentry *dentry, const char *name,
 					const void *value, size_t size,
 					int flags)
@@ -7063,6 +7070,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(inode_getxattr, selinux_inode_getxattr),
 	LSM_HOOK_INIT(inode_listxattr, selinux_inode_listxattr),
 	LSM_HOOK_INIT(inode_removexattr, selinux_inode_removexattr),
+	LSM_HOOK_INIT(inode_set_acl, selinux_inode_set_acl),
 	LSM_HOOK_INIT(inode_getsecurity, selinux_inode_getsecurity),
 	LSM_HOOK_INIT(inode_setsecurity, selinux_inode_setsecurity),
 	LSM_HOOK_INIT(inode_listsecurity, selinux_inode_listsecurity),
-- 
2.34.1