namei: make permission helpers idmapped mount aware

The two helpers inode_permission() and generic_permission() are used by
the vfs to perform basic permission checking by verifying that the
caller is privileged over an inode. In order to handle idmapped mounts
we extend the two helpers with an additional user namespace argument.
On idmapped mounts the two helpers will make sure to map the inode
according to the mount's user namespace and then peform identical
permission checks to inode_permission() and generic_permission(). If the
initial user namespace is passed nothing changes so non-idmapped mounts
will see identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-6-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 12 insertions, 5 deletions

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 4ca6d53c6f0a..5d9fe2fb2953 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -345,10 +345,13 @@ EXPORT_SYMBOL(posix_acl_from_mode);
  * by the acl. Returns -E... otherwise.
  */
 int
-posix_acl_permission(struct inode *inode, const struct posix_acl *acl, int want)
+posix_acl_permission(struct user_namespace *mnt_userns, struct inode *inode,
+		     const struct posix_acl *acl, int want)
 {
 	const struct posix_acl_entry *pa, *pe, *mask_obj;
 	int found = 0;
+	kuid_t uid;
+	kgid_t gid;
 
 	want &= MAY_READ | MAY_WRITE | MAY_EXEC;
 
@@ -356,22 +359,26 @@ posix_acl_permission(struct inode *inode, const struct posix_acl *acl, int want)
                 switch(pa->e_tag) {
                         case ACL_USER_OBJ:
 				/* (May have been checked already) */
-				if (uid_eq(inode->i_uid, current_fsuid()))
+				uid = i_uid_into_mnt(mnt_userns, inode);
+				if (uid_eq(uid, current_fsuid()))
                                         goto check_perm;
                                 break;
                         case ACL_USER:
-				if (uid_eq(pa->e_uid, current_fsuid()))
+				uid = kuid_into_mnt(mnt_userns, pa->e_uid);
+				if (uid_eq(uid, current_fsuid()))
                                         goto mask;
 				break;
                         case ACL_GROUP_OBJ:
-                                if (in_group_p(inode->i_gid)) {
+				gid = i_gid_into_mnt(mnt_userns, inode);
+				if (in_group_p(gid)) {
 					found = 1;
 					if ((pa->e_perm & want) == want)
 						goto mask;
                                 }
 				break;
                         case ACL_GROUP:
-				if (in_group_p(pa->e_gid)) {
+				gid = kgid_into_mnt(mnt_userns, pa->e_gid);
+				if (in_group_p(gid)) {
 					found = 1;
 					if ((pa->e_perm & want) == want)
 						goto mask;