attr: add in_group_or_capable()

In setattr_{copy,prepare}() we need to perform the same permission
checks to determine whether we need to drop the setgid bit or not.
Instead of open-coding it twice add a simple helper the encapsulates the
logic. We will reuse this helpers to make dropping the setgid bit during
write operations more consistent in a follow up patch.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

1 files changed, 24 insertions, 4 deletions

diff --git a/fs/inode.c b/fs/inode.c
index b608528efd3a..55299b710c45 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2488,6 +2488,28 @@ struct timespec64 current_time(struct inode *inode)
 EXPORT_SYMBOL(current_time);
 
 /**
+ * in_group_or_capable - check whether caller is CAP_FSETID privileged
+ * @mnt_userns: user namespace of the mount @inode was found from
+ * @inode:	inode to check
+ * @vfsgid:	the new/current vfsgid of @inode
+ *
+ * Check wether @vfsgid is in the caller's group list or if the caller is
+ * privileged with CAP_FSETID over @inode. This can be used to determine
+ * whether the setgid bit can be kept or must be dropped.
+ *
+ * Return: true if the caller is sufficiently privileged, false if not.
+ */
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+			 const struct inode *inode, vfsgid_t vfsgid)
+{
+	if (vfsgid_in_group_p(vfsgid))
+		return true;
+	if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		return true;
+	return false;
+}
+
+/**
  * mode_strip_sgid - handle the sgid bit for non-directories
  * @mnt_userns: User namespace of the mount the inode was created from
  * @dir: parent directory inode
@@ -2508,11 +2530,9 @@ umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
 		return mode;
 	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
 		return mode;
-	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
-		return mode;
-	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+	if (in_group_or_capable(mnt_userns, dir,
+				i_gid_into_vfsgid(mnt_userns, dir)))
 		return mode;
-
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);