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>

fs/attr.c

@@ -18,6 +18,8 @@
 #include <linux/evm.h>
 #include <linux/ima.h>
 
+#include "internal.h"
+
 /**
  * chown_ok - verify permissions to chown inode
  * @mnt_userns:	user namespace of the mount @inode was found from
@@ -140,8 +142,7 @@ int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
 			vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
 
 		/* Also check the setgid bit! */
-		if (!vfsgid_in_group_p(vfsgid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode, vfsgid))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -251,9 +252,8 @@ void setattr_copy(struct user_namespace *mnt_userns, struct inode *inode,
 		inode->i_ctime = attr->ia_ctime;
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
-		vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
-		if (!vfsgid_in_group_p(vfsgid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode,
+					 i_gid_into_vfsgid(mnt_userns, inode)))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}

fs/inode.c

@@ -2487,6 +2487,28 @@ 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
@@ -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);

fs/internal.h

@@ -151,6 +151,8 @@ extern int vfs_open(const struct path *, struct file *);
  */
 extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern int dentry_needs_remove_privs(struct dentry *dentry);
+bool in_group_or_capable(struct user_namespace *mnt_userns,
+			 const struct inode *inode, vfsgid_t vfsgid);
 
 /*
  * fs-writeback.c