attr: add in_group_or_capable()

commit 11c2a8700c upstream.

[backported to 5.10.y, prior to idmapped mounts]

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>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/attr.c

@@ -18,6 +18,8 @@
 #include <linux/evm.h>
 #include <linux/ima.h>
 
+#include "internal.h"
+
 static bool chown_ok(const struct inode *inode, kuid_t uid)
 {
 	if (uid_eq(current_fsuid(), inode->i_uid) &&
@@ -90,9 +92,8 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 		if (!inode_owner_or_capable(inode))
 			return -EPERM;
 		/* Also check the setgid bit! */
-		if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
+		if (!in_group_or_capable(inode, (ia_valid & ATTR_GID) ?
-				inode->i_gid) &&
+						attr->ia_gid : inode->i_gid))
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -193,9 +194,7 @@ void setattr_copy(struct inode *inode, const struct iattr *attr)
 		inode->i_ctime = attr->ia_ctime;
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
-
+		if (!in_group_or_capable(inode, inode->i_gid))
-		if (!in_group_p(inode->i_gid) &&
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}

fs/inode.c

@@ -2379,6 +2379,26 @@ int vfs_ioc_fssetxattr_check(struct inode *inode, const struct fsxattr *old_fa,
 }
 EXPORT_SYMBOL(vfs_ioc_fssetxattr_check);
 
+/**
+ * in_group_or_capable - check whether caller is CAP_FSETID privileged
+ * @inode:	inode to check
+ * @gid:	the new/current gid of @inode
+ *
+ * Check wether @gid 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(const struct inode *inode, kgid_t gid)
+{
+	if (in_group_p(gid))
+		return true;
+	if (capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		return true;
+	return false;
+}
+
 /**
  * mode_strip_sgid - handle the sgid bit for non-directories
  * @dir: parent directory inode
@@ -2398,11 +2418,8 @@ umode_t mode_strip_sgid(const struct inode *dir, umode_t mode)
 		return mode;
 	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
 		return mode;
-	if (in_group_p(dir->i_gid))
+	if (in_group_or_capable(dir, dir->i_gid))
 		return mode;
-	if (capable_wrt_inode_uidgid(dir, CAP_FSETID))
-		return mode;
-
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);

fs/internal.h

@@ -149,6 +149,7 @@ extern int vfs_open(const struct path *, struct file *);
 extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern void inode_add_lru(struct inode *inode);
 extern int dentry_needs_remove_privs(struct dentry *dentry);
+bool in_group_or_capable(const struct inode *inode, kgid_t gid);
 
 /*
  * fs-writeback.c