[PATCH 5.15 07/11] attr: add in_group_or_capable()

[Leah Rumancik <leah.rumancik@gmail.com>]

From: Christian Brauner <brauner@kernel.org>

commit 11c2a8700cdcabf9b639b7204a1e38e2a0b6798e upstream.

[backport to 5.15.y, prior to vfsgid_t]

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>
Tested-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/attr.c     |  8 ++++----
 fs/inode.c    | 28 ++++++++++++++++++++++++----
 fs/internal.h |  2 ++
 3 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index f581c4d00897..686840aa91c8 100644
--- a/fs/attr.c
+++ b/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
@@ -141,8 +143,7 @@ int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
 			mapped_gid = i_gid_into_mnt(mnt_userns, inode);
 
 		/* Also check the setgid bit! */
-		if (!in_group_p(mapped_gid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode, mapped_gid))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -257,8 +258,7 @@ void setattr_copy(struct user_namespace *mnt_userns, struct inode *inode,
 	if (ia_valid & ATTR_MODE) {
 		umode_t mode = attr->ia_mode;
 		kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
-		if (!in_group_p(kgid) &&
-		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_or_capable(mnt_userns, inode, kgid))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}
diff --git a/fs/inode.c b/fs/inode.c
index 957b2d18ec29..a71fb82279bb 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2321,6 +2321,28 @@ 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
+ * @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(struct user_namespace *mnt_userns,
+			 const struct inode *inode, kgid_t gid)
+{
+	if (in_group_p(gid))
+		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
@@ -2342,11 +2364,9 @@ 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_mnt(mnt_userns, dir)))
 		return mode;
-
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);
diff --git a/fs/internal.h b/fs/internal.h
index 9075490f21a6..c89814727281 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -150,6 +150,8 @@ 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(struct user_namespace *mnt_userns,
+			 const struct inode *inode, kgid_t gid);
 
 /*
  * fs-writeback.c
-- 
2.40.0.rc0.216.gc4246ad0f0-goog