linux-kernel - Re: [PATCH] Clarify the rootid_owns

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Message-ID: <aR0JrOvDxDKZPELd@mail.hallyn.com>
Date: Tue, 18 Nov 2025 18:05:00 -0600
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Paul Moore <paul@...l-moore.com>
Cc: "Serge E. Hallyn" <serge@...lyn.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-security-module@...r.kernel.org,
	Ryan Foster <foster.ryan.r@...il.com>,
	Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH] Clarify the rootid_owns_currentns

On Tue, Nov 18, 2025 at 10:47:06AM -0500, Paul Moore wrote:
> On Tue, Nov 18, 2025 at 9:16 AM Serge E. Hallyn <serge@...lyn.com> wrote:
> > On Fri, Nov 14, 2025 at 03:33:19PM -0600, Serge E. Hallyn wrote:
> > > Split most of the rootid_owns_currentns() functionality
> > > into a more generic rootid_owns_ns() function which
> > > will be easier to write tests for.
> > >
> > > Rename the functions and variables to make clear that
> > > the ids being tested could be any uid.
> > >
> > > Signed-off-by: Serge Hallyn <serge@...lyn.com>
> > > CC: Ryan Foster <foster.ryan.r@...il.com>
> > > CC: Christian Brauner <brauner@...nel.org>
> >
> > Paul, Christian, let me know if you have any objections, else I will
> > queue this up in caps-next.
> 
> Seems reasonable to me, but it would be good to fix the parameter doc
> bug that the kernel test robot identified.  I suspect it is just the
> extra vertical comment space between the top one line summary and the
> parameter list.

Actually I think it was probably the use of - instead of : after the
parameter name, but I went ahead and changed both, thanks.

Pushed the below patch for linux-next.

Subject: [PATCH 1/1] Clarify the rootid_owns_currentns

Split most of the rootid_owns_currentns() functionality
into a more generic rootid_owns_ns() function which
will be easier to write tests for.

Rename the functions and variables to make clear that
the ids being tested could be any uid.

Signed-off-by: Serge Hallyn <serge@...lyn.com>
CC: Ryan Foster <foster.ryan.r@...il.com>
CC: Christian Brauner <brauner@...nel.org>

---
v2: change the function parameter documentation to mollify the bot.
---
 security/commoncap.c | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 6bd4adeb4795..496e054c5d37 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -358,17 +358,17 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
 	return error;
 }
 
-static bool rootid_owns_currentns(vfsuid_t rootvfsuid)
+/**
+ * kuid_root_in_ns - check whether the given kuid is root in the given ns
+ * @kuid: the kuid to be tested
+ * @ns: the user namespace to test against
+ *
+ * Returns true if @kuid represents the root user in @ns, false otherwise.
+ */
+static bool kuid_root_in_ns(kuid_t kuid, struct user_namespace *ns)
 {
-	struct user_namespace *ns;
-	kuid_t kroot;
-
-	if (!vfsuid_valid(rootvfsuid))
-		return false;
-
-	kroot = vfsuid_into_kuid(rootvfsuid);
-	for (ns = current_user_ns();; ns = ns->parent) {
-		if (from_kuid(ns, kroot) == 0)
+	for (;; ns = ns->parent) {
+		if (from_kuid(ns, kuid) == 0)
 			return true;
 		if (ns == &init_user_ns)
 			break;
@@ -377,6 +377,16 @@ static bool rootid_owns_currentns(vfsuid_t rootvfsuid)
 	return false;
 }
 
+static bool vfsuid_root_in_currentns(vfsuid_t vfsuid)
+{
+	kuid_t kuid;
+
+	if (!vfsuid_valid(vfsuid))
+		return false;
+	kuid = vfsuid_into_kuid(vfsuid);
+	return kuid_root_in_ns(kuid, current_user_ns());
+}
+
 static __u32 sansflags(__u32 m)
 {
 	return m & ~VFS_CAP_FLAGS_EFFECTIVE;
@@ -481,7 +491,7 @@ int cap_inode_getsecurity(struct mnt_idmap *idmap,
 		goto out_free;
 	}
 
-	if (!rootid_owns_currentns(vfsroot)) {
+	if (!vfsuid_root_in_currentns(vfsroot)) {
 		size = -EOVERFLOW;
 		goto out_free;
 	}
@@ -722,7 +732,7 @@ int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
 	/* Limit the caps to the mounter of the filesystem
 	 * or the more limited uid specified in the xattr.
 	 */
-	if (!rootid_owns_currentns(rootvfsuid))
+	if (!vfsuid_root_in_currentns(rootvfsuid))
 		return -ENODATA;
 
 	cpu_caps->permitted.val = le32_to_cpu(caps->data[0].permitted);
-- 
2.34.1