linux-kernel - [PATCH v2 09/11] fs: reorder capability check last

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Message-ID: <20250302160657.127253-8-cgoettsche@seltendoof.de>
Date: Sun,  2 Mar 2025 17:06:45 +0100
From: Christian Göttsche <cgoettsche@...tendoof.de>
To: 
Cc: Christian Göttsche <cgzones@...glemail.com>,
	Serge Hallyn <serge@...lyn.com>,
	Jan Kara <jack@...e.com>,
	Julia Lawall <Julia.Lawall@...ia.fr>,
	Nicolas Palix <nicolas.palix@...g.fr>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	cocci@...ia.fr,
	Christian Brauner <brauner@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org
Subject: [PATCH v2 09/11] fs: reorder capability check last

From: Christian Göttsche <cgzones@...glemail.com>

capable() calls refer to enabled LSMs whether to permit or deny the
request.  This is relevant in connection with SELinux, where a
capability check results in a policy decision and by default a denial
message on insufficient permission is issued.
It can lead to three undesired cases:
  1. A denial message is generated, even in case the operation was an
     unprivileged one and thus the syscall succeeded, creating noise.
  2. To avoid the noise from 1. the policy writer adds a rule to ignore
     those denial messages, hiding future syscalls, where the task
     performs an actual privileged operation, leading to hidden limited
     functionality of that task.
  3. To avoid the noise from 1. the policy writer adds a rule to permit
     the task the requested capability, while it does not need it,
     violating the principle of least privilege.

Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
Reviewed-by: Serge Hallyn <serge@...lyn.com>
Reviewed-by: Christian Brauner <brauner@...nel.org>
---
 fs/fhandle.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 3e092ae6d142..5b77b38f0510 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -303,9 +303,9 @@ static inline int may_decode_fh(struct handle_to_path_ctx *ctx,
 	if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
 		ctx->flags = HANDLE_CHECK_PERMS;
 	else if (is_mounted(root->mnt) &&
+		 !has_locked_children(real_mount(root->mnt), root->dentry) &&
 		 ns_capable(real_mount(root->mnt)->mnt_ns->user_ns,
-			    CAP_SYS_ADMIN) &&
-		 !has_locked_children(real_mount(root->mnt), root->dentry))
+			    CAP_SYS_ADMIN))
 		ctx->flags = HANDLE_CHECK_PERMS | HANDLE_CHECK_SUBTREE;
 	else
 		return -EPERM;
-- 
2.47.2