linux-kernel - [PATCH AUTOSEL 5.15] umount: Allow superblock owners to force umount

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Message-Id: <20250331143231.1667887-1-sashal@kernel.org>
Date: Mon, 31 Mar 2025 10:32:31 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Trond Myklebust <trond.myklebust@...merspace.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Christian Brauner <brauner@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org
Subject: [PATCH AUTOSEL 5.15] umount: Allow superblock owners to force umount

From: Trond Myklebust <trond.myklebust@...merspace.com>

[ Upstream commit e1ff7aa34dec7e650159fd7ca8ec6af7cc428d9f ]

Loosen the permission check on forced umount to allow users holding
CAP_SYS_ADMIN privileges in namespaces that are privileged with respect
to the userns that originally mounted the filesystem.

Signed-off-by: Trond Myklebust <trond.myklebust@...merspace.com>
Link: https://lore.kernel.org/r/12f212d4ef983714d065a6bb372fbb378753bf4c.1742315194.git.trond.myklebust@hammerspace.com
Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/namespace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 22af4b6c737f4..642baef4d9aaa 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1734,6 +1734,7 @@ static void warn_mandlock(void)
 static int can_umount(const struct path *path, int flags)
 {
 	struct mount *mnt = real_mount(path->mnt);
+	struct super_block *sb = path->dentry->d_sb;
 
 	if (!may_mount())
 		return -EPERM;
@@ -1743,7 +1744,7 @@ static int can_umount(const struct path *path, int flags)
 		return -EINVAL;
 	if (mnt->mnt.mnt_flags & MNT_LOCKED) /* Check optimistically */
 		return -EINVAL;
-	if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
+	if (flags & MNT_FORCE && !ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 	return 0;
 }
-- 
2.39.5