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Subject[PATCH 5.12 040/127] fs/mount_setattr: tighten permission checks
From: Christian Brauner <>

commit 2ca4dcc4909d787ee153272f7efc2bff3b498720 upstream.

We currently don't have any filesystems that support idmapped mounts
which are mountable inside a user namespace. That was a deliberate
decision for now as a userns root can just mount the filesystem
themselves. So enforce this restriction explicitly until there's a real
use-case for this. This way we can notice it and will have a chance to
adapt and audit our translation helpers and fstests appropriately if we
need to support such filesystems.

Cc: Christoph Hellwig <>
Cc: Al Viro <>
Suggested-by: Seth Forshee <>
Signed-off-by: Christian Brauner <>
Signed-off-by: Greg Kroah-Hartman <>
fs/namespace.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3853,8 +3853,12 @@ static int can_idmap_mount(const struct
if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
return -EINVAL;

+ /* Don't yet support filesystem mountable in user namespaces. */
+ if (m->mnt_sb->s_user_ns != &init_user_ns)
+ return -EINVAL;
/* We're not controlling the superblock. */
- if (!ns_capable(m->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
+ if (!capable(CAP_SYS_ADMIN))
return -EPERM;

/* Mount has already been visible in the filesystem hierarchy. */

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