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fs/mount_setattr: tighten permission checks

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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 <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
Suggested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
This commit is contained in:
Christian Brauner 2021-05-11 16:30:15 +02:00
parent 6efb943b86
commit 2ca4dcc490
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 5 additions and 1 deletions
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6
fs/namespace.c
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@ -3855,8 +3855,12 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
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. */