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use uniform permission checks for all mount propagation changes

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do_change_type() and do_set_group() are operating on different
aspects of the same thing - propagation graph.  The latter
asks for mounts involved to be mounted in namespace(s) the caller
has CAP_SYS_ADMIN for.  The former is a mess - originally it
didn't even check that mount *is* mounted.  That got fixed,
but the resulting check turns out to be too strict for userland -
in effect, we check that mount is in our namespace, having already
checked that we have CAP_SYS_ADMIN there.

What we really need (in both cases) is
	* only touch mounts that are mounted.  That's a must-have
constraint - data corruption happens if it get violated.
	* don't allow to mess with a namespace unless you already
have enough permissions to do so (i.e. CAP_SYS_ADMIN in its userns).

That's an equivalent of what do_set_group() does; let's extract that
into a helper (may_change_propagation()) and use it in both
do_set_group() and do_change_type().

Fixes: 12f147ddd6 "do_change_type(): refuse to operate on unmounted/not ours mounts"
Acked-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Tested-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2025-08-14 01:44:31 -04:00
parent da025cdb97
commit cffd044187
1 changed files with 20 additions and 14 deletions
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34
fs/namespace.c
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@ -2859,6 +2859,19 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
return attach_recursive_mnt(mnt, p, mp);
}
static int may_change_propagation(const struct mount *m)
{
struct mnt_namespace *ns = m->mnt_ns;
// it must be mounted in some namespace
if (IS_ERR_OR_NULL(ns)) // is_mounted()
return -EINVAL;
// and the caller must be admin in userns of that namespace
if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
return -EPERM;
return 0;
}
/*
* Sanity check the flags to change_mnt_propagation.
*/
@ -2895,10 +2908,10 @@ static int do_change_type(struct path *path, int ms_flags)
return -EINVAL;
namespace_lock();
if (!check_mnt(mnt)) {
err = -EINVAL;
err = may_change_propagation(mnt);
if (err)
goto out_unlock;
}
if (type == MS_SHARED) {
err = invent_group_ids(mnt, recurse);
if (err)
@ -3344,18 +3357,11 @@ static int do_set_group(struct path *from_path, struct path *to_path)
namespace_lock();
err = -EINVAL;
/* To and From must be mounted */
if (!is_mounted(&from->mnt))
err = may_change_propagation(from);
if (err)
goto out;
if (!is_mounted(&to->mnt))
goto out;
err = -EPERM;
/* We should be allowed to modify mount namespaces of both mounts */
if (!ns_capable(from->mnt_ns->user_ns, CAP_SYS_ADMIN))
goto out;
if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN))
err = may_change_propagation(to);
if (err)
goto out;
err = -EINVAL;