From cffd0441872e7f6b1fce5e78fb1c99187a291330 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Thu, 14 Aug 2025 01:44:31 -0400 Subject: [PATCH] use uniform permission checks for all mount propagation changes 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: 12f147ddd6de "do_change_type(): refuse to operate on unmounted/not ours mounts" Acked-by: Andrei Vagin Reviewed-by: Pavel Tikhomirov Tested-by: Pavel Tikhomirov Reviewed-by: Christian Brauner Signed-off-by: Al Viro --- fs/namespace.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 1c97f93d1865..88db58061919 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -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)) - 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)) + err = may_change_propagation(from); + if (err) goto out; - if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN)) + err = may_change_propagation(to); + if (err) goto out; err = -EINVAL; -- 2.25.1