Re: [PATCH for 4.4.y-cip] cgroup-v1: Require capabilities to set release_agent

Masami Ichikawa

On Wed, Feb 16, 2022 at 9:11 PM Michal Koutný <mkoutny@...> wrote:

Thanks for sharing this Masami, I've been looking into pre-cgroup_ns
backport too.
Thank you for the review.

On Wed, Feb 16, 2022 at 08:40:37AM +0900, Masami Ichikawa <masami256@...> wrote:
[masami: Backport patch from 4.9. Adjust to use current_user_ns() to get current user_ns.
Fix conflict in cgroup_release_agent_write().]
The condition to allow modifying release_agent is two-fold:
a) caller is capabable(CAP_SYS_ADMIN),
b) cgroup_ns is owned by init_user_ns.

In pre-cgroup_ns kernels, it is IMO safer to consider all (=the only)
cgroup_ns owned by init_user_ns.

So the (positive) condition translates into capable(CAP_SYS_ADMIN) only.
I see. Thank you for the details.
If we treat cgroup_ns as only cgroup_ns that are owned by
init_user_ns, we don't have to have an extra check.

Additionally, there's invariant/implication
capable(CAP_XXX) -> (current_user_ns() == &init_user_ns) ,
so the expression
(current_user_ns() != &init_user_ns) || !capable(CAP_SYS_ADMIN)
simplifies to
!capable(CAP_SYS_ADMIN) .

@@ -2839,6 +2856,14 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,

BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);

+ /*
+ * Release agent gets called with all capabilities,
+ * require capabilities to set release agent.
+ */
+ if ((of->file->f_cred->user_ns != &init_user_ns) ||
+ !capable(CAP_SYS_ADMIN))
+ return -EPERM;
Following the reasoning above, the check simplifies too but it should be
be against the opener, not the writer:
file_ns_capable(of->file, &init_user_ns, CAP_SYS_ADMIN)
Thanks. I got it.

(It seems to be to be incorrect even in the original commit.
So I'll send a patch there to rectify that.)


* Masami Ichikawa
* personal: masami256@...
* fedora project: masami@...

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