2022-02-17 20:22:37

by Michal Koutný

Subject: [PATCH] cgroup-v1: Correct privileges check in release_agent writes

The idea is to check: a) the owning user_ns of cgroup_ns, b)
capabilities in init_user_ns.

The commit 24f600856418 ("cgroup-v1: Require capabilities to set
release_agent") got this wrong in the write handler of release_agent
since it checked user_ns of the opener (may be different from the owning
user_ns of cgroup_ns).
Secondly, to avoid possibly confused deputy, the capability of the
opener must be checked.

Fixes: 24f600856418 ("cgroup-v1: Require capabilities to set release_agent")
Cc: [email protected]
Link: https://lore.kernel.org/stable/[email protected]/
Signed-off-by: Michal Koutný <[email protected]>
---
kernel/cgroup/cgroup-v1.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 0e877dbcfeea..afc6c0e9c966 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -546,6 +546,7 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
char *buf, size_t nbytes, loff_t off)
{
struct cgroup *cgrp;
+ struct cgroup_file_ctx *ctx;

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

@@ -553,8 +554,9 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
* 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))
+ ctx = of->priv;
+ if ((ctx->ns->user_ns != &init_user_ns) ||
+ !file_ns_capable(of->file, &init_user_ns, CAP_SYS_ADMIN))
return -EPERM;

cgrp = cgroup_kn_lock_live(of->kn, false);
--
2.34.1

2022-02-18 01:30:43

by Masami Ichikawa(CIP)

Subject: Re: [PATCH] cgroup-v1: Correct privileges check in release_agent writes

On Fri, Feb 18, 2022 at 1:11 AM Michal Koutný <[email protected]> wrote:
>
> The idea is to check: a) the owning user_ns of cgroup_ns, b)
> capabilities in init_user_ns.
>
> The commit 24f600856418 ("cgroup-v1: Require capabilities to set
> release_agent") got this wrong in the write handler of release_agent
> since it checked user_ns of the opener (may be different from the owning
> user_ns of cgroup_ns).
> Secondly, to avoid possibly confused deputy, the capability of the
> opener must be checked.
>
> Fixes: 24f600856418 ("cgroup-v1: Require capabilities to set release_agent")
> Cc: [email protected]
> Link: https://lore.kernel.org/stable/[email protected]/
> Signed-off-by: Michal Koutný <[email protected]>
> ---
> kernel/cgroup/cgroup-v1.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
> index 0e877dbcfeea..afc6c0e9c966 100644
> --- a/kernel/cgroup/cgroup-v1.c
> +++ b/kernel/cgroup/cgroup-v1.c
> @@ -546,6 +546,7 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
> char *buf, size_t nbytes, loff_t off)
> {
> struct cgroup *cgrp;
> + struct cgroup_file_ctx *ctx;
>
> BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
>
> @@ -553,8 +554,9 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
> * 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))
> + ctx = of->priv;
> + if ((ctx->ns->user_ns != &init_user_ns) ||
> + !file_ns_capable(of->file, &init_user_ns, CAP_SYS_ADMIN))
> return -EPERM;
>
> cgrp = cgroup_kn_lock_live(of->kn, false);
> --
> 2.34.1

Thank you. Looks good to me.

Reviewed-by: Masami Ichikawa(CIP) <[email protected]>

2022-02-22 18:36:28

by Tejun Heo

Subject: Re: [PATCH] cgroup-v1: Correct privileges check in release_agent writes

On Thu, Feb 17, 2022 at 05:11:28PM +0100, Michal Koutn? wrote:
> The idea is to check: a) the owning user_ns of cgroup_ns, b)
> capabilities in init_user_ns.
>
> The commit 24f600856418 ("cgroup-v1: Require capabilities to set
> release_agent") got this wrong in the write handler of release_agent
> since it checked user_ns of the opener (may be different from the owning
> user_ns of cgroup_ns).
> Secondly, to avoid possibly confused deputy, the capability of the
> opener must be checked.
>
> Fixes: 24f600856418 ("cgroup-v1: Require capabilities to set release_agent")
> Cc: [email protected]
> Link: https://lore.kernel.org/stable/[email protected]/
> Signed-off-by: Michal Koutn? <[email protected]>

Applied to cgroup/for-5.17-fixes.

Thanks.

--
tejun