linux-kernel - Re: [RFC][PATCH 1/4] lsm: Clarify documentation of vm_enough

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Message-ID: <CAHC9VhQjnwbFgAoFgTaLQP7YnNDNyP1i0i8H++HZWj930pW=-A@mail.gmail.com>
Date:   Tue, 15 Nov 2022 21:11:43 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, revest@...omium.org,
        jackmanb@...omium.org, jmorris@...ei.org, serge@...lyn.com,
        bpf@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [RFC][PATCH 1/4] lsm: Clarify documentation of vm_enough_memory hook

On Tue, Nov 15, 2022 at 12:57 PM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
>
> From: Roberto Sassu <roberto.sassu@...wei.com>
>
> include/linux/lsm_hooks.h reports the result of the LSM infrastructure to
> the callers, not what LSMs should return to the LSM infrastructure.
>
> Clarify that and add that returning 1 from the LSMs means calling
> __vm_enough_memory() with cap_sys_admin set, 0 without.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> Reviewed-by: KP Singh <kpsingh@...nel.org>
> ---
>  include/linux/lsm_hooks.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 4ec80b96c22e..f40b82ca91e7 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -1411,7 +1411,9 @@
>   *     Check permissions for allocating a new virtual mapping.
>   *     @mm contains the mm struct it is being added to.
>   *     @pages contains the number of pages.
> - *     Return 0 if permission is granted.
> + *     Return 0 if permission is granted by LSMs to the caller. LSMs should
> + *     return 1 if __vm_enough_memory() should be called with
> + *     cap_sys_admin set, 0 if not.

I think this is a nice addition, but according to the code, any value
greater than zero will trigger the caller-should-have-CAP_SYS_ADMIN
behavior, not just 1.  I suggest updating the comment.

-- 
paul-moore.com