Re: [PATCH] perf/core: fix unconditional security_locked_down() call

[Paul Moore]

On Wed, Feb 24, 2021 at 4:59 PM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
>
> Currently, the lockdown state is queried unconditionally, even though
> its result is used only if the PERF_SAMPLE_REGS_INTR bit is set in
> attr.sample_type. While that doesn't matter in case of the Lockdown LSM,
> it causes trouble with the SELinux's lockdown hook implementation.
>
> SELinux implements the locked_down hook with a check whether the current
> task's type has the corresponding "lockdown" class permission
> ("integrity" or "confidentiality") allowed in the policy. This means
> that calling the hook when the access control decision would be ignored
> generates a bogus permission check and audit record.
>
> Fix this by checking sample_type first and only calling the hook when
> its result would be honored.
>
> Fixes: b0c8fdc7fdb7 ("lockdown: Lock down perf when in confidentiality mode")
> Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> ---
>  kernel/events/core.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)

Perf/core folks, do you want to pull this in via your tree?  If I
don't hear anything in the next day I'll pull this in via the
selinux/next tree.

Reviewed-by: Paul Moore <paul@xxxxxxxxxxxxxx>

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 129dee540a8b..0f857307e9bd 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -11796,12 +11796,12 @@ SYSCALL_DEFINE5(perf_event_open,
>                         return err;
>         }
>
> -       err = security_locked_down(LOCKDOWN_PERF);
> -       if (err && (attr.sample_type & PERF_SAMPLE_REGS_INTR))
> -               /* REGS_INTR can leak data, lockdown must prevent this */
> -               return err;
> -
> -       err = 0;
> +       /* REGS_INTR can leak data, lockdown must prevent this */
> +       if (attr.sample_type & PERF_SAMPLE_REGS_INTR) {
> +               err = security_locked_down(LOCKDOWN_PERF);
> +               if (err)
> +                       return err;
> +       }
>
>         /*
>          * In cgroup mode, the pid argument is used to pass the fd
> --
> 2.29.2

-- 
paul moore
www.paul-moore.com