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Date:   Tue, 12 Jan 2021 08:46:29 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, jannh@...gle.com, keescook@...omium.org,
        jeffv@...gle.com, minchan@...nel.org, shakeelb@...gle.com,
        rientjes@...gle.com, edgararriaga@...gle.com, timmurray@...gle.com,
        linux-mm@...ck.org, selinux@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for
 process_madvise

On Mon 11-01-21 09:06:22, Suren Baghdasaryan wrote:
> process_madvise currently requires ptrace attach capability.
> PTRACE_MODE_ATTACH gives one process complete control over another
> process. It effectively removes the security boundary between the
> two processes (in one direction). Granting ptrace attach capability
> even to a system process is considered dangerous since it creates an
> attack surface. This severely limits the usage of this API.
> The operations process_madvise can perform do not affect the correctness
> of the operation of the target process; they only affect where the data
> is physically located (and therefore, how fast it can be accessed).

Yes it doesn't influence the correctness but it is still a very
sensitive operation because it can allow a targeted side channel timing
attacks so we should be really careful.

> What we want is the ability for one process to influence another process
> in order to optimize performance across the entire system while leaving
> the security boundary intact.
> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ
> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata
> and CAP_SYS_NICE for influencing process performance.

I have to say that ptrace modes are rather obscure to me. So I cannot
really judge whether MODE_READ is sufficient. My understanding has
always been that this is requred to RO access to the address space. But
this operation clearly has a visible side effect. Do we have any actual
documentation for the existing modes?

I would be really curious to hear from Jann and Oleg (now Cced).

Is CAP_SYS_NICE requirement really necessary?

> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Acked-by: Minchan Kim <minchan@...nel.org>
> Acked-by: David Rientjes <rientjes@...gle.com>
> ---
>  mm/madvise.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6a660858784b..a9bcd16b5d95 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1197,12 +1197,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  		goto release_task;
>  	}
>  
> -	mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> +	/* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */
> +	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
>  	if (IS_ERR_OR_NULL(mm)) {
>  		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
>  		goto release_task;
>  	}
>  
> +	/*
> +	 * Require CAP_SYS_NICE for influencing process performance. Note that
> +	 * only non-destructive hints are currently supported.
> +	 */
> +	if (!capable(CAP_SYS_NICE)) {
> +		ret = -EPERM;
> +		goto release_mm;
> +	}
> +
>  	total_len = iov_iter_count(&iter);
>  
>  	while (iov_iter_count(&iter)) {
> @@ -1217,6 +1227,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  	if (ret == 0)
>  		ret = total_len - iov_iter_count(&iter);
>  
> +release_mm:
>  	mmput(mm);
>  release_task:
>  	put_task_struct(task);
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
> 

-- 
Michal Hocko
SUSE Labs

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