Re: [PATCH mm-unstable] mm/madvise: remove CAP_SYS_ADMIN requirement for process_madvise(MADV_COLLAPSE)

[Michal Hocko <mhocko@suse.com>]

Hasn't this been discussed during the MADV_COLLAPSE submission? What has
changed? Does this need more time to settle with the consensus?

On Mon 01-08-22 14:09:46, Zach O'Keefe wrote:
> process_madvise(MADV_COLLAPSE) currently requires CAP_SYS_ADMIN when not
> acting on the caller's own mm.  This is maximally restrictive, and
> perpetuates existing issues with CAP_SYS_ADMIN.  Remove this requirement.
> 
> When acting on an external process' memory, the biggest concerns for
> process_madvise(MADV_COLLAPSE) are (1) being able to influence process
> performance by moving memory, possibly between nodes, that is mapped
> into the address space of external process(es), (2) defeat of
> address-space-layout randomization, and (3), being able to increase
> process RSS and memcg usage, possibly causing memcg OOM.
> 
> process_madvise(2) already enforces CAP_SYS_NICE and PTRACE_MODE_READ (in
> PTRACE_MODE_FSCREDS mode).  A process with these credentials can already
> accomplish (1) and (2) via move_pages(MPOL_MF_MOVE_ALL), and (3) via
> process_madvise(MADV_WILLNEED).
> 
> process_madvise(MADV_COLLAPSE) may also circumvent sysfs THP settings.
> When acting on one's own memory (which is equivalent to
> madvise(MADV_COLLAPSE)), this is deemed acceptable, since aside from the
> possibility of hoarding available hugepages (which is currently already
> possible) no harm to the system can be done.  When acting on an external
> process' memory, circumventing sysfs THP settings should provide no
> additional threat compared to the ones listed.  As such, imposing
> additional capabilities (such as CAP_SETUID, as a way to ensure the
> caller could have just altered the sysfs THP settings themselves)
> provides no extra protection.
> 
> Fixes: 7ec952341312 ("mm/madvise: add MADV_COLLAPSE to process_madvise()")
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> ---
>  mm/madvise.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index f9e11b6c9916..af97100a0727 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1170,16 +1170,14 @@ madvise_behavior_valid(int behavior)
>  	}
>  }
>  
> -static bool
> -process_madvise_behavior_valid(int behavior, struct task_struct *task)
> +static bool process_madvise_behavior_valid(int behavior)
>  {
>  	switch (behavior) {
>  	case MADV_COLD:
>  	case MADV_PAGEOUT:
>  	case MADV_WILLNEED:
> -		return true;
>  	case MADV_COLLAPSE:
> -		return task == current || capable(CAP_SYS_ADMIN);
> +		return true;
>  	default:
>  		return false;
>  	}
> @@ -1457,7 +1455,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  		goto free_iov;
>  	}
>  
> -	if (!process_madvise_behavior_valid(behavior, task)) {
> +	if (!process_madvise_behavior_valid(behavior)) {
>  		ret = -EINVAL;
>  		goto release_task;
>  	}
> -- 
> 2.37.1.455.g008518b4e5-goog

-- 
Michal Hocko
SUSE Labs