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@xxxxxxxxxx>
> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> 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
