On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 04.03.21 01:03, Shakeel Butt wrote: > > On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > >> > >> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > >>> > >>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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). > >>>> 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. > >>>> > >>>> Cc: stable@xxxxxxxxxxxxxxx # 5.10+ > >>>> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > >>>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > >>>> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> > >>>> Acked-by: David Rientjes <rientjes@xxxxxxxxxx> > >>>> --- > >>>> changes in v3 > >>>> - Added Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > >>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > >>>> - cc'ed stable@xxxxxxxxxxxxxxx # 5.10+ per Andrew's request > >>>> - cc'ed linux-security-module@xxxxxxxxxxxxxxx per James Morris's request > >>>> > >>>> mm/madvise.c | 13 ++++++++++++- > >>>> 1 file changed, 12 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/mm/madvise.c b/mm/madvise.c > >>>> index df692d2e35d4..01fef79ac761 100644 > >>>> --- a/mm/madvise.c > >>>> +++ b/mm/madvise.c > >>>> @@ -1198,12 +1198,22 @@ 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. > >>> > >>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? > >> > >> Non-destructive in this context means the data is not lost and can be > >> recovered. I follow the logic described in > >> https://lwn.net/Articles/794704/ where Minchan was introducing > >> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > >> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > >> would be considered destructive hints. > >> Note that process_madvise_behavior_valid() allows only MADV_COLD and > >> MADV_PAGEOUT at the moment, which are both non-destructive. > >> > > > > There is a plan to support MADV_DONTNEED for this syscall. Do we need > > to change these access checks again with that support? > > Eh, I absolutely don't think letting another process discard memory in > another process' address space is a good idea. The target process can > observe that easily and might even run into real issues. > > What's the use case? > Userspace oom reaper. Please look at https://lore.kernel.org/linux-api/20201014183943.GA1489464@xxxxxxxxxx/T/
