On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 05.03.21 18:45, Shakeel Butt wrote: > > 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/ > > > > Thanks, somehow I missed that (not that it really changed my opinion on > the approach while skimming over the discussion :) will have a more > detailed look) The latest version of that patchset is: https://lore.kernel.org/patchwork/patch/1344419/ Yeah, memory reaping is a special case when we are operating on a dying process to speed up the release of its memory. I don't know if for that particular case we need to make the checks stricter. It's a dying process anyway and the data is being destroyed. Allowing to speed up that process probably can still use CAP_SYS_NICE. > > -- > Thanks, > > David / dhildenb >
