On Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 05.03.21 19:08, Suren Baghdasaryan wrote: > > 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. > > I know, unrelated discussion (sorry, I don't have above thread in my > archive anymore due to automatic cleanups ...) , but introducing > MADV_DONTEED on a remote processes, having to tweak range logic because > we always want to apply it to the whole MM, just to speed up memory > reaping sounds like completely abusing madvise()/process_madvise() to me. > > You want different semantics than MADV_DONTNEED. You want different > semantics than madvise. > > Simple example: mlock()ed pages in the target process. MADV_DONTNEED > would choke on that. For the use case of reaping, you certainly don't care. > > I am not sure if process_madvise() is the right interface to enforce > discarding of all target memory. > > > Not to mention that MADV_FREE doesn't make any sense IMHO for memory > reaping ... no to mention exposing this via process_madvise(). Yeah, that was the last comment from Christoph Hellwig on https://lore.kernel.org/patchwork/patch/1344418/ I'll be rethinking the whole approach. Previously I proposed couple different approaches that would make reaping a part of the kill by adding a new flag for pidfd_send_signal: https://lore.kernel.org/patchwork/patch/1338196/ https://lore.kernel.org/patchwork/patch/1060407/ but maybe a separate syscall for reaping is indeed the right way to go... > > -- > Thanks, > > David / dhildenb >
