> Am 05.03.2021 um 19:36 schrieb Suren Baghdasaryan <surenb@xxxxxxxxxx>: > > 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... Yeah, most likely! >
