On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand david@redhat.com wrote:
On 05.03.21 18:45, Shakeel Butt wrote:
On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand david@redhat.com wrote:
On 04.03.21 01:03, Shakeel Butt wrote:
On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan surenb@google.com wrote:
On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt shakeelb@google.com wrote:
On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan surenb@google.com 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@vger.kernel.org # 5.10+ > Signed-off-by: Suren Baghdasaryan surenb@google.com > Reviewed-by: Kees Cook keescook@chromium.org > Acked-by: Minchan Kim minchan@kernel.org > Acked-by: David Rientjes rientjes@google.com > --- > changes in v3 > - Added Reviewed-by: Kees Cook keescook@chromium.org > - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f... > - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > - cc'ed linux-security-module@vger.kernel.org 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@google.com/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