Re: [PATCH 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

On Mon, Jan 11, 2021 at 9:05 AM Suren Baghdasaryan [off-list ref] wrote:

On Mon, Jan 11, 2021 at 2:20 AM Florian Weimer [off-list ref] wrote:

quoted

* Suren Baghdasaryan:

quoted

diff --git a/mm/madvise.c b/mm/madvise.c
index 6a660858784b..c2d600386902 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -1197,12 +1197,22 @@ 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;
      }

Shouldn't this depend on the requested behavior?  Several operations
directly result in observable changes, and go beyond performance tuning.

Thanks for the comment Florian.
process_madvise supports only MADV_COLD and MADV_PAGEOUT hints which
are both non-destructive (see process_madvise_behavior_valid()
function). Maybe you meant something else by "observable changes", if
so please clarify.
Thanks,
Suren.

V2 with Minchan's fix is posted at:
https://lore.kernel.org/lkml/20210111170622.2613577-1-surenb@google.com/T/#u (local)

quoted

Thanks,
Florian
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