On Wed, Mar 12, 2025 at 9:45 AM Kees Cook kees@kernel.org wrote:
On Wed, Mar 12, 2025 at 03:50:40PM +0000, Lorenzo Stoakes wrote:
What about madvise() with MADV_DONTNEED on a r/o VMA that's not faulted in? That's a no-op right? But it's not permitted.
Madvise's semantics are about behavior, while mprotect is about attributes. To me: madvise is like "make this VMA do that" while mprotect is about "update this VMA's attributes to a new value".
It is more difficult to determine if a behavior is no-op, so I don't intend to apply the same no-op concept to madvise().
Hmm, yes, that's a good example. Thank you!
So now we have an inconsistency between the two calls.
Yeah, I see your concern now.
I don't know what you mean by 'ergonomic'?
I was thinking about idempotent-ness. Like, some library setting up a memory region, it can't call its setup routine twice if the second time through (where no changes are made) it gets rejected. But I think this is likely just a userspace problem: check for the VMAs before blindly trying to do it again. (This is strictly an imagined situation.)
Yes.
We also don't have a system call to query the "mprotect" attributes, so it is understandable that userspace can rely on idempotents of the mprotect.
My reply seemed to get truncated at the end here :) So let me ask again - do you have a practical case in mind for this?
I noticed there were idempotent mprotects last year while working on applying mseal on stack in Android. I assume this might not be the only instance since mprotect gets called a lot in general.
Blocking this won't improve security, it could actually hinder the adoption of mseal, i.e. force apps to make code change.
-Jeff
Sorry, I didn't have any reply to that part, so I left it off. If Jeff has a specific case in mind, I'll let him answer that part. :)
-Kees
-- Kees Cook