Re: [PATCH V2] mm: Allow userland to request that the kernel clear memory on release

[Matthew Garrett <mjg59@google.com>]

On Thu, Apr 25, 2019 at 5:43 AM Jann Horn <jannh@google.com> wrote:
> An interesting effect of this is that it will be possible to set this
> on a CoW anon VMA in a fork() child, and then the semantics in the
> parent will be subtly different - e.g. if the parent vmsplice()d a
> CoWed page into a pipe, then forked an unprivileged child, the child
> set MADV_WIPEONRELEASE on its VMA, the parent died somehow, and then
> the child died, the page in the pipe would be zeroed out. A child
> should not be able to affect its parent like this, I think. If this
> was an mmap() flag instead of a madvise() command, that issue could be
> avoided. Alternatively, if adding more mmap() flags doesn't work,
> perhaps you could scan the VMA and ensure that it contains no pages
> yet, or something like that?

I /think/ my argument here would be not to do that? I agree that it's
unexpected, but I guess the other alternative would be to force a copy
on any existing COW pages in the VMA at madvise() time, and maybe also
at fork() time (sort of like the behaviour of MADV_WIPEONFORK, but
copying the page rather than providing a new zero page)

> I think all the callers have a reference to the VMA, so perhaps you
> could add a VMA parameter to page_remove_rmap() and then look at the
> VMA in there?

I'll dig into that, thanks!