Re: [RFC PATCH 3/4] mm: madvise: implement lightweight guard page mechanism

[Jann Horn]

On Fri, Sep 27, 2024 at 2:51 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
> Implement a new lightweight guard page feature, that is regions of userland
> virtual memory that, when accessed, cause a fatal signal to arise.
[...]
> ---
>  arch/alpha/include/uapi/asm/mman.h     |   3 +
>  arch/mips/include/uapi/asm/mman.h      |   3 +
>  arch/parisc/include/uapi/asm/mman.h    |   3 +
>  arch/xtensa/include/uapi/asm/mman.h    |   3 +
>  include/uapi/asm-generic/mman-common.h |   3 +

I kinda wonder if we could start moving the parts of those headers
that are the same for all architectures to include/uapi/linux/mman.h
instead... but that's maybe out of scope for this series.

[...]
> diff --git a/mm/madvise.c b/mm/madvise.c
> index e871a72a6c32..7216e10723ae 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -60,6 +60,7 @@ static int madvise_need_mmap_write(int behavior)
>         case MADV_POPULATE_READ:
>         case MADV_POPULATE_WRITE:
>         case MADV_COLLAPSE:
> +       case MADV_GUARD_UNPOISON: /* Only poisoning needs a write lock. */

What does poisoning need a write lock for? anon_vma_prepare() doesn't
need it (it only needs mmap_lock held for reading),
zap_page_range_single() doesn't need it, and pagewalk also doesn't
need it as long as the range being walked is covered by a VMA, which
it is...

I see you set PGWALK_WRLOCK in guard_poison_walk_ops with a comment
saying "We might need to install an anon_vma" - is that referring to
an older version of the patch where the anon_vma_prepare() call was
inside the pagewalk callback or something like that? Either way,
anon_vma_prepare() doesn't need write locks (it can't, it has to work
from the page fault handling path).

>                 return 0;
>         default:
>                 /* be safe, default to 1. list exceptions explicitly */
[...]
> +static long madvise_guard_poison(struct vm_area_struct *vma,
> +                                struct vm_area_struct **prev,
> +                                unsigned long start, unsigned long end)
> +{
> +       long err;
> +       bool retried = false;
> +
> +       *prev = vma;
> +       if (!is_valid_guard_vma(vma, /* allow_locked = */false))
> +               return -EINVAL;
> +
> +       /*
> +        * Optimistically try to install the guard poison pages first. If any
> +        * non-guard pages are encountered, give up and zap the range before
> +        * trying again.
> +        */
> +       while (true) {
> +               unsigned long num_installed = 0;
> +
> +               /* Returns < 0 on error, == 0 if success, > 0 if zap needed. */
> +               err = walk_page_range_mm(vma->vm_mm, start, end,
> +                                        &guard_poison_walk_ops,
> +                                        &num_installed);
> +               /*
> +                * If we install poison markers, then the range is no longer
> +                * empty from a page table perspective and therefore it's
> +                * appropriate to have an anon_vma.
> +                *
> +                * This ensures that on fork, we copy page tables correctly.
> +                */
> +               if (err >= 0 && num_installed > 0) {
> +                       int err_anon = anon_vma_prepare(vma);

I'd move this up, to before we create poison PTEs. There's no harm in
attaching an anon_vma to the VMA even if the rest of the operation
fails; and I think it would be weird to have error paths that don't
attach an anon_vma even though they .

> +                       if (err_anon)
> +                               err = err_anon;
> +               }
> +
> +               if (err <= 0)
> +                       return err;
> +
> +               if (!retried)
> +                       /*
> +                        * OK some of the range have non-guard pages mapped, zap
> +                        * them. This leaves existing guard pages in place.
> +                        */
> +                       zap_page_range_single(vma, start, end - start, NULL);
> +               else
> +                       /*
> +                        * If we reach here, then there is a racing fault that
> +                        * has populated the PTE after we zapped. Give up and
> +                        * let the user know to try again.
> +                        */
> +                       return -EAGAIN;

Hmm, yeah, it would be nice if we could avoid telling userspace to
loop on -EAGAIN but I guess we don't have any particularly good
options here? Well, we could bail out with -EINTR if a (fatal?) signal
is pending and otherwise keep looping... if we'd tell userspace "try
again on -EAGAIN", we might as well do that in the kernel...

(Personally I would put curly braces around these branches because
they occupy multiple lines, though the coding style doesn't explicitly
say that, so I guess maybe it's a matter of personal preference...
adding curly braces here would match what is done, for example, in
relocate_vma_down().)

> +               retried = true;
> +       }
> +}
> +
> +static int guard_unpoison_pte_entry(pte_t *pte, unsigned long addr,
> +                                   unsigned long next, struct mm_walk *walk)
> +{
> +       pte_t ptent = ptep_get(pte);
> +
> +       if (is_guard_pte_marker(ptent)) {
> +               /* Simply clear the PTE marker. */
> +               pte_clear_not_present_full(walk->mm, addr, pte, true);

I think that last parameter probably should be "false"? The sparc code
calls it "fullmm", which is a term the MM code uses when talking about
operations that remove all mappings in the entire mm_struct because
the process has died, which allows using some faster special-case
version of TLB shootdown or something along those lines.

> +               update_mmu_cache(walk->vma, addr, pte);
> +       }
> +
> +       return 0;
> +}
> +
> +static const struct mm_walk_ops guard_unpoison_walk_ops = {
> +       .pte_entry              = guard_unpoison_pte_entry,
> +       .walk_lock              = PGWALK_RDLOCK,
> +};

It is a _little_ weird that unpoisoning creates page tables when they
don't already exist, which will also prevent creating THP entries on
fault in such areas afterwards... but I guess it doesn't really matter
given that poisoning has that effect, too, and you probably usually
won't call MADV_GUARD_UNPOISON on an area that hasn't been poisoned
before... so I guess this is not an actionable comment.