On Fri, Mar 07, 2025 at 12:33:06PM +0000, Ryan Roberts wrote:
> Instead of writing a pte directly into the table, use the set_pte_at()
> helper, which gives the arch visibility of the change.
>
> In this instance we are guaranteed that the pte was originally none and
> is being modified to a not-present pte, so there was unlikely to be a
> bug in practice (at least not on arm64). But it's bad practice to write
> the page table memory directly without arch involvement.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> mm/madvise.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 388dc289b5d1..6170f4acc14f 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1101,7 +1101,7 @@ static int guard_install_set_pte(unsigned long addr, unsigned long next,
> unsigned long *nr_pages = (unsigned long *)walk->private;
>
> /* Simply install a PTE marker, this causes segfault on access. */
> - *ptep = make_pte_marker(PTE_MARKER_GUARD);
> + set_pte_at(walk->mm, addr, ptep, make_pte_marker(PTE_MARKER_GUARD));
I agree with you, but I think perhaps the arg name here is misleading :) If
you look at mm/pagewalk.c and specifically, in walk_pte_range_inner():
if (ops->install_pte && pte_none(ptep_get(pte))) {
pte_t new_pte;
err = ops->install_pte(addr, addr + PAGE_SIZE, &new_pte,
walk);
if (err)
break;
set_pte_at(walk->mm, addr, pte, new_pte);
...
}
So the ptep being assigned here is a stack value, new_pte, which we simply
assign to, and _then_ the page walker code handles the set_pte_at() for us.
So we are indeed doing the right thing here, just in a different place :P
> (*nr_pages)++;
>
> return 0;
> --
> 2.43.0
>
