Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries

[Jason A. Donenfeld]

On Fri, Aug 7, 2020 at 10:40 AM Joerg Roedel <joro@xxxxxxxxxx> wrote:
>
> From: Joerg Roedel <jroedel@xxxxxxx>
>
> The code for preallocate_vmalloc_pages() was written under the
> assumption that the p4d_offset() and pud_offset() functions will perform
> present checks before dereferencing the parent entries.
>
> This assumption is wrong an leads to a bug in the code which causes the
> physical address found in the PGD be used as a page-table page, even if
> the PGD is not present.
>
> So the code flow currently is:
>
>         pgd = pgd_offset_k(addr);
>         p4d = p4d_offset(pgd, addr);
>         if (p4d_none(*p4d))
>                 p4d = p4d_alloc(&init_mm, pgd, addr);
>
> This lacks a check for pgd_none() at least, the correct flow would be:
>
>         pgd = pgd_offset_k(addr);
>         if (pgd_none(*pgd))
>                 p4d = p4d_alloc(&init_mm, pgd, addr);
>         else
>                 p4d = p4d_offset(pgd, addr);
>
> But this is the same flow that the p4d_alloc() and the pud_alloc()
> functions use internally, so there is no need to duplicate them.
>
> Remove the p?d_none() checks from the function and just call into
> p4d_alloc() and pud_alloc() to correctly pre-allocate the PGD entries.
>
> Reported-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> Fixes: 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area")
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  arch/x86/mm/init_64.c | 31 +++++++++++++------------------
>  1 file changed, 13 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3f4e29a78f2b..449e071240e1 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1253,28 +1253,23 @@ static void __init preallocate_vmalloc_pages(void)
>                 p4d_t *p4d;
>                 pud_t *pud;
>
> -               p4d = p4d_offset(pgd, addr);
> -               if (p4d_none(*p4d)) {
> -                       /* Can only happen with 5-level paging */
> -                       p4d = p4d_alloc(&init_mm, pgd, addr);
> -                       if (!p4d) {
> -                               lvl = "p4d";
> -                               goto failed;
> -                       }
> -               }
> +               lvl = "p4d";
> +               p4d = p4d_alloc(&init_mm, pgd, addr);
> +               if (!p4d)
> +                       goto failed;
>
> +               /*
> +                * With 5-level paging the P4D level is not folded. So the PGDs
> +                * are now populated and there is no need to walk down to the
> +                * PUD level.
> +                */
>                 if (pgtable_l5_enabled())
>                         continue;
>
> -               pud = pud_offset(p4d, addr);
> -               if (pud_none(*pud)) {
> -                       /* Ends up here only with 4-level paging */
> -                       pud = pud_alloc(&init_mm, p4d, addr);
> -                       if (!pud) {
> -                               lvl = "pud";
> -                               goto failed;
> -                       }
> -               }
> +               lvl = "pud";
> +               pud = pud_alloc(&init_mm, p4d, addr);
> +               if (!pud)
> +                       goto failed;
>         }
>
>         return;
> --
> 2.26.2


This appears to fix the issue, so:

Tested-by: Jason A. Donenfeld <Jason@xxxxxxxxx>