linux-kernel - Re: [PATCH 32/39] x86/pgtable/pae: Use separate kernel PMDs for user page-table

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Date:   Fri, 5 Oct 2018 16:06:47 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        gregkh <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        dhgutteridge@...patico.ca, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 32/39] x86/pgtable/pae: Use separate kernel PMDs for user page-table

On Wed, Jul 18, 2018 at 11:43 AM Joerg Roedel <joro@...tes.org> wrote:
>  arch/x86/mm/pgtable.c | 100 ++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 81 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index db6fb77..8e4e63d 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -182,6 +182,14 @@ static void pgd_dtor(pgd_t *pgd)
>   */
>  #define PREALLOCATED_PMDS      UNSHARED_PTRS_PER_PGD
>
> +/*
> + * We allocate separate PMDs for the kernel part of the user page-table
> + * when PTI is enabled. We need them to map the per-process LDT into the
> + * user-space page-table.
> + */
> +#define PREALLOCATED_USER_PMDS  (static_cpu_has(X86_FEATURE_PTI) ? \
> +                                       KERNEL_PGD_PTRS : 0)

>   * Xen paravirt assumes pgd table should be in one page. 64 bit kernel also
>   * assumes that pgd should be in one page.
> @@ -376,6 +431,7 @@ static inline void _pgd_free(pgd_t *pgd)
>  pgd_t *pgd_alloc(struct mm_struct *mm)
>  {
>         pgd_t *pgd;
> +       pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
>         pmd_t *pmds[PREALLOCATED_PMDS];
>

This commit from back in July now causes a build warning after the patch
from Kees that enables -Wvla:

In file included from /git/arm-soc/include/linux/kernel.h:15,
                 from /git/arm-soc/include/asm-generic/bug.h:18,
                 from /git/arm-soc/arch/x86/include/asm/bug.h:83,
                 from /git/arm-soc/include/linux/bug.h:5,
                 from /git/arm-soc/include/linux/mmdebug.h:5,
                 from /git/arm-soc/include/linux/mm.h:9,
                 from /git/arm-soc/arch/x86/mm/pgtable.c:2:
/git/arm-soc/arch/x86/mm/pgtable.c: In function 'pgd_alloc':
/git/arm-soc/include/linux/build_bug.h:29:45: error: ISO C90 forbids
variable length array 'u_pmds' [-Werror=vla]
 #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:(-!!(e)); }))
                                             ^
/git/arm-soc/arch/x86/include/asm/cpufeature.h:85:5: note: in
expansion of macro 'BUILD_BUG_ON_ZERO'
     BUILD_BUG_ON_ZERO(NCAPINTS != 19))
     ^~~~~~~~~~~~~~~~~
/git/arm-soc/arch/x86/include/asm/cpufeature.h:111:32: note: in
expansion of macro 'REQUIRED_MASK_BIT_SET'
  (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 : \
                                ^~~~~~~~~~~~~~~~~~~~~
/git/arm-soc/arch/x86/include/asm/cpufeature.h:129:27: note: in
expansion of macro 'cpu_has'
 #define boot_cpu_has(bit) cpu_has(&boot_cpu_data, bit)
                           ^~~~~~~
/git/arm-soc/arch/x86/include/asm/cpufeature.h:209:3: note: in
expansion of macro 'boot_cpu_has'
   boot_cpu_has(bit) :    \
   ^~~~~~~~~~~~
/git/arm-soc/arch/x86/mm/pgtable.c:190:34: note: in expansion of macro
'static_cpu_has'
 #define PREALLOCATED_USER_PMDS  (static_cpu_has(X86_FEATURE_PTI) ? \
                                  ^~~~~~~~~~~~~~
/git/arm-soc/arch/x86/mm/pgtable.c:431:16: note: in expansion of macro
'PREALLOCATED_USER_PMDS'
  pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
                ^~~~~~~~~~~~~~~~~~~~~~

       Arnd