Re: [tip:x86/urgent] x86/mm: Avoid VLA in pgd_alloc()

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Oct 9, 2018 at 9:03 AM tip-bot for Kees Cook <tipbot@zytor.com> wrote:
>
> Commit-ID:  184d47f0fd365108bd06ab26cdb3450b716269fd
> Gitweb:     https://git.kernel.org/tip/184d47f0fd365108bd06ab26cdb3450b716269fd
> Author:     Kees Cook <keescook@chromium.org>
> AuthorDate: Mon, 8 Oct 2018 16:54:34 -0700
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 9 Oct 2018 08:55:07 +0200
>
> x86/mm: Avoid VLA in pgd_alloc()
>
> Arnd Bergmann reported that turning on -Wvla found a new (unintended) VLA usage:
>
>   arch/x86/mm/pgtable.c: In function 'pgd_alloc':
>   include/linux/build_bug.h:29:45: error: ISO C90 forbids variable length array 'u_pmds' [-Werror=vla]
>   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) ? \
>                                     ^~~~~~~~~~~~~~
>   arch/x86/mm/pgtable.c:431:16: note: in expansion of macro 'PREALLOCATED_USER_PMDS'
>     pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
>                 ^~~~~~~~~~~~~~~~~~~~~~
>
> Use the actual size of the array that is used for X86_FEATURE_PTI,
> which is known at build time, instead of the variable size.
>
> [ mingo: Squashed original fix with followup fix to avoid bisection breakage, wrote new changelog. ]
>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Original-written-by: Arnd Bergmann <arnd@arndb.de>
> Reported-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for fixing up my incorrect fix, looks correct to me now.

       Arnd