linux-kernel - Re: [PATCH] x86: use the correct function type for native_set

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Message-ID: <201909231626.A912664DA1@keescook>
Date:   Mon, 23 Sep 2019 16:27:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: use the correct function type for native_set_fixmap

On Fri, Sep 13, 2019 at 02:14:02PM -0700, Sami Tolvanen wrote:
> We call native_set_fixmap indirectly through the function pointer
> struct pv_mmu_ops::set_fixmap, which expects the first parameter to be
> 'unsigned' instead of 'enum fixed_addresses'. This patch changes the
> function type for native_set_fixmap to match the pointer, which fixes
> indirect call mismatches with Control-Flow Integrity (CFI) checking.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Is it correct that pv_mmu_ops can't be changed since this is an external
API?

Assuming so, then:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  arch/x86/include/asm/fixmap.h | 2 +-
>  arch/x86/mm/pgtable.c         | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index 9da8cccdf3fb..6a295acd3de6 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -157,7 +157,7 @@ extern pte_t *kmap_pte;
>  extern pte_t *pkmap_page_table;
>  
>  void __native_set_fixmap(enum fixed_addresses idx, pte_t pte);
> -void native_set_fixmap(enum fixed_addresses idx,
> +void native_set_fixmap(unsigned /* enum fixed_addresses */ idx,
>  		       phys_addr_t phys, pgprot_t flags);
>  
>  #ifndef CONFIG_PARAVIRT_XXL
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index 44816ff6411f..d0ad35e3de74 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -647,8 +647,8 @@ void __native_set_fixmap(enum fixed_addresses idx, pte_t pte)
>  	fixmaps_set++;
>  }
>  
> -void native_set_fixmap(enum fixed_addresses idx, phys_addr_t phys,
> -		       pgprot_t flags)
> +void native_set_fixmap(unsigned /* enum fixed_addresses */ idx,
> +		       phys_addr_t phys, pgprot_t flags)
>  {
>  	/* Sanitize 'prot' against any unsupported bits: */
>  	pgprot_val(flags) &= __default_kernel_pte_mask;
> -- 
> 2.23.0.237.gc6a4ce50a0-goog
> 

-- 
Kees Cook