linux-kernel - Re: [PATCH v3] x86: Ensure input to pfn_to

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Date:   Thu, 23 Nov 2023 11:15:02 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Michael Roth <michael.roth@....com>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v3] x86: Ensure input to pfn_to_kaddr() is treated as a
 64-bit type


* Michael Roth <michael.roth@....com> wrote:

> On 64-bit platforms, the pfn_to_kaddr() macro requires that the input
> value is 64-bits in order to ensure that valid address bits don't get
> lost when shifting that input by PAGE_SHIFT to calculate the physical
> address to provide a virtual address for.
> 
> One such example is in pvalidate_pages() (used by SEV-SNP guests), where
> the GFN in the struct used for page-state change requests is a 40-bit
> bit-field, so attempts to pass this GFN field directly into
> pfn_to_kaddr() ends up causing guest crashes when dealing with addresses
> above the 1TB range due to the above.
> 
> Fix this issue with SEV-SNP guests, as well as any similar cases that
> might cause issues in current/future code, by using a inline function,
> instead of the macro, so that the input is implicitly cast to the
> expected 64-bit input type prior to performing the shift operation.
> 
> While it might be argued that the issue is on the caller side, other
> archs/macros have taken similar approaches to deal with instances like
> this, such as commit e48866647b48 ("ARM: 8396/1: use phys_addr_t in
> pfn_to_kaddr()").
> 
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Fixes: 6c3211796326 ("x86/sev: Add SNP-specific unaccepted memory support")
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---
> v3:
>   - Use an inline function instead of a macro to handle the cast (Peter)
> v2:
>   - Move the cast down into pfn_to_kaddr() to fix other possible
>     instances (Dave)
>   - Rename from "x86/sev: Fix overflow when computing address for
>     PVALIDATE"
> 
>  arch/x86/include/asm/page.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> index d18e5c332cb9..aa8930da42fd 100644
> --- a/arch/x86/include/asm/page.h
> +++ b/arch/x86/include/asm/page.h
> @@ -66,10 +66,14 @@ static inline void copy_user_page(void *to, void *from, unsigned long vaddr,
>   * virt_addr_valid(kaddr) returns true.
>   */
>  #define virt_to_page(kaddr)	pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
> -#define pfn_to_kaddr(pfn)      __va((pfn) << PAGE_SHIFT)
>  extern bool __virt_addr_valid(unsigned long kaddr);
>  #define virt_addr_valid(kaddr)	__virt_addr_valid((unsigned long) (kaddr))
>  
> +static inline void *pfn_to_kaddr(unsigned long pfn)
> +{
> +	return __va(pfn << PAGE_SHIFT);
> +}

Makes sense I guess.

Note that in the tip:x86/mm commit I changed it to __always_inline, as we 
typically do for such low level MM helpers. Delta patch below.

Thanks,

	Ingo

==============>

 arch/x86/include/asm/page.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
index aa8930da42fd..1b93ff80b43b 100644
--- a/arch/x86/include/asm/page.h
+++ b/arch/x86/include/asm/page.h
@@ -69,7 +69,7 @@ static inline void copy_user_page(void *to, void *from, unsigned long vaddr,
 extern bool __virt_addr_valid(unsigned long kaddr);
 #define virt_addr_valid(kaddr)	__virt_addr_valid((unsigned long) (kaddr))
 
-static inline void *pfn_to_kaddr(unsigned long pfn)
+static __always_inline void *pfn_to_kaddr(unsigned long pfn)
 {
 	return __va(pfn << PAGE_SHIFT);
 }