Re: [PATCH 2/3] x86/microcode/AMD: Make __verify_patch_size() return bool

[Nikolay Borisov <nik.borisov@suse.com>]

On 14.11.24 г. 16:01 ч., Borislav Petkov wrote:
> On Thu, Nov 14, 2024 at 03:19:33PM +0200, Nikolay Borisov wrote:
>> For the older families we have a hard upper bound so we want to ensure that
>> the size in the header is strictly <= than buf_size, which in turn must be
>> <= max_size .
>>
>>
>> i.e Is it not valid to have sh_psize < buf_size rather than strictly equal ?
> 
> Let's look at all possible cases:
> 
> * sh_psize > min_t(sh_psize, buf_size) == buf_size -- means the buffer is
> truncated so the patch is incomplete
> 
> * sh_psize < min_t(sh_psize, buf_size) == buf_size -- this is actually ok
> because we're working with the whole buffer and there can be other patches
> following. Now I remember why I had ">" there.
> 
> * sh_psize > min_t(u32, buf_size, max_size) == buf_size -- truncated buffer
> 
> * sh_psize < min_t(u32, buf_size, max_size) == buf_size -- that's ok
> 
> * sh_psize > min_t(u32, buf_size, max_size) == max_size -- some mismatch, fail
> 
> * sh_psize < min_t(u32, buf_size, max_size) == max_size -- ditto.
> 
> So this needs more staring and I need to make it more readable.
> 
> Btw, one more spot:
> 
> diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
> index 01ea25f31c0c..7554d83f00e6 100644
> --- a/arch/x86/kernel/cpu/microcode/amd.c
> +++ b/arch/x86/kernel/cpu/microcode/amd.c
> @@ -303,7 +303,7 @@ static bool __verify_patch_size(u32 sh_psize, size_t buf_size)
>   		break;
>   	default:
>   		WARN(1, "%s: WTF family: 0x%x\n", __func__, family);
> -		return 0;
> +		return false;
>   	}
>   
>   	return sh_psize == min_t(u32, buf_size, max_size);
> 
> ---
> 
> IOW, I'm thinking about something like this (pasting the whole function here):
> 
> static bool __verify_patch_size(u32 sh_psize, size_t buf_size)
> {
>          u8 family = x86_family(bsp_cpuid_1_eax);
>          u32 max_size;
> 
>          if (family >= 0x15)
>                  goto ret;
> 
> #define F1XH_MPB_MAX_SIZE 2048
> #define F14H_MPB_MAX_SIZE 1824
> 
>          switch (family) {
>          case 0x10 ... 0x12:
>                  max_size = F1XH_MPB_MAX_SIZE;
>                  break;
>          case 0x14:
>                  max_size = F14H_MPB_MAX_SIZE;
>                  break;
>          default:
>                  WARN(1, "%s: WTF family: 0x%x\n", __func__, family);
>                  return false;
>          }
> 
>          if (sh_psize != max_size)
>                  return false;

Isn't sh_psize < max_size valid here?

>                  
> ret:
>          /* Working with the whole buffer so < is ok. */
>          return sh_psize <= buf_size;
> }
>