Re: [Xen-devel] [RFC] x86/boot: Don't use BDA value if it's suspiciously small

[Sylvain Munaut <s.munaut@xxxxxxxxxxxxxxxxxxxx>]

Hi,

On Fri, Aug 26, 2016 at 3:06 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 26.08.16 at 11:09, <s.munaut@xxxxxxxxxxxxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/boot/head.S
>> +++ b/xen/arch/x86/boot/head.S
>> @@ -108,6 +108,8 @@ __start:
>>          shl     $10-4,%edx
>>          cmp     %eax,%edx           /* compare with BDA value */
>>          cmovb   %edx,%eax           /* and use the smaller */
>> +        cmp     $0x1000,%eax        /* or if the BDA value is too small */
>> +        cmovb   %edx,%eax           /* (and probably not valid) */
>
> Considering there is a bounds check of the EBDA values a few
> lines up from here (against 0x4000) I don't think I see how this
> code can help, assuming the given explanation is applicable.

Because in my case, the EBDA value is rejected, it then falls back to
reading the "base memory size" from the BDA, but that also reads as
0x00.
Then because 0 is smaller than the value from multiboot, it actually
takes that. Then decrements it by 0x1000 (which doesn't go well) and
try to place the trampoline there ...


> In any event is bounding by 0x1000 likely not enough, as placing
> the trampoline at address zero is unlikely to be a good idea.

I just took that value since that was the lower bound used for the
multiboot value.

What would be reasonable as a lower bound for validity check ?


Cheers,

   Sylvain

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