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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

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

Jan


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