Marcelo Tosatti wrote:
> There is a race where VCPU0 is shadowing a pagetable entry while VCPU1
> is updating it, which results in a stale shadow copy.
>
> Fix that by comparing the contents of the cached guest pte with the
> current guest pte after write-protecting the guest pagetable.
>
> Attached program kvm_shadow_race.c demonstrates the problem.
>
>
Where is it?
> Signed-off-by: Marcelo Tosatti <[EMAIL PROTECTED]>
>
>
> diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
> index 72d4816..4fece01 100644
> --- a/drivers/kvm/paging_tmpl.h
> +++ b/drivers/kvm/paging_tmpl.h
> @@ -66,6 +66,7 @@ struct guest_walker {
> int level;
> gfn_t table_gfn[PT_MAX_FULL_LEVELS];
> pt_element_t pte;
> + gpa_t pte_gpa;
>
I think this needs to be an array like table_gfn[]. The guest may play
with the pde (and upper entries) as well as the pte.
>
> + kvm_read_guest(vcpu->kvm, walker->pte_gpa, &curr_pte, sizeof(curr_pte));
> +
> + if (curr_pte != walker->pte)
> + return 0;
> +
>
'return NULL'
It would also be preferable to read the pte only if we shadowed the page
just now. Perhaps pass the pte and the index to kvm_mmu_get_page()
which would use them as a guard when the page is being shadowed:
if (lookup page succeeds)
return it
shadow page
write protect it
if (guard check succeeds)
return it
else
return NULL
or perhaps have kvm_mmu_get_page() return an additional bool signifying
it is a new page. but this is ugly.
>
> - ++vcpu->stat.pf_fixed;
> + if (shadow_pte)
> + ++vcpu->stat.pf_fixed;
>
This is a very rare case; it isn't worth being so accurate maintaining
the statistics.
--
error compiling committee.c: too many arguments to function
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