Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled

Kalra, Ashish
2022-11-23T18:32:29Z

Hello Boris,

On 11/23/2022 5:40 AM, Borislav Petkov wrote:
> On Tue, Nov 22, 2022 at 05:44:47AM -0600, Kalra, Ashish wrote:
>> It is important to note that if invalid address/len are supplied, the
>> failure will happen at the initial stage itself of transitioning these pages
>> to firmware state.
>
> /me goes and checks out your v6 tree based on 5.18.
>
> Lemme choose one:
>
> static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
> {
> ...
>
> inpages = sev_pin_memory(kvm, params.uaddr, params.len, &npages, 1);
>
> ...
>
> for (i = 0; i < npages; i++) {
> pfn = page_to_pfn(inpages[i]);
>
> ...
>
> ret = __sev_issue_cmd(argp->sev_fd, SEV_CMD_SNP_LAUNCH_UPDATE, &data, error);
> if (ret) {
> /*
> * If the command failed then need to reclaim the page.
> */
> snp_page_reclaim(pfn);
>
> and here it would leak the pages if it cannot reclaim them.
>
> Now how did you get those?
>
> Through params.uaddr and params.len which come from userspace:
>
> if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data, sizeof(params)))
> return -EFAULT;
>
>
> Now, think about it, can userspace be trusted?
>
> Exactly.
>
> Yeah, yeah, I see it does is_hva_registered() but userspace can just as
> well supply the wrong region which fits.

Yes, that's right.

Also, before sev_issue_cmd() above, there is a call to
rmp_make_private() to make these pages transition to firmware state
before we issue the LAUNCH_UPDATE command as below:

ret = rmp_make_private(pfn, gfn << PAGE_SHIFT, level,
sev_get_asid(kvm), true);
if (ret) {
ret = -EFAULT;
goto e_unpin;

}
...
ret = __sev_issue_cmd(argp->sev_fd, SEV_CMD_SNP_LAUNCH_UPDATE,
&data, error);

So in case, the userspace provided an invalid/incorrect range, this
transition would have failed and there would not have been a need to do
any reclaim, so there are no pages leaked here.

This is also the reason why we need to reclaim pages if the subsequent
LAUNCH_UPDATE command fails, as now the pages are in F/W state because
of the rmp_make_private() call and they are now unsafe to be used by the
host.

>
>> In such a case the kernel panic is justifiable,
>
> So userspace can supply whatever it wants and you'd panic?
>
> You surely don't mean that.
>

No, we don't want to do that.

>> but again if incorrect addresses are supplied, the failure will happen
>> at the initial stage of transitioning these pages to firmware state
>> and there is no need to reclaim.

This is the case i mentioned above, rmp_make_private() before the
firmware command is the initial stage of transitioning the pages to
firmware state before issuing the firmware command.

>
> See above.
>
>> Or, otherwise dump a warning and let the pages not be freed/returned
>> back to the page allocator.
>>
>> It is either innocent pages or kernel panic or an innocent host
>> process crash (these are the choices to make).
>
> No, it is make the kernel as resilient as possible. Which means, no
> panic, add the pages to a not-to-be-used-anymore list and scream loudly
> with warning messages when it must leak pages so that people can fix the
> issue.
>
> Ok?
>

Right, yes, i totally agree.

So now we are adding these pages to an internal not-to-be-used-anymore
list and printing warnings and ensuring no panics as we don't allow
these pages to be released back to the page allocator.

Thanks,
Ashish