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Re: [PATCH][4.17] x86/shadow: drop (replace) bogus assertions
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Fri, 14 Oct 2022 12:30:29 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Tim Deegan <tim@xxxxxxx>, Henry Wang <Henry.Wang@xxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>
- Delivery-date: Fri, 14 Oct 2022 10:30:44 +0000
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On Fri, Oct 14, 2022 at 10:49:55AM +0200, Jan Beulich wrote:
> The addition of a call to shadow_blow_tables() from shadow_teardown()
> has resulted in the "no vcpus" related assertion becoming triggerable:
> If domain_create() fails with at least one page successfully allocated
> in the course of shadow_enable(), or if domain_create() succeeds and
> the domain is then killed without ever invoking XEN_DOMCTL_max_vcpus.
>
> The assertion's comment was bogus anyway: Shadow mode has been getting
> enabled before allocation of vCPU-s for quite some time. Convert the
> assertion to a conditional: As long as there are no vCPU-s, there's
> nothing to blow away.
>
> Fixes: e7aa55c0aab3 ("x86/p2m: free the paging memory pool preemptively")
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> A similar assertion/comment pair exists in _shadow_prealloc(); the
> comment is similarly bogus, and the assertion could in principle trigger
> e.g. when shadow_alloc_p2m_page() is called early enough. Replace those
> at the same time by a similar early return, here indicating failure to
> the caller (which will generally lead to the domain being crashed in
> shadow_prealloc()).
It's my understanding we do care about this because a control domain
could try to populate the p2m before calling XEN_DOMCTL_max_vcpus, and
hence could trigger the ASSERT, as otherwise asserting would be fine.
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> While in shadow_blow_tables() the option exists to simply remove the
> assertion without adding a new conditional (the two loops simply will
> do nothing), the same isn't true for _shadow_prealloc(): There we
> would then trigger the ASSERT_UNREACHABLE() near the end of the
> function.
I think it's fine to exit early.
Thanks, Roger.
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