Messages in this thread | | | Date | Mon, 18 May 2020 08:36:25 -0700 | From | "Luck, Tony" <> | Subject | Re: [PATCH] x86/mm: Don't try to change poison pages to uncacheable in a guest |
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On Mon, May 18, 2020 at 03:48:13PM +0200, Borislav Petkov wrote: > Hi, > > lemme try to reply to three emails at once. > > First of all, the two of you: pls do not top-post.
Sorry. Phone e-mail client is dumb.
> On Sat, May 16, 2020 at 6:52 PM Luck, Tony <tony.luck@intel.com> wrote: > > But the guest isn’t likely to do the right thing with a page fault. > > The guest just accessed a page that it knows is poisoned (VMM just told > > it once that it was poisoned). There is no reason that the VMM should > > let the guest actually touch the poison a second time. But if the guest > > does, then the guest should get the expected response. I.e. another > > machine check. > > So Jue says below that the VMM has unmapped the guest page from the EPT. > So the guest cannot touch the poison anymore. > > How is then possible for the guest to touch it again if the page is not > mapped anymore?
The VMM wants to make sure that the guest can't touch the poison (this is important because not every touch of poison results in a recoverable machine check. If the guest's next touch is an unaligned access that crosses from the poison cache line to a non-poisoned line then h/w will signal a fatal machinecheck and the whole machine will go down).
> The guest should get a #PF when the page is unmapped and cause a new > page to be mapped.
The VMM gets the page fault (because the unmapping of the guest physical address is at the VMM EPT level). The VMM can't map a new page into that guest physical address because it has no way to replace the contents of the old page. The VMM could pass the #PF to the guest, but that would just confuse the guest (its page tables all say that the page is still valid). In this particular case the page is part of the 1:1 kernel map. So the kernel will OOPS (I think).
> On Sun, May 17, 2020 at 07:36:00AM -0700, Jue Wang wrote: > > The stack is from guest MCE handler's processing of the first MCE injected. > > Aha, so you've flipped the functions order in the trace. It all starts > at > > set_mce_nospec(m->addr >> PAGE_SHIFT); > > Now it makes sense. > > > Note before the first MCE is injected into guest (i.e., after the host MCE > > handler successfully finished MCE handling and notified VMM via SIGBUS with > > BUS_MCEERR_AR), VMM unmaps the guest page from EPT. > > Ok, good. > > > The guest MCE handler finished the "normal" MCE handling and recovery > > (memory_failure() in mm/memory_failure.cc) successfully, it's the aftermath > > below leading to the stack trace: > > https://github.com/torvalds/linux/blob/5a9ffb954a3933d7867f4341684a23e008d6839b/arch/x86/kernel/cpu/mce/core.c#L1101 > > On Sun, May 17, 2020 at 08:33:00AM -0700, Jue Wang wrote: > > In other words, it's the *do_memory_failure -> set_mce_nospec* flow of > > guest MCE handler acting on the *first* MCE injection. As a result, the > > guest panics and resets *whenever* there is an MCE injected, even when the > > injected MCE is recoverable. > > So IIUC that set_mce_nospec() thing should check whether m->addr is > mapped first and only then mark it _uc and whatever monkey business it > does. Not this blanket
PLease explain how a guest (that doesn't even know that it is a guest) is going to figure out that the EPT tables (that it has no way to access) have marked this page invalid in guest physical address space.
> if am I a guest? > > test. > > Imagine a hypervisor which doesn't set X86_FEATURE_HYPERVISOR, i.e., > CPUID(1)[EDX, bit 31]?
Guest is going to be screwed in this case.
-Tony
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