KVM: SVM: Don't inject #UD if KVM attempts to skip SEV guest insn
commit cb49631a upstream. Don't inject a #UD if KVM attempts to "emulate" to skip an instruction for an SEV guest, and instead resume the guest and hope that it can make forward progress. When commit 04c40f34 ("KVM: SVM: Inject #UD on attempted emulation for SEV guest w/o insn buffer") added the completely arbitrary #UD behavior, there were no known scenarios where a well-behaved guest would induce a VM-Exit that triggered emulation, i.e. it was thought that injecting #UD would be helpful. However, now that KVM (correctly) attempts to re-inject INT3/INTO, e.g. if a #NPF is encountered when attempting to deliver the INT3/INTO, an SEV guest can trigger emulation without a buffer, through no fault of its own. Resuming the guest and retrying the INT3/INTO is architecturally wrong, e.g. the vCPU will incorrectly re-hit code #DBs, but for SEV guests there is literally no other option that has a chance of making forward progress. Drop the #UD injection for all "skip" emulation, not just those related to INT3/INTO, even though that means that the guest will likely end up in an infinite loop instead of getting a #UD (the vCPU may also crash, e.g. if KVM emulated everything about an instruction except for advancing RIP). There's no evidence that suggests that an unexpected #UD is actually better than hanging the vCPU, e.g. a soft-hung vCPU can still respond to IRQs and NMIs to generate a backtrace. Reported-by:Wu Zongyo <wuzongyo@mail.ustc.edu.cn> Closes: https://lore.kernel.org/all/8eb933fd-2cf3-d7a9-32fe-2a1d82eac42a@mail.ustc.edu.cn Fixes: 6ef88d6e ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction") Cc: stable@vger.kernel.org Cc: Tom Lendacky <thomas.lendacky@amd.com> Link: https://lore.kernel.org/r/20230825013621.2845700-2-seanjc@google.com Signed-off-by:
Sean Christopherson <seanjc@google.com> Signed-off-by:
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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3988692a
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