FAILED: patch "[PATCH] x86/virt: Eat faults on VMXOFF in reboot flows" failed to apply to 4.4-stable tree

FAILED: patch "[PATCH] x86/virt: Eat faults on VMXOFF in reboot flows" failed to apply to 4.4-stable tree

[gregkh]

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From aec511ad153556640fb1de38bfe00c69464f997f Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Wed, 30 Dec 2020 16:26:54 -0800
Subject: [PATCH] x86/virt: Eat faults on VMXOFF in reboot flows

Silently ignore all faults on VMXOFF in the reboot flows as such faults
are all but guaranteed to be due to the CPU not being in VMX root.
Because (a) VMXOFF may be executed in NMI context, e.g. after VMXOFF but
before CR4.VMXE is cleared, (b) there's no way to query the CPU's VMX
state without faulting, and (c) the whole point is to get out of VMX
root, eating faults is the simplest way to achieve the desired behaior.

Technically, VMXOFF can fault (or fail) for other reasons, but all other
fault and failure scenarios are mode related, i.e. the kernel would have
to magically end up in RM, V86, compat mode, at CPL>0, or running with
the SMI Transfer Monitor active.  The kernel is beyond hosed if any of
those scenarios are encountered; trying to do something fancy in the
error path to handle them cleanly is pointless.

Fixes: 1e9931146c74 ("x86: asm/virtext.h: add cpu_vmxoff() inline function")
Reported-by: David P. Reed <dpreed@deepplum.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201231002702.2223707-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h
index 9aad0e0876fb..fda3e7747c22 100644
--- a/arch/x86/include/asm/virtext.h
+++ b/arch/x86/include/asm/virtext.h
@@ -30,15 +30,22 @@ static inline int cpu_has_vmx(void)
 }
 
 
-/** Disable VMX on the current CPU
+/**
+ * cpu_vmxoff() - Disable VMX on the current CPU
  *
- * vmxoff causes a undefined-opcode exception if vmxon was not run
- * on the CPU previously. Only call this function if you know VMX
- * is enabled.
+ * Disable VMX and clear CR4.VMXE (even if VMXOFF faults)
+ *
+ * Note, VMXOFF causes a #UD if the CPU is !post-VMXON, but it's impossible to
+ * atomically track post-VMXON state, e.g. this may be called in NMI context.
+ * Eat all faults as all other faults on VMXOFF faults are mode related, i.e.
+ * faults are guaranteed to be due to the !post-VMXON check unless the CPU is
+ * magically in RM, VM86, compat mode, or at CPL>0.
  */
 static inline void cpu_vmxoff(void)
 {
-	asm volatile ("vmxoff");
+	asm_volatile_goto("1: vmxoff\n\t"
+			  _ASM_EXTABLE(1b, %l[fault]) :::: fault);
+fault:
 	cr4_clear_bits(X86_CR4_VMXE);
 }