linux-kernel - [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <25d5c6de22cabf6a4ecb44ce077f33aa5f10b4d4.1583547371.git.luto@kernel.org>
Date:   Fri,  6 Mar 2020 18:16:27 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org
Subject: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

The ABI is broken and we cannot support it properly.  Turn it off.

If this causes a meaningful performance regression for someone, KVM
can introduce an improved ABI that is supportable.

Cc: stable@...r.kernel.org
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/kvm.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 93ab0cbd304e..71f9f39f93da 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -318,11 +318,16 @@ static void kvm_guest_cpu_init(void)
 
 		pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));
 
-#ifdef CONFIG_PREEMPTION
-		pa |= KVM_ASYNC_PF_SEND_ALWAYS;
-#endif
 		pa |= KVM_ASYNC_PF_ENABLED;
 
+		/*
+		 * We do not set KVM_ASYNC_PF_SEND_ALWAYS.  With the current
+		 * KVM paravirt ABI, if an async page fault occurs on an early
+		 * memory access in the normal (sync) #PF path or in an NMI
+		 * that happens early in the #PF code, the combination of CR2
+		 * and the APF reason field will be corrupted.
+		 */
+
 		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
 			pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
-- 
2.24.1