Messages in this thread Patch in this message | | | Date | Thu, 29 Dec 2011 02:59:15 +0100 | From | Stephan Bärwolf <> | Subject | KVM guest-kernel panics double fault |
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Hello guys,
I am sorry to disturb you this short before New Year, but I think this shouldn't wait until next year.
After experiencing crashes in virtual maschines and considering kernel / qemu / kvm / cpu -bugs, I discovered the following (see patch) issue. Because unpriviledged users can crash VMs, I think it is a serious one and needs short-term attention.
The patch I wrote is against 3.2-rc7 but I always tested with linux 3.1.6. Hopfully it solve the problems to your satisfaction.
regards and a happy new year, Stephan Bärwolf
Subject: [PATCH] KVM: fix missing "illegal instruction"-trap in guests within non-64bit protected modes
On hosts without this patch, 32bit guests will crash for example by simply executing following nasm-demo-application:
[bits 32] global _start SECTION .text _start: syscall
(I am not sure if this can be exploited in more worse ways, like breaking out of VMs in more complex szenarios? But I tested it with win32 and linux - both always crashed)
Disassembly of section .text:
00000000 <_start>: 0: 0f 05 syscall
The reason seems a missing "invalid opcode"-trap (int6) for the syscall opcode "0f05", which is not available on 32bit cpus. Intel's "Intel 64 and IA-32 Architecture Software Developers Manual" (http://www.intel.com/content/dam/doc/manual/ 64-ia-32-architectures-software-developer-manual-325462.pdf) documents on page 1804 (4-586) "syscall" is only available in 64bit longmode. So "syscall" must trap in real- and virtual 8086 -mode, as also in all non-64bit protected-modes.
The last ones (16 & 32bit protected mode) are not beeing checked by kvm and so causing a missing trap as an double-fault-panic on 32bit guests.
Also an initially not observed problem can be explained with this bug: On 64bit guests (x86_64) 32bit compat-programs are able to syscall their kernel via "0f05" correctly, althought native (not virtualized) systems would also trap!
This patch solves the described problem by extending the checking of cpu's operational mode.
Screenshots of a i686 testing VM before and after applying this patch are available under:
http://matrixstorm.com/software/linux/kvm/20111229/before.jpg http://matrixstorm.com/software/linux/kvm/20111229/after.jpg
From 4de09b4bdba4927b8e248daa1bbfacaf3752fb6e Mon Sep 17 00:00:00 2001 From: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> Date: Thu, 29 Dec 2011 00:50:46 +0000 Subject: [PATCH] KVM: fix missing "illegal instruction"-trap in guests within non-64bit protected modes
On hosts without this patch, 32bit guests will crash for example by simply executing following nasm-demo-application:
[bits 32] global _start SECTION .text _start: syscall
(I am not sure if this can be exploited in more worse ways, like breaking out of VMs in more complex szenarios? But I tested it with win32 and linux - both always crashed)
Disassembly of section .text:
00000000 <_start>: 0: 0f 05 syscall
The reason seems a missing "invalid opcode"-trap (int6) for the syscall opcode "0f05", which is not available on 32bit cpus. Intel's "Intel 64 and IA-32 Architecture Software Developers Manual" (http://www.intel.com/content/dam/doc/manual/ 64-ia-32-architectures-software-developer-manual-325462.pdf) documents on page 1804 (4-586) "syscall" is only available in 64bit longmode. So "syscall" must trap in real- and virtual 8086 -mode, as also in all non-64bit protected-modes.
The last ones (16 & 32bit protected mode) are not beeing checked by kvm and so causing a missing trap as an double-fault-panic on 32bit guests.
Also an initially not observed problem can be explained with this bug: On 64bit guests (x86_64) 32bit compat-programs are able to syscall their kernel via "0f05" correctly, althought native (not virtualized) systems would also trap!
This patch solves the described problem by extending the checking of cpu's operational mode.
Screenshots of a i686 testing VM before and after applying this patch are available under:
http://matrixstorm.com/software/linux/kvm/20111229/before.jpg http://matrixstorm.com/software/linux/kvm/20111229/after.jpg
Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> --- arch/x86/kvm/emulate.c | 8 ++++++++ 1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index f1e3be1..60f6ffc 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -1886,7 +1886,15 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt) u64 efer = 0; /* syscall is not available in real mode */ + /* + "0f05" is also not available in + all non-64-bit protected modes (16& + 32bit) or virtual 8086 mode + Only 64bit longmode supports this opcode + */ if (ctxt->mode == X86EMUL_MODE_REAL || + ctxt->mode == X86EMUL_MODE_PROT16 || + ctxt->mode == X86EMUL_MODE_PROT32 || ctxt->mode == X86EMUL_MODE_VM86) return emulate_ud(ctxt); -- 1.7.3.4
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