Re: [PATCH v2 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Tue, Oct 24, 2023 at 12:36:01PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 24, 2023 at 01:08:21AM -0700, Pawan Gupta wrote:
> 
> > +.macro CLEAR_CPU_BUFFERS
> > +	ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@", X86_FEATURE_CLEAR_CPU_BUF
> > +		/* nopl __KERNEL_DS(%rax) */
> > +		.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;
> > +.Lverw_arg_\@:	.word __KERNEL_DS;
> > +.Ldo_verw_\@:	verw _ASM_RIP(.Lverw_arg_\@);
> > +.Lskip_verw_\@:
> > +.endm
> 
> Why can't this be:
> 
> 	ALTERNATIVE "". "verw _ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF
> 
> And have that mds_verw_sel thing be out-of-line ? That gives much better
> code for the case where we don't need this.

Overall the code generated with this approach is much better. But, in my
testing I am seeing an issue with runtime patching in 32-bit mode, when
mitigations are off. Instead of NOPs I am seeing random instruction. I
don't see any issue with 64-bit mode.

config1: mitigations=on, 32-bit mode, post-boot

entry_SYSENTER_32:
   ...
   0xc1a3748e <+222>:   pop    %eax
   0xc1a3748f <+223>:   verw   0xc1a38240
   0xc1a37496 <+230>:   sti
   0xc1a37497 <+231>:   sysexit

---------------------------------------------

config2: mitigations=off, 32-bit mode, post-boot

entry_SYSENTER_32:
   ...
   0xc1a3748e <+222>:   pop    %eax
   0xc1a3748f <+223>:   lea    0x0(%esi,%eiz,1),%esi   <---- Doesn't look right
   0xc1a37496 <+230>:   sti
   0xc1a37497 <+231>:   sysexit

---------------------------------------------

config3: 32-bit mode, pre-boot objdump

entry_SYSENTER_32:
   ...
   c8e:       58                      pop    %eax
   c8f:       90                      nop
   c90:       90                      nop
   c91:       90                      nop
   c92:       90                      nop
   c93:       90                      nop
   c94:       90                      nop
   c95:       90                      nop
   c96:       fb                      sti
   c97:       0f 35                   sysexit

These tests were done with below patch:

-----8<-----
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Date: Mon, 23 Oct 2023 15:04:56 -0700
Subject: [PATCH] x86/bugs: Add asm helpers for executing VERW

MDS mitigation requires clearing the CPU buffers before returning to
user. This needs to be done late in the exit-to-user path. Current
location of VERW leaves a possibility of kernel data ending up in CPU
buffers for memory accesses done after VERW such as:

  1. Kernel data accessed by an NMI between VERW and return-to-user can
     remain in CPU buffers ( since NMI returning to kernel does not
     execute VERW to clear CPU buffers.
  2. Alyssa reported that after VERW is executed,
     CONFIG_GCC_PLUGIN_STACKLEAK=y scrubs the stack used by a system
     call. Memory accesses during stack scrubbing can move kernel stack
     contents into CPU buffers.
  3. When caller saved registers are restored after a return from
     function executing VERW, the kernel stack accesses can remain in
     CPU buffers(since they occur after VERW).

To fix this VERW needs to be moved very late in exit-to-user path.

In preparation for moving VERW to entry/exit asm code, create macros
that can be used in asm. Also make them depend on a new feature flag
X86_FEATURE_CLEAR_CPU_BUF.

Reported-by: Alyssa Milburn <alyssa.milburn@xxxxxxxxx>
Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/nospec-branch.h | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 58cb9495e40f..f21fc0f12737 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -308,10 +308,10 @@
 #define X86_FEATURE_SMBA		(11*32+21) /* "" Slow Memory Bandwidth Allocation */
 #define X86_FEATURE_BMEC		(11*32+22) /* "" Bandwidth Monitoring Event Configuration */
 #define X86_FEATURE_USER_SHSTK		(11*32+23) /* Shadow stack support for user mode applications */
-
 #define X86_FEATURE_SRSO		(11*32+24) /* "" AMD BTB untrain RETs */
 #define X86_FEATURE_SRSO_ALIAS		(11*32+25) /* "" AMD BTB untrain RETs through aliasing */
 #define X86_FEATURE_IBPB_ON_VMEXIT	(11*32+26) /* "" Issue an IBPB only on VMEXIT */
+#define X86_FEATURE_CLEAR_CPU_BUF	(11*32+27) /* "" Clear CPU buffers */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index c55cc243592e..ed8218e2d9a7 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -13,6 +13,7 @@
 #include <asm/unwind_hints.h>
 #include <asm/percpu.h>
 #include <asm/current.h>
+#include <asm/segment.h>
 
 /*
  * Call depth tracking for Intel SKL CPUs to address the RSB underflow
@@ -329,6 +330,29 @@
 #endif
 .endm
 
+/*
+ * Macros to execute VERW instruction that mitigate transient data sampling
+ * attacks such as MDS. On affected systems a microcode update overloaded VERW
+ * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
+ *
+ * Note: Only the memory operand variant of VERW clears the CPU buffers.
+ */
+.pushsection .rodata
+.align 64
+mds_verw_sel:
+	.word __KERNEL_DS
+ 	.byte 0xcc
+.align 64
+.popsection
+
+.macro EXEC_VERW
+	verw _ASM_RIP(mds_verw_sel)
+.endm
+
+.macro CLEAR_CPU_BUFFERS
+	ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
+.endm
+
 #else /* __ASSEMBLY__ */
 
 #define ANNOTATE_RETPOLINE_SAFE					\
-- 
2.34.1