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

Pawan Gupta
2023-10-24T08:08:33Z

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@intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
arch/x86/include/asm/cpufeatures.h | 2 +-
arch/x86/include/asm/nospec-branch.h | 19 +++++++++++++++++++
2 files changed, 20 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..c269ee74682c 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -329,6 +329,25 @@
#endif
.endm

+/*
+ * Macro to execute VERW instruction to 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. To
+ * handle the case when VERW is executed after user registers are restored, use
+ * RIP to point the memory operand to a part NOPL instruction that contains
+ * __KERNEL_DS.
+ */
+.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
+
#else /* __ASSEMBLY__ */

#define ANNOTATE_RETPOLINE_SAFE \

--
2.34.1