[PATCH 2/7] x86/bugs: Add asm helpers for executing VERW

[Nikolay Borisov <nik.borisov@suse.com>]

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

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 VERW patching depend on a new feature
flag X86_FEATURE_CLEAR_CPU_BUF.

Reported-by: Alyssa Milburn <alyssa.milburn@intel.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-1-a6216d83edb7%40linux.intel.com
Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
---
 arch/x86/entry/Makefile              |  2 +-
 arch/x86/entry/entry.S               | 23 +++++++++++++++++++++++
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/nospec-branch.h | 14 ++++++++++++++
 4 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 arch/x86/entry/entry.S

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index 06fc70cf5433..b8da38e81e96 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -7,7 +7,7 @@ OBJECT_FILES_NON_STANDARD_entry_64_compat.o := y
 
 CFLAGS_syscall_64.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_32.o		+= $(call cc-option,-Wno-override-init,)
-obj-y				:= entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
+obj-y				:= entry.o entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
 obj-y				+= common.o
 
 obj-y				+= vdso/
diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
new file mode 100644
index 000000000000..21bf05354670
--- /dev/null
+++ b/arch/x86/entry/entry.S
@@ -0,0 +1,23 @@
+#include <linux/linkage.h>
+#include <asm/export.h>
+#include <asm/segment.h>
+#include <asm/cache.h>
+
+
+/*
+ * Define the VERW operand that is disguised as entry code so that
+ * it can be referenced with KPTI enabled. This ensure VERW can be
+ * used late in exit-to-user path after page tables are switched.
+ */
+.pushsection .entry.text, "ax"
+
+.align L1_CACHE_BYTES, 0xcc
+SYM_CODE_START_NOALIGN(mds_verw_sel)
+      .word __KERNEL_DS
+.align L1_CACHE_BYTES, 0xcc
+SYM_CODE_END(mds_verw_sel)
+/* For KVM */
+EXPORT_SYMBOL_GPL(mds_verw_sel);
+
+.popsection
+
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index f42286e9a2b1..6d024db8384c 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -96,7 +96,7 @@
 #define X86_FEATURE_SYSCALL32		( 3*32+14) /* "" syscall in IA32 userspace */
 #define X86_FEATURE_SYSENTER32		( 3*32+15) /* "" sysenter in IA32 userspace */
 #define X86_FEATURE_REP_GOOD		( 3*32+16) /* REP microcode works well */
-/* FREE!                                ( 3*32+17) */
+#define X86_FEATURE_CLEAR_CPU_BUF	( 3*32+17) /* "" Clear CPU buffers using VERW */
 #define X86_FEATURE_LFENCE_RDTSC	( 3*32+18) /* "" LFENCE synchronizes RDTSC */
 #define X86_FEATURE_ACC_POWER		( 3*32+19) /* AMD Accumulated Power Mechanism */
 #define X86_FEATURE_NOPL		( 3*32+20) /* The NOPL (0F 1F) instructions */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index c8819358a332..ba069ed16f94 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -175,6 +175,18 @@
 .Lskip_rsb_\@:
 .endm
 
+/*
+ * Macro 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.
+ */
+.macro CLEAR_CPU_BUFFERS
+	ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF
+	verw _ASM_RIP(mds_verw_sel)
+.Lskip_verw_\@:
+.endm
 #else /* __ASSEMBLY__ */
 
 #define ANNOTATE_RETPOLINE_SAFE					\
@@ -346,6 +358,8 @@ DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 
 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
 
+extern u16 mds_verw_sel;
+
 #include <asm/segment.h>
 
 /**
-- 
2.34.1