[PATCH 5.10.y v2 02/11] x86/bugs: Add asm helpers for executing VERW

Pawan Gupta
2024-03-12T22:40:34Z

commit baf8361e54550a48a7087b603313ad013cc13386 upstream.

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.

[pawan: - Runtime patch jmp instead of verw in macro CLEAR_CPU_BUFFERS
due to lack of relative addressing support for relocations
in kernels < v6.5.
- Add UNWIND_HINT_EMPTY to avoid warning:
arch/x86/entry/entry.o: warning: objtool: mds_verw_sel+0x0: unreachable instruction]

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
---
arch/x86/entry/entry.S | 23 +++++++++++++++++++++++
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/nospec-branch.h | 15 +++++++++++++++
3 files changed, 39 insertions(+)

diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index bfb7bcb362bc..09e99d13fc0b 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -6,6 +6,9 @@
#include <linux/linkage.h>
#include <asm/export.h>
#include <asm/msr-index.h>
+#include <asm/unwind_hints.h>
+#include <asm/segment.h>
+#include <asm/cache.h>

.pushsection .noinstr.text, "ax"

@@ -20,3 +23,23 @@ SYM_FUNC_END(entry_ibpb)
EXPORT_SYMBOL_GPL(entry_ibpb);

.popsection
+
+/*
+ * 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)
+ UNWIND_HINT_EMPTY
+ ANNOTATE_NOENDBR
+ .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 5a54c3685a06..90079ab77fb5 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -300,6 +300,7 @@
#define X86_FEATURE_USE_IBPB_FW (11*32+16) /* "" Use IBPB during runtime firmware calls */
#define X86_FEATURE_RSB_VMEXIT_LITE (11*32+17) /* "" Fill RSB on VM exit when EIBRS is enabled */
#define X86_FEATURE_MSR_TSX_CTRL (11*32+18) /* "" MSR IA32_TSX_CTRL (Intel) implemented */
+#define X86_FEATURE_CLEAR_CPU_BUF (11*32+19) /* "" Clear CPU buffers using VERW */

#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 */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7711ba5342a1..abbddf7c3290 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -182,6 +182,19 @@
#endif
.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 \
@@ -362,6 +375,8 @@ 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