[PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[Pawan Gupta]

This is the backport of recently upstreamed series that moves VERW
execution to a later point in exit-to-user path. This is needed because
in some cases it may be possible for data accessed after VERW executions
may end into MDS affected CPU buffers. Moving VERW closer to ring
transition reduces the attack surface.

Patch 1/6 includes a minor fix that is queued for upstream:
https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@tip-bot2/

Patch 1,2,5 and 6 needed conflict resolution.

I saw a few new warnings:

  arch/x86/entry/entry.o: warning: objtool: mds_verw_sel+0x0: unreachable instruction

I tried using REACHABLE, but that did not fix the warning.

For the below warning:

  vmlinux.o: warning: objtool: .altinstr_replacement+0x17: unsupported relocation in alternatives section

not sure if this is related to this series or a pre-existing warning, I
will check later without this series.

I am not too concerned because the alternative did substitute verw
correctly:

entry_SYSCALL_64:
...
   0xffffffff8200013d <+253>:   swapgs
   0xffffffff82000140 <+256>:   verw   0xffffffff82000000
   0xffffffff82000148 <+264>:   sysretq
   0xffffffff8200014b <+267>:   int3

---
Pawan Gupta (5):
      x86/bugs: Add asm helpers for executing VERW
      x86/entry_64: Add VERW just before userspace transition
      x86/entry_32: Add VERW just before userspace transition
      x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key
      KVM/VMX: Move VERW closer to VMentry for MDS mitigation

Sean Christopherson (1):
      KVM/VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. VMLAUNCH

 Documentation/x86/mds.rst            | 38 +++++++++++++++++++++++++-----------
 arch/x86/entry/entry.S               | 22 +++++++++++++++++++++
 arch/x86/entry/entry_32.S            |  3 +++
 arch/x86/entry/entry_64.S            | 11 +++++++++++
 arch/x86/entry/entry_64_compat.S     |  1 +
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/entry-common.h  |  1 -
 arch/x86/include/asm/nospec-branch.h | 25 ++++++++++++------------
 arch/x86/kernel/cpu/bugs.c           | 15 ++++++--------
 arch/x86/kernel/nmi.c                |  3 ---
 arch/x86/kvm/vmx/run_flags.h         |  7 +++++--
 arch/x86/kvm/vmx/vmenter.S           |  9 ++++++---
 arch/x86/kvm/vmx/vmx.c               | 12 ++++++++----
 13 files changed, 103 insertions(+), 46 deletions(-)
---
base-commit: 81e1dc2f70014b9523dd02ca763788e4f81e5bac
change-id: 20240226-delay-verw-backport-6-1-y-4b0cec84087c

[PATCH 6.1.y 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

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.

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               | 22 ++++++++++++++++++++++
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/nospec-branch.h | 13 +++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index bfb7bcb362bc..4f16ef37fdbb 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 +23,22 @@ 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)
+	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 b122708792c4..b60f24b30cb9 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -304,7 +304,7 @@
 #define X86_FEATURE_UNRET		(11*32+15) /* "" AMD BTB untrain return */
 #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_CLEAR_CPU_BUF	(11*32+18) /* "" Clear CPU buffers using VERW */
 
 #define X86_FEATURE_MSR_TSX_CTRL	(11*32+20) /* "" MSR IA32_TSX_CTRL (Intel) implemented */
 
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2f123d4fb85b..a582f100385e 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -194,6 +194,17 @@
 #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 "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
+.endm
+
 #else /* __ASSEMBLY__ */
 
 #define ANNOTATE_RETPOLINE_SAFE					\
@@ -373,6 +384,8 @@ DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
 
+extern u16 mds_verw_sel;
+
 #include <asm/segment.h>
 
 /**

-- 
2.34.1

Re: [PATCH 6.1.y 1/6] x86/bugs: Add asm helpers for executing VERW

[Jiri Slaby]

On 27. 02. 24, 9:00, Pawan Gupta wrote:
> commit baf8361e54550a48a7087b603313ad013cc13386 upstream.
...
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -194,6 +194,17 @@
>   #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 "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF

The same here. Except 6.1 (I didn't check stable-6.1) does not have:
commit 270a69c4485d7d07516d058bcc0473c90ee22185
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed Feb 8 18:10:52 2023 +0100

     x86/alternative: Support relocations in alternatives

yet. You likely need to backport that too.

regards,
-- 
js
suse labs

Re: [PATCH 6.1.y 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Tue, Feb 27, 2024 at 09:19:13AM +0100, Jiri Slaby wrote:
> On 27. 02. 24, 9:00, Pawan Gupta wrote:
> > commit baf8361e54550a48a7087b603313ad013cc13386 upstream.
> ...
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -194,6 +194,17 @@
> >   #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 "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
> 
> The same here. Except 6.1 (I didn't check stable-6.1) does not have:
> commit 270a69c4485d7d07516d058bcc0473c90ee22185
> Author: Peter Zijlstra <peterz@infradead.org>
> Date:   Wed Feb 8 18:10:52 2023 +0100
> 
>     x86/alternative: Support relocations in alternatives
> 
> yet. You likely need to backport that too.

I will backport this patch too if rip-relative turns out to be a must.
At this point, I am not sure if VERW buffer clearing behavior will be
impacted by fixed addressing.

[PATCH 6.1.y 2/6] x86/entry_64: Add VERW just before userspace transition

[Pawan Gupta]

commit 3c7501722e6b31a6e56edd23cea5e77dbb9ffd1a upstream.

Mitigation for MDS is to use VERW instruction to clear any secrets in
CPU Buffers. Any memory accesses after VERW execution can still remain
in CPU buffers. It is safer to execute VERW late in return to user path
to minimize the window in which kernel data can end up in CPU buffers.
There are not many kernel secrets to be had after SWITCH_TO_USER_CR3.

Add support for deploying VERW mitigation after user register state is
restored. This helps minimize the chances of kernel data ending up into
CPU buffers after executing VERW.

Note that the mitigation at the new location is not yet enabled.

  Corner case not handled
  =======================
  Interrupts returning to kernel don't clear CPUs buffers since the
  exit-to-user path is expected to do that anyways. But, there could be
  a case when an NMI is generated in kernel after the exit-to-user path
  has cleared the buffers. This case is not handled and NMI returning to
  kernel don't clear CPU buffers because:

  1. It is rare to get an NMI after VERW, but before returning to userspace.
  2. For an unprivileged user, there is no known way to make that NMI
     less rare or target it.
  3. It would take a large number of these precisely-timed NMIs to mount
     an actual attack.  There's presumably not enough bandwidth.
  4. The NMI in question occurs after a VERW, i.e. when user state is
     restored and most interesting data is already scrubbed. Whats left
     is only the data that NMI touches, and that may or may not be of
     any interest.

  [ pawan: resolved conflict for hunk swapgs_restore_regs_and_return_to_usermode in backport ]

Suggested-by: Dave Hansen <dave.hansen@intel.com>
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-v8-2-a6216d83edb7%40linux.intel.com
---
 arch/x86/entry/entry_64.S        | 11 +++++++++++
 arch/x86/entry/entry_64_compat.S |  1 +
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9953d966d124..c2383c2880ec 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -223,6 +223,7 @@ syscall_return_via_sysret:
 SYM_INNER_LABEL(entry_SYSRETQ_unsafe_stack, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
 	swapgs
+	CLEAR_CPU_BUFFERS
 	sysretq
 SYM_INNER_LABEL(entry_SYSRETQ_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
@@ -656,6 +657,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
 	/* Restore RDI. */
 	popq	%rdi
 	swapgs
+	CLEAR_CPU_BUFFERS
 	jmp	.Lnative_iret
 
 
@@ -767,6 +769,8 @@ native_irq_return_ldt:
 	 */
 	popq	%rax				/* Restore user RAX */
 
+	CLEAR_CPU_BUFFERS
+
 	/*
 	 * RSP now points to an ordinary IRET frame, except that the page
 	 * is read-only and RSP[31:16] are preloaded with the userspace
@@ -1493,6 +1497,12 @@ nmi_restore:
 	std
 	movq	$0, 5*8(%rsp)		/* clear "NMI executing" */
 
+	/*
+	 * Skip CLEAR_CPU_BUFFERS here, since it only helps in rare cases like
+	 * NMI in kernel after user state is restored. For an unprivileged user
+	 * these conditions are hard to meet.
+	 */
+
 	/*
 	 * iretq reads the "iret" frame and exits the NMI stack in a
 	 * single instruction.  We are returning to kernel mode, so this
@@ -1511,6 +1521,7 @@ SYM_CODE_START(ignore_sysret)
 	UNWIND_HINT_EMPTY
 	ENDBR
 	mov	$-ENOSYS, %eax
+	CLEAR_CPU_BUFFERS
 	sysretl
 SYM_CODE_END(ignore_sysret)
 #endif
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index d6c08d8986b1..4bcd009a232b 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -272,6 +272,7 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
 	xorl	%r9d, %r9d
 	xorl	%r10d, %r10d
 	swapgs
+	CLEAR_CPU_BUFFERS
 	sysretl
 SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR

-- 
2.34.1

[PATCH 6.1.y 3/6] x86/entry_32: Add VERW just before userspace transition

[Pawan Gupta]

commit a0e2dab44d22b913b4c228c8b52b2a104434b0b3 upstream.

As done for entry_64, add support for executing VERW late in exit to
user path for 32-bit mode.

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-v8-3-a6216d83edb7%40linux.intel.com
---
 arch/x86/entry/entry_32.S | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index e309e7156038..ee5def1060c8 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -912,6 +912,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
 	BUG_IF_WRONG_CR3 no_user_check=1
 	popfl
 	popl	%eax
+	CLEAR_CPU_BUFFERS
 
 	/*
 	 * Return back to the vDSO, which will pop ecx and edx.
@@ -981,6 +982,7 @@ restore_all_switch_stack:
 
 	/* Restore user state */
 	RESTORE_REGS pop=4			# skip orig_eax/error_code
+	CLEAR_CPU_BUFFERS
 .Lirq_return:
 	/*
 	 * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
@@ -1173,6 +1175,7 @@ SYM_CODE_START(asm_exc_nmi)
 
 	/* Not on SYSENTER stack. */
 	call	exc_nmi
+	CLEAR_CPU_BUFFERS
 	jmp	.Lnmi_return
 
 .Lnmi_from_sysenter_stack:

-- 
2.34.1

[PATCH 6.1.y 4/6] x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key

[Pawan Gupta]

commit 6613d82e617dd7eb8b0c40b2fe3acea655b1d611 upstream.

The VERW mitigation at exit-to-user is enabled via a static branch
mds_user_clear. This static branch is never toggled after boot, and can
be safely replaced with an ALTERNATIVE() which is convenient to use in
asm.

Switch to ALTERNATIVE() to use the VERW mitigation late in exit-to-user
path. Also remove the now redundant VERW in exc_nmi() and
arch_exit_to_user_mode().

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-v8-4-a6216d83edb7%40linux.intel.com
---
 Documentation/x86/mds.rst            | 38 +++++++++++++++++++++++++-----------
 arch/x86/include/asm/entry-common.h  |  1 -
 arch/x86/include/asm/nospec-branch.h | 12 ------------
 arch/x86/kernel/cpu/bugs.c           | 15 ++++++--------
 arch/x86/kernel/nmi.c                |  3 ---
 arch/x86/kvm/vmx/vmx.c               |  2 +-
 6 files changed, 34 insertions(+), 37 deletions(-)

diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 5d4330be200f..e801df0bb3a8 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -95,6 +95,9 @@ The kernel provides a function to invoke the buffer clearing:
 
     mds_clear_cpu_buffers()
 
+Also macro CLEAR_CPU_BUFFERS can be used in ASM late in exit-to-user path.
+Other than CFLAGS.ZF, this macro doesn't clobber any registers.
+
 The mitigation is invoked on kernel/userspace, hypervisor/guest and C-state
 (idle) transitions.
 
@@ -138,17 +141,30 @@ Mitigation points
 
    When transitioning from kernel to user space the CPU buffers are flushed
    on affected CPUs when the mitigation is not disabled on the kernel
-   command line. The migitation is enabled through the static key
-   mds_user_clear.
-
-   The mitigation is invoked in prepare_exit_to_usermode() which covers
-   all but one of the kernel to user space transitions.  The exception
-   is when we return from a Non Maskable Interrupt (NMI), which is
-   handled directly in do_nmi().
-
-   (The reason that NMI is special is that prepare_exit_to_usermode() can
-    enable IRQs.  In NMI context, NMIs are blocked, and we don't want to
-    enable IRQs with NMIs blocked.)
+   command line. The mitigation is enabled through the feature flag
+   X86_FEATURE_CLEAR_CPU_BUF.
+
+   The mitigation is invoked just before transitioning to userspace after
+   user registers are restored. This is done to minimize the window in
+   which kernel data could be accessed after VERW e.g. via an NMI after
+   VERW.
+
+   **Corner case not handled**
+   Interrupts returning to kernel don't clear CPUs buffers since the
+   exit-to-user path is expected to do that anyways. But, there could be
+   a case when an NMI is generated in kernel after the exit-to-user path
+   has cleared the buffers. This case is not handled and NMI returning to
+   kernel don't clear CPU buffers because:
+
+   1. It is rare to get an NMI after VERW, but before returning to userspace.
+   2. For an unprivileged user, there is no known way to make that NMI
+      less rare or target it.
+   3. It would take a large number of these precisely-timed NMIs to mount
+      an actual attack.  There's presumably not enough bandwidth.
+   4. The NMI in question occurs after a VERW, i.e. when user state is
+      restored and most interesting data is already scrubbed. Whats left
+      is only the data that NMI touches, and that may or may not be of
+      any interest.
 
 
 2. C-State transition
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 11203a9fe0a8..ffe72790ceaf 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -91,7 +91,6 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 
 static __always_inline void arch_exit_to_user_mode(void)
 {
-	mds_user_clear_cpu_buffers();
 	amd_clear_divider();
 }
 #define arch_exit_to_user_mode arch_exit_to_user_mode
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index a582f100385e..7f5001afd786 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -377,7 +377,6 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
-DECLARE_STATIC_KEY_FALSE(mds_user_clear);
 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
@@ -411,17 +410,6 @@ static __always_inline void mds_clear_cpu_buffers(void)
 	asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
 }
 
-/**
- * mds_user_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
- *
- * Clear CPU buffers if the corresponding static key is enabled
- */
-static __always_inline void mds_user_clear_cpu_buffers(void)
-{
-	if (static_branch_likely(&mds_user_clear))
-		mds_clear_cpu_buffers();
-}
-
 /**
  * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
  *
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 13dffc43ded0..d1895930e6eb 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -110,9 +110,6 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 /* Control unconditional IBPB in switch_mm() */
 DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
-/* Control MDS CPU buffer clear before returning to user space */
-DEFINE_STATIC_KEY_FALSE(mds_user_clear);
-EXPORT_SYMBOL_GPL(mds_user_clear);
 /* Control MDS CPU buffer clear before idling (halt, mwait) */
 DEFINE_STATIC_KEY_FALSE(mds_idle_clear);
 EXPORT_SYMBOL_GPL(mds_idle_clear);
@@ -251,7 +248,7 @@ static void __init mds_select_mitigation(void)
 		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
 			mds_mitigation = MDS_MITIGATION_VMWERV;
 
-		static_branch_enable(&mds_user_clear);
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 
 		if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
 		    (mds_nosmt || cpu_mitigations_auto_nosmt()))
@@ -355,7 +352,7 @@ static void __init taa_select_mitigation(void)
 	 * For guests that can't determine whether the correct microcode is
 	 * present on host, enable the mitigation for UCODE_NEEDED as well.
 	 */
-	static_branch_enable(&mds_user_clear);
+	setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 
 	if (taa_nosmt || cpu_mitigations_auto_nosmt())
 		cpu_smt_disable(false);
@@ -423,7 +420,7 @@ static void __init mmio_select_mitigation(void)
 	 */
 	if (boot_cpu_has_bug(X86_BUG_MDS) || (boot_cpu_has_bug(X86_BUG_TAA) &&
 					      boot_cpu_has(X86_FEATURE_RTM)))
-		static_branch_enable(&mds_user_clear);
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 	else
 		static_branch_enable(&mmio_stale_data_clear);
 
@@ -483,12 +480,12 @@ static void __init md_clear_update_mitigation(void)
 	if (cpu_mitigations_off())
 		return;
 
-	if (!static_key_enabled(&mds_user_clear))
+	if (!boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
 		goto out;
 
 	/*
-	 * mds_user_clear is now enabled. Update MDS, TAA and MMIO Stale Data
-	 * mitigation, if necessary.
+	 * X86_FEATURE_CLEAR_CPU_BUF is now enabled. Update MDS, TAA and MMIO
+	 * Stale Data mitigation, if necessary.
 	 */
 	if (mds_mitigation == MDS_MITIGATION_OFF &&
 	    boot_cpu_has_bug(X86_BUG_MDS)) {
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index cec0bfa3bc04..ed6cce6c3950 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -522,9 +522,6 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 		write_cr2(this_cpu_read(nmi_cr2));
 	if (this_cpu_dec_return(nmi_state))
 		goto nmi_restart;
-
-	if (user_mode(regs))
-		mds_user_clear_cpu_buffers();
 }
 
 #if defined(CONFIG_X86_64) && IS_ENABLED(CONFIG_KVM_INTEL)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 57c1374fdfd4..3b76f1bf001e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7123,7 +7123,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	/* L1D Flush includes CPU buffer clear to mitigate MDS */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (static_branch_unlikely(&mds_user_clear))
+	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
 		mds_clear_cpu_buffers();
 	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
 		 kvm_arch_has_assigned_device(vcpu->kvm))

-- 
2.34.1

[PATCH 6.1.y 5/6] KVM/VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. VMLAUNCH

[Pawan Gupta]

From: Sean Christopherson <seanjc@google.com>

commit 706a189dcf74d3b3f955e9384785e726ed6c7c80 upstream.

Use EFLAGS.CF instead of EFLAGS.ZF to track whether to use VMRESUME versus
VMLAUNCH.  Freeing up EFLAGS.ZF will allow doing VERW, which clobbers ZF,
for MDS mitigations as late as possible without needing to duplicate VERW
for both paths.

  [ pawan: resolved merge conflict in __vmx_vcpu_run in backport. ]

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-5-a6216d83edb7%40linux.intel.com
---
 arch/x86/kvm/vmx/run_flags.h | 7 +++++--
 arch/x86/kvm/vmx/vmenter.S   | 6 +++---
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/run_flags.h b/arch/x86/kvm/vmx/run_flags.h
index edc3f16cc189..6a9bfdfbb6e5 100644
--- a/arch/x86/kvm/vmx/run_flags.h
+++ b/arch/x86/kvm/vmx/run_flags.h
@@ -2,7 +2,10 @@
 #ifndef __KVM_X86_VMX_RUN_FLAGS_H
 #define __KVM_X86_VMX_RUN_FLAGS_H
 
-#define VMX_RUN_VMRESUME	(1 << 0)
-#define VMX_RUN_SAVE_SPEC_CTRL	(1 << 1)
+#define VMX_RUN_VMRESUME_SHIFT		0
+#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT	1
+
+#define VMX_RUN_VMRESUME		BIT(VMX_RUN_VMRESUME_SHIFT)
+#define VMX_RUN_SAVE_SPEC_CTRL		BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT)
 
 #endif /* __KVM_X86_VMX_RUN_FLAGS_H */
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 0b5db4de4d09..42c0b2c3aee1 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -106,7 +106,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	mov (%_ASM_SP), %_ASM_AX
 
 	/* Check if vmlaunch or vmresume is needed */
-	testb $VMX_RUN_VMRESUME, %bl
+	bt   $VMX_RUN_VMRESUME_SHIFT, %bx
 
 	/* Load guest registers.  Don't clobber flags. */
 	mov VCPU_RCX(%_ASM_AX), %_ASM_CX
@@ -128,8 +128,8 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
-	/* Check EFLAGS.ZF from 'testb' above */
-	jz .Lvmlaunch
+	/* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
+	jnc .Lvmlaunch
 
 	/*
 	 * After a successful VMRESUME/VMLAUNCH, control flow "magically"

-- 
2.34.1

[PATCH 6.1.y 6/6] KVM/VMX: Move VERW closer to VMentry for MDS mitigation

[Pawan Gupta]

commit 43fb862de8f628c5db5e96831c915b9aebf62d33 upstream.

During VMentry VERW is executed to mitigate MDS. After VERW, any memory
access like register push onto stack may put host data in MDS affected
CPU buffers. A guest can then use MDS to sample host data.

Although likelihood of secrets surviving in registers at current VERW
callsite is less, but it can't be ruled out. Harden the MDS mitigation
by moving the VERW mitigation late in VMentry path.

Note that VERW for MMIO Stale Data mitigation is unchanged because of
the complexity of per-guest conditional VERW which is not easy to handle
that late in asm with no GPRs available. If the CPU is also affected by
MDS, VERW is unconditionally executed late in asm regardless of guest
having MMIO access.

  [ pawan: conflict resolved in backport ]

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-6-a6216d83edb7%40linux.intel.com
---
 arch/x86/kvm/vmx/vmenter.S |  3 +++
 arch/x86/kvm/vmx/vmx.c     | 12 ++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 42c0b2c3aee1..0b2cad66dee1 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -128,6 +128,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Load guest RAX.  This kills the @regs pointer! */
 	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
+	/* Clobbers EFLAGS.ZF */
+	CLEAR_CPU_BUFFERS
+
 	/* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
 	jnc .Lvmlaunch
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3b76f1bf001e..5c1590855ffc 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -407,7 +407,8 @@ static __always_inline void vmx_enable_fb_clear(struct vcpu_vmx *vmx)
 
 static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
 {
-	vmx->disable_fb_clear = vmx_fb_clear_ctrl_available;
+	vmx->disable_fb_clear = !cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF) &&
+		vmx_fb_clear_ctrl_available;
 
 	/*
 	 * If guest will not execute VERW, there is no need to set FB_CLEAR_DIS
@@ -7120,11 +7121,14 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 {
 	guest_state_enter_irqoff();
 
-	/* L1D Flush includes CPU buffer clear to mitigate MDS */
+	/*
+	 * L1D Flush includes CPU buffer clear to mitigate MDS, but VERW
+	 * mitigation for MDS is done late in VMentry and is still
+	 * executed in spite of L1D Flush. This is because an extra VERW
+	 * should not matter much after the big hammer L1D Flush.
+	 */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
-	else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
-		mds_clear_cpu_buffers();
 	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
 		 kvm_arch_has_assigned_device(vcpu->kvm))
 		mds_clear_cpu_buffers();

-- 
2.34.1

Re: [PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[Jiri Slaby]

On 27. 02. 24, 9:00, Pawan Gupta wrote:
> This is the backport of recently upstreamed series that moves VERW
> execution to a later point in exit-to-user path. This is needed because
> in some cases it may be possible for data accessed after VERW executions
> may end into MDS affected CPU buffers. Moving VERW closer to ring
> transition reduces the attack surface.
> 
> Patch 1/6 includes a minor fix that is queued for upstream:
> https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@tip-bot2/

Ah, you note it here.

But you should include this patch on its own instead of merging it to 
1/6. You might need to wait until it is in linus' tree, though.

regards,
-- 
js
suse labs

Re: [PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[Pawan Gupta]

On Tue, Feb 27, 2024 at 09:21:33AM +0100, Jiri Slaby wrote:
> On 27. 02. 24, 9:00, Pawan Gupta wrote:
> > This is the backport of recently upstreamed series that moves VERW
> > execution to a later point in exit-to-user path. This is needed because
> > in some cases it may be possible for data accessed after VERW executions
> > may end into MDS affected CPU buffers. Moving VERW closer to ring
> > transition reduces the attack surface.
> > 
> > Patch 1/6 includes a minor fix that is queued for upstream:
> > https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@tip-bot2/
> 
> Ah, you note it here.
> 
> But you should include this patch on its own instead of merging it to 1/6.

Thats exactly what I would have done ideally, but the backports to
stable kernels < 6.6 wont work without this patch. And this patch is
going to take some time before it can be merged upstream.

> You might need to wait until it is in linus' tree, though.

Ok will wait.

Re: [PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[Greg KH]

On Tue, Feb 27, 2024 at 12:56:38AM -0800, Pawan Gupta wrote:
> On Tue, Feb 27, 2024 at 09:21:33AM +0100, Jiri Slaby wrote:
> > On 27. 02. 24, 9:00, Pawan Gupta wrote:
> > > This is the backport of recently upstreamed series that moves VERW
> > > execution to a later point in exit-to-user path. This is needed because
> > > in some cases it may be possible for data accessed after VERW executions
> > > may end into MDS affected CPU buffers. Moving VERW closer to ring
> > > transition reduces the attack surface.
> > > 
> > > Patch 1/6 includes a minor fix that is queued for upstream:
> > > https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@tip-bot2/
> > 
> > Ah, you note it here.
> > 
> > But you should include this patch on its own instead of merging it to 1/6.
> 
> Thats exactly what I would have done ideally, but the backports to
> stable kernels < 6.6 wont work without this patch. And this patch is
> going to take some time before it can be merged upstream.

It's in the tip-urgent branch, doesn't that mean it will go to Linus
this week?  If it's in linux-next, and you _KNOW_ it will go to Linus
this week, and the git id is stable, then we can consider the
application of it to the stable tree now.

But if it's not working for anyone without this change (i.e. in Linus's
branch), why would you want these changes in a stable tree?

thanks,

greg k-h

Re: [PATCH 6.1.y 0/6] Delay VERW - 6.1.y backport

[Pawan Gupta]

On Tue, Feb 27, 2024 at 10:09:06AM +0100, Greg KH wrote:
> On Tue, Feb 27, 2024 at 12:56:38AM -0800, Pawan Gupta wrote:
> > On Tue, Feb 27, 2024 at 09:21:33AM +0100, Jiri Slaby wrote:
> > > On 27. 02. 24, 9:00, Pawan Gupta wrote:
> > > > This is the backport of recently upstreamed series that moves VERW
> > > > execution to a later point in exit-to-user path. This is needed because
> > > > in some cases it may be possible for data accessed after VERW executions
> > > > may end into MDS affected CPU buffers. Moving VERW closer to ring
> > > > transition reduces the attack surface.
> > > > 
> > > > Patch 1/6 includes a minor fix that is queued for upstream:
> > > > https://lore.kernel.org/lkml/170899674562.398.6398007479766564897.tip-bot2@tip-bot2/
> > > 
> > > Ah, you note it here.
> > > 
> > > But you should include this patch on its own instead of merging it to 1/6.
> > 
> > Thats exactly what I would have done ideally, but the backports to
> > stable kernels < 6.6 wont work without this patch. And this patch is
> > going to take some time before it can be merged upstream.
> 
> It's in the tip-urgent branch, doesn't that mean it will go to Linus
> this week?  If it's in linux-next, and you _KNOW_ it will go to Linus
> this week, and the git id is stable, then we can consider the
> application of it to the stable tree now.

If we use the approach in Nikolay's backport, we can avoid dependency on
any other patch. Basically alternative patching jmp instead of VERW:

+.macro CLEAR_CPU_BUFFERS
+       ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF
+       verw _ASM_RIP(mds_verw_sel)
+.Lskip_verw_\@:
+.endm

https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@suse.com/

I will send the backports with this approach.