Re: [PATCH v6 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Tue, Jan 23, 2024 at 11:41:01PM -0800, Pawan Gupta wrote:
> 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@xxxxxxxxx>
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/entry/entry.S               | 22 ++++++++++++++++++++++
>  arch/x86/include/asm/cpufeatures.h   |  2 +-
>  arch/x86/include/asm/nospec-branch.h | 15 +++++++++++++++
>  3 files changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index 8c8d38f0cb1d..bd8e77c5a375 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -6,6 +6,9 @@
>  #include <linux/export.h>
>  #include <linux/linkage.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
> +
> +/*
> + * Defines the VERW operand that is disguised as entry code so that
> + * it can be referenced with KPTI enabled. This ensures 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_UNDEFINED
> +	ANNOTATE_NOENDBR
> +	.word __KERNEL_DS
> +.align L1_CACHE_BYTES, 0xcc
> +SYM_CODE_END(mds_verw_sel);
> +/* For KVM */
> +EXPORT_SYMBOL_GPL(mds_verw_sel);

I realized this needs an extern:

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 0a8fa023a804..9daf92071f77 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -550,6 +550,8 @@ 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>
 
 /**