[MODERATED] Re: [patch V6 07/14] MDS basics 7

On Fri, Mar 01, 2019 at 10:47:45PM +0100, speck for Thomas Gleixner wrote:
> Subject: [patch V6 07/14] x86/speculation/mds: Clear CPU buffers on exit to user
> From: Thomas Gleixner <[email protected]>
> 
> Add a static key which controls the invocation of the CPU buffer clear
> mechanism on exit to user space and add the call into
> prepare_exit_to_usermode() and do_nmi() right before actually returning.
> 
> Add documentation which kernel to user space transition this covers and
> explain why some corner cases are not mitigated.
> 
> Signed-off-by: Thomas Gleixner <[email protected]>
> Reviewed-by: Greg Kroah-Hartman <[email protected]>
> 
> ---
> V4 --> v5: Use an inline helper instead of open coding it.
> 	   Rework the documentation paragraph about exceptions.
> 
> V3 --> V4: Add #DS mitigation and document that the #MC corner case
>        	   is really not interesting.
> 
> V3: Add NMI conditional on user regs and update documentation accordingly.
>     Use the static branch scheme suggested by Peter. Fix typos ...
> ---
>  Documentation/x86/mds.rst            |   52 +++++++++++++++++++++++++++++++++++
>  arch/x86/entry/common.c              |    3 ++
>  arch/x86/include/asm/nospec-branch.h |   13 ++++++++
>  arch/x86/kernel/cpu/bugs.c           |    3 ++
>  arch/x86/kernel/nmi.c                |    4 ++
>  arch/x86/kernel/traps.c              |    7 ++++
>  6 files changed, 82 insertions(+)

...

> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -366,6 +366,13 @@ dotraplinkage void do_double_fault(struc
>  		regs->ip = (unsigned long)general_protection;
>  		regs->sp = (unsigned long)&gpregs->orig_ax;
>  
> +		/*
> +		 * This situation can be triggered by userspace via
> +		 * modify_ldt(2) and the return does not take the regular
> +		 * user space exit, so a CPU buffer clear is required when
> +		 * MDS mitigation is enabled.
> +		 */
> +		mds_user_clear_cpu_buffers();
>  		return;
>  	}
>  #endif

Looks like the traps.c change is missing a hunk, see below. Otherwise:

arch/x86/kernel/traps.c: In function ‘do_double_fault’:
arch/x86/kernel/traps.c:375:3: error: implicit declaration of function ‘mds_user_clear_cpu_buffers’ [-Werror=implicit-function-declaration]
   mds_user_clear_cpu_buffers();
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:276: arch/x86/kernel/traps.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [scripts/Makefile.build:492: arch/x86/kernel] Error 2
make: *** [Makefile:1043: arch/x86] Error 2
make: *** Waiting for unfinished jobs....

---
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 5942060dba9a..ce33f7f672d6 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -61,6 +61,7 @@
 #include <asm/mpx.h>
 #include <asm/vm86.h>
 #include <asm/umip.h>
+#include <asm/nospec-branch.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
---

with that

Reviewed-by: Borislav Petkov <[email protected]>

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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