Re: [Xen-devel] [PATCH v3 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> This demotes an OOPS and likely panic due to a failed non-"safe" MSR
> access to a WARN and, for RDMSR, a return value of zero.  If
> panic_on_oops is set, then failed unsafe MSR accesses will still
> oops and panic.
> 
> To be clear, this type of failure should *not* happen.  This patch
> exists to minimize the chance of nasty undebuggable failures due on
> systems that used to work due to a now-fixed CONFIG_PARAVIRT=y bug.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/msr.h | 10 ++++++++--
>  arch/x86/mm/extable.c      | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 41 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 93fb7c1cffda..1487054a1a70 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -92,7 +92,10 @@ static inline unsigned long long native_read_msr(unsigned 
> int msr)
>  {
>       DECLARE_ARGS(val, low, high);
>  
> -     asm volatile("rdmsr" : EAX_EDX_RET(val, low, high) : "c" (msr));
> +     asm volatile("1: rdmsr\n"
> +                  "2:\n"
> +                  _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_rdmsr_unsafe)
> +                  : EAX_EDX_RET(val, low, high) : "c" (msr));
>       if (msr_tracepoint_active(__tracepoint_read_msr))
>               do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), 0);
>       return EAX_EDX_VAL(val, low, high);
> @@ -119,7 +122,10 @@ static inline unsigned long long 
> native_read_msr_safe(unsigned int msr,
>  static inline void native_write_msr(unsigned int msr,
>                                   unsigned low, unsigned high)
>  {
> -     asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
> +     asm volatile("1: wrmsr\n"
> +                  "2:\n"
> +                  _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
> +                  : : "c" (msr), "a"(low), "d" (high) : "memory");
>       if (msr_tracepoint_active(__tracepoint_read_msr))
>               do_trace_write_msr(msr, ((u64)high << 32 | low), 0);
>  }
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 9dd7e4b7fcde..f310714e6e6d 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -49,6 +49,39 @@ bool ex_handler_ext(const struct exception_table_entry 
> *fixup,
>  }
>  EXPORT_SYMBOL(ex_handler_ext);
>  
> +bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
> +                          struct pt_regs *regs, int trapnr)
> +{
> +     WARN(1, "unsafe MSR access error: RDMSR from 0x%x",
> +          (unsigned int)regs->cx);

Btw., instead of the safe/unsafe naming (which has an emotional and security 
secondary attribute), shouldn't we move this over to a _check() (or 
_checking()) 
naming instead that we do in other places in the kernel?

I.e.:

        rdmsr(msr, l, h);

and:

        if (rdmsr_check(msr, l, h)) {
                ...
        }

and then we could name the helpers as _check() and _nocheck() - which is 
neutral 
naming.

Thanks,

        Ingo

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