Re: [Xen-devel] [PATCH] x86_emulate: Always truncate %eip out of long mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

Ping?

>>> On 15.12.15 at 09:53, <JBeulich@xxxxxxxx> wrote:
>>>> On 10.12.15 at 21:03, <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -570,8 +570,10 @@ do{ asm volatile (                                      
>>  
> 
>>                \
>>  /* Fetch next part of the instruction being emulated. */
>>  #define insn_fetch_bytes(_size)                                         \
>>  ({ unsigned long _x = 0, _eip = _regs.eip;                              \
>> -   if ( !mode_64bit() ) _eip = (uint32_t)_eip; /* ignore upper dword */ \
>> -   _regs.eip += (_size); /* real hardware doesn't truncate */           \
>> +   _regs.eip += (_size);                                                \
>> +   if ( !mode_64bit() ) { /* Truncate eip to def_ad_bytes (2 or 4). */  \
>> +       _eip      &= ~((1UL << (def_ad_bytes * 8)) - 1);                 \
>> +       _regs.eip &= ~((1UL << (def_ad_bytes * 8)) - 1); };              \
> 
> Did you test this? The ~ seems quite wrong, and this would also
> cause undefined behavior in 32-bit compilation of the emulator
> test... Anyway, how about the following replacement patch,
> along the lines of what I think I had described in reply to the first
> variant of the patch?
> 
> Jan
> 
> x86emul: fix rIP handling
> 
> Deal with rIP just like with any other register: Truncate to designated
> width upon entry, and write back the zero-extended 32-bit value when
> emulating 32-bit code, and leave the upper 48 bits unchanged for 16-bit
> code.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -570,7 +570,6 @@ do{ asm volatile (
>  /* Fetch next part of the instruction being emulated. */
>  #define insn_fetch_bytes(_size)                                         \
>  ({ unsigned long _x = 0, _eip = _regs.eip;                              \
> -   if ( !mode_64bit() ) _eip = (uint32_t)_eip; /* ignore upper dword */ \
>     _regs.eip += (_size); /* real hardware doesn't truncate */           \
>     generate_exception_if((uint8_t)(_regs.eip -                          \
>                                     ctxt->regs->eip) > MAX_INST_LEN,     \
> @@ -1481,6 +1480,10 @@ x86_emulate(
>  #endif
>      }
>  
> +    /* Truncate rIP to def_ad_bytes (2 or 4) if necessary. */
> +    if ( def_ad_bytes < sizeof(_regs.eip) )
> +        _regs.eip &= (1UL << (def_ad_bytes * 8)) - 1;
> +
>      /* Prefix bytes. */
>      for ( ; ; )
>      {
> @@ -3793,6 +3796,21 @@ x86_emulate(
>  
>      /* Commit shadow register state. */
>      _regs.eflags &= ~EFLG_RF;
> +    switch ( __builtin_expect(def_ad_bytes, sizeof(_regs.eip)) )
> +    {
> +        uint16_t ip;
> +
> +    case 2:
> +        ip = _regs.eip;
> +        _regs.eip = ctxt->regs->eip;
> +        *(uint16_t *)&_regs.eip = ip;
> +        break;
> +#ifdef __x86_64__
> +    case 4:
> +        _regs.rip = _regs._eip;
> +        break;
> +#endif
> +    }
>      *ctxt->regs = _regs;
>  
>   done:




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