Re: [PATCH v8 06/28] x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b

[Borislav Petkov]

On Fri, Aug 18, 2017 at 05:27:47PM -0700, Ricardo Neri wrote:
> Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual volume 2A states that when ModRM.mod !=11b and
> ModRM.rm = 100b indexed register-indirect addressing is used. In other
> words, a SIB byte follows the ModRM byte. In the specific case of
> SIB.index = 100b, the scale*index portion of the computation of the
> effective address is null. To signal callers of this particular situation,
> get_reg_offset() can return -EDOM (-EINVAL continues to indicate that an
> error when decoding the SIB byte).
> 
> An example of this situation can be the following instruction:
> 
>    8b 4c 23 80       mov -0x80(%rbx,%riz,1),%rcx
>    ModRM:            0x4c [mod:1b][reg:1b][rm:100b]
>    SIB:              0x23 [scale:0b][index:100b][base:11b]
>    Displacement:     0x80  (1-byte, as per ModRM.mod = 1b)
> 
> The %riz 'register' indicates a null index.
> 
> In long mode, a REX prefix may be used. When a REX prefix is present,
> REX.X adds a fourth bit to the register selection of SIB.index. This gives
> the ability to refer to all the 16 general purpose registers. When REX.X is
> 1b and SIB.index is 100b, the index is indicated in %r12. In our example,
> this would look like:
> 
>    42 8b 4c 23 80    mov -0x80(%rbx,%r12,1),%rcx
>    REX:              0x42 [W:0b][R:0b][X:1b][B:0b]
>    ModRM:            0x4c [mod:1b][reg:1b][rm:100b]
>    SIB:              0x23 [scale:0b][.X: 1b, index:100b][.B:0b, base:11b]
>    Displacement:     0x80  (1-byte, as per ModRM.mod = 1b)
> 
> %r12 is a valid register to use in the scale*index part of the effective
> address computation.
> 
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Nathan Howard <liverlint@xxxxxxxxx>
> Cc: Adan Hawthorn <adanhawthorn@xxxxxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/mm/mpx.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)

Reviewed-by: Borislav Petkov <bp@xxxxxxx>

-- 
Regards/Gruss,
    Boris.

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