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

[Ricardo Neri <ricardo.neri-calderon@linux.intel.com>]

On Thu, 2017-08-31 at 21:38 +0200, Borislav Petkov wrote:
> 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@suse.de>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> > Cc: Colin Ian King <colin.king@canonical.com>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Nathan Howard <liverlint@gmail.com>
> > Cc: Adan Hawthorn <adanhawthorn@gmail.com>
> > Cc: Joe Perches <joe@perches.com>
> > Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> > Cc: x86@kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> >  arch/x86/mm/mpx.c | 22 ++++++++++++++++++++--
> >  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> Reviewed-by: Borislav Petkov <bp@suse.de>

Thanks for your review!