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 <[email protected]> > Cc: Andy Lutomirski <[email protected]> > Cc: Dave Hansen <[email protected]> > Cc: Adam Buchbinder <[email protected]> > Cc: Colin Ian King <[email protected]> > Cc: Lorenzo Stoakes <[email protected]> > Cc: Qiaowei Ren <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Nathan Howard <[email protected]> > Cc: Adan Hawthorn <[email protected]> > Cc: Joe Perches <[email protected]> > Cc: Ravi V. Shankar <[email protected]> > Cc: [email protected] > Signed-off-by: Ricardo Neri <[email protected]> > --- > arch/x86/mm/mpx.c | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-)
Reviewed-by: Borislav Petkov <[email protected]> -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
