From mboxrd@z Thu Jan 1 00:00:00 1970 From: Borislav Petkov Subject: Re: [PATCH v7 04/26] x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b Date: Wed, 24 May 2017 15:37:37 +0200 Message-ID: <20170524133737.GC7275@nazgul.tnic> References: <20170505181724.55000-1-ricardo.neri-calderon@linux.intel.com> <20170505181724.55000-5-ricardo.neri-calderon@linux.intel.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <20170505181724.55000-5-ricardo.neri-calderon@linux.intel.com> Sender: linux-msdos-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="utf-8" To: Ricardo Neri Cc: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Andrew Morton , Brian Gerst , Chris Metcalf , Dave Hansen , Paolo Bonzini , Liang Z Li , Masami Hiramatsu , Huang Rui , Jiri Slaby , Jonathan Corbet , "Michael S. Tsirkin" , Paul Gortmaker , Vlastimil Babka , Chen Yucong , Alexandre Julliard , Stas On Fri, May 05, 2017 at 11:17:02AM -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) > > Cc: Borislav Petkov > Cc: Andy Lutomirski > Cc: Dave Hansen > Cc: Adam Buchbinder > Cc: Colin Ian King > Cc: Lorenzo Stoakes > Cc: Qiaowei Ren > Cc: Peter Zijlstra > Cc: Nathan Howard > Cc: Adan Hawthorn > Cc: Joe Perches > Cc: Ravi V. Shankar > Cc: x86@kernel.org > Signed-off-by: Ricardo Neri > --- > arch/x86/mm/mpx.c | 20 ++++++++++++++++++-- > 1 file changed, 18 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c > index ebdead8..7397b81 100644 > --- a/arch/x86/mm/mpx.c > +++ b/arch/x86/mm/mpx.c > @@ -110,6 +110,14 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs, > regno = X86_SIB_INDEX(insn->sib.value); > if (X86_REX_X(insn->rex_prefix.value)) > regno += 8; <--- newline. > + /* > + * If ModRM.mod !=3 and SIB.index (regno=4) the scale*index > + * portion of the address computation is null. This is > + * true only if REX.X is 0. In such a case, the SIB index > + * is used in the address computation. > + */ > + if (X86_MODRM_MOD(insn->modrm.value) != 3 && regno == 4) > + return -EDOM; > break; > > case REG_TYPE_BASE: > @@ -159,11 +167,19 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs) > goto out_err; > > indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX); > - if (indx_offset < 0) <--- newline. > + /* > + * A negative offset generally means a error, except an > + * -EDOM, which means that the contents of the register > + * should not be used as index. > + */ > + if (indx_offset == -EDOM) > + indx = 0; > + else if (indx_offset < 0) > goto out_err; > + else > + indx = regs_get_register(regs, indx_offset); > > base = regs_get_register(regs, base_offset); > - indx = regs_get_register(regs, indx_offset); > eff_addr = base + indx * (1 << X86_SIB_SCALE(sib)); > } else { > addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM); > -- > 2.9.3 > -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --