On Wed, 2017-05-24 at 15:37 +0200, Borislav Petkov wrote:
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 <bp(a)suse.de> Cc: Andy Lutomirski <luto(a)kernel.org> Cc: Dave Hansen <dave.hansen(a)linux.intel.com> Cc: Adam Buchbinder <adam.buchbinder(a)gmail.com> Cc: Colin Ian King <colin.king(a)canonical.com> Cc: Lorenzo Stoakes <lstoakes(a)gmail.com> Cc: Qiaowei Ren <qiaowei.ren(a)intel.com> Cc: Peter Zijlstra <peterz(a)infradead.org> Cc: Nathan Howard <liverlint(a)gmail.com> Cc: Adan Hawthorn <adanhawthorn(a)gmail.com> Cc: Joe Perches <joe(a)perches.com> Cc: Ravi V. Shankar <ravi.v.shankar(a)intel.com> Cc: x86(a)kernel.org Signed-off-by: Ricardo Neri <ricardo.neri-calderon(a)linux.intel.com> --- 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. I will add a new line here.
+ /* + * 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.
I will add a new line here.
+ /* + * 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.
Thanks for reviewing! BR, Ricardo
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