On Tue, Mar 07, 2017 at 04:32:36PM -0800, 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 a SIB byte is used and the base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part of the ModRM byte is zero, the value of such register will not be used as part of the address computation. To signal this, a -EDOM error is returned to indicate callers that they should ignore the value.
Also, for this particular case, a displacement of 32-bits should follow the SIB byte if the mod part of ModRM is equal to zero. The instruction decoder ensures that this is the case.
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 | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-)
diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c index d9e92d6..ef7eb67 100644 --- a/arch/x86/mm/mpx.c +++ b/arch/x86/mm/mpx.c @@ -121,6 +121,17 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
case REG_TYPE_BASE: regno = X86_SIB_BASE(insn->sib.value); + /* + * If mod is 0 and register R/EBP (regno=5) is indicated in the + * base part of the SIB byte,
you can simply say here: "if SIB.base == 5, the base of the register-indirect addressing is 0."
the value of such register should + * not be used in the address computation. Also, a 32-bit
Not "Also" but "In this case, a 32-bit displacement..."
+ * displacement is expected in this case; the instruction + * decoder takes care of it. This is true for both R13 and + * R/EBP as REX.B will not be decoded.
You don't need that sentence as the only thing that matters is ModRM.mod being 0.
+ */ + if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
The 0 test we normally do with the ! (also flip parts of if-condition): if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
+ return -EDOM; + if (X86_REX_B(insn->rex_prefix.value)) regno += 8; break; @@ -161,16 +172,21 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs) eff_addr = regs_get_register(regs, addr_offset); } else { if (insn->sib.nbytes) { + /* + * Negative values in the base and index offset means + * an error when decoding the SIB byte. Except -EDOM, + * which means that the registers should not be used + * in the address computation. + */ base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE); - if (base_offset < 0) + if (unlikely(base_offset == -EDOM)) + base = 0; + else if (unlikely(base_offset < 0))
Bah, unlikely's in something which is not really a hot path. They only encumber readability, no need for them. -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --