From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ricardo Neri Subject: [v3 PATCH 01/10] x86/mpx: Do not use SIB index if index points to R/ESP Date: Wed, 25 Jan 2017 12:23:44 -0800 Message-ID: <20170125202353.101059-2-ricardo.neri-calderon@linux.intel.com> References: <20170125202353.101059-1-ricardo.neri-calderon@linux.intel.com> Return-path: In-Reply-To: <20170125202353.101059-1-ricardo.neri-calderon@linux.intel.com> Sender: linux-msdos-owner@vger.kernel.org List-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Andy Lutomirski , Borislav Petkov Cc: 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 , Fenghua Yu , Stas Sergeev , "Ravi V. Shankar" , Shuah Khan , linux-kernel@vger.kern Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software Developer's Manual volume 2A states that when memory addressing is used (i.e., mod part of ModR/M is not 3), a SIB byte is used and the index of the SIB byte points to the R/ESP (i.e., index = 4), the index should not be used in the computation of the memory address. In these cases the address is simply the value present in the register pointed by the base part of the SIB byte plus the displacement byte. An example of such instruction could be insn -0x80(%rsp) This is represented as: [opcode] 4c 23 80 ModR/M=0x4c: mod: 0x1, reg: 0x1: r/m: 0x4(R/ESP) SIB=0x23: sc: 0, index: 0x100(R/ESP), base: 0x11(R/EBX): Displacement -0x80 The correct address is (base) + displacement; no index is used. We can achieve the desired effect of not using the index by making get_reg_offset return a negative offset in this particular case. A negative offset indicates callers that they should not use the index to calculate the address. This is equivalent to using a index of zero when multiplying it by the base. Care is taken to allow R12 to be used as index, which is a valid scenario. Cc: Dave Hansen Cc: Adam Buchbinder Cc: Colin Ian King Cc: Lorenzo Stoakes Cc: Qiaowei Ren Cc: Ravi V. Shankar Cc: x86@kernel.org Signed-off-by: Ricardo Neri --- arch/x86/mm/mpx.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c index af59f80..9d15f6b 100644 --- a/arch/x86/mm/mpx.c +++ b/arch/x86/mm/mpx.c @@ -109,6 +109,13 @@ 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; + /* + * If mod !=3, register R/ESP (regno=4) is not used as index in + * the address computation. Check is done after looking at REX.X + * This is because R12 (regno=12) can be used as an index. + */ + if (regno == 4 && X86_MODRM_MOD(insn->modrm.value) != 3) + return -EINVAL; break; case REG_TYPE_BASE: @@ -157,11 +164,16 @@ 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); + /* + * A negative offset means that the register cannot be + * be used as an index. + */ if (indx_offset < 0) - goto out_err; + indx = 0; + else + indx = regs_get_register(regs, indx_offset); base = regs_get_register(regs, base_offset); - indx = regs_get_register(regs, indx_offset); addr = base + indx * (1 << X86_SIB_SCALE(sib)); } else { addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM); -- 2.9.3