Messages in this thread Patch in this message |  | | From | Ricardo Neri <> | Subject | [PATCH v8 16/28] x86/insn-eval: Indicate a 32-bit displacement if ModRM.mod is 0 and ModRM.rm is 101b | Date | Fri, 18 Aug 2017 17:27:57 -0700 |
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Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software Developer's Manual volume 2A states that when ModRM.mod is zero and ModRM.rm is 101b, a 32-bit displacement follows the ModRM byte. This means that none of the registers are used in the computation of the effective address. A return value of -EDOM indicates callers that they should not use the value of registers when computing the effective address for the instruction.
In long mode, the effective address is given by the 32-bit displacement plus the location of the next instruction. In protected mode, only the displacement is used.
The instruction decoder takes care of obtaining the displacement.
Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Adam Buchbinder <adam.buchbinder@gmail.com> Cc: Colin Ian King <colin.king@canonical.com> Cc: Lorenzo Stoakes <lstoakes@gmail.com> Cc: Qiaowei Ren <qiaowei.ren@intel.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Adrian Hunter <adrian.hunter@intel.com> Cc: Kees Cook <keescook@chromium.org> Cc: Thomas Garnier <thgarnie@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Borislav Petkov <bp@suse.de> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com> Cc: x86@kernel.org Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> --- arch/x86/lib/insn-eval.c | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-)
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c index a8e12bd0aecd..04f696c3793e 100644 --- a/arch/x86/lib/insn-eval.c +++ b/arch/x86/lib/insn-eval.c @@ -360,6 +360,14 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs, switch (type) { case REG_TYPE_RM: regno = X86_MODRM_RM(insn->modrm.value); + + /* + * ModRM.mod == 0 and ModRM.rm == 5 means a 32-bit displacement + * follows the ModRM byte. + */ + if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5) + return -EDOM; + if (X86_REX_B(insn->rex_prefix.value)) regno += 8; break; @@ -706,10 +714,22 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs) eff_addr = base + indx * (1 << X86_SIB_SCALE(sib)); } else { addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM); - if (addr_offset < 0) - goto out_err; - eff_addr = regs_get_register(regs, addr_offset); + /* + * -EDOM means that we must ignore the address_offset. + * In such a case, in 64-bit mode the effective address + * relative to the RIP of the following instruction. + */ + if (addr_offset == -EDOM) { + if (user_64bit_mode(regs)) + eff_addr = (long)regs->ip + insn->length; + else + eff_addr = 0; + } else if (addr_offset < 0) { + goto out_err; + } else { + eff_addr = regs_get_register(regs, addr_offset); + } } eff_addr += insn->displacement.value; -- 2.13.0
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