From 250bbd12c2fe1221ec96d8087d63e982d4f2180a Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Thu, 24 Apr 2014 19:08:24 +0200 Subject: [PATCH] uprobes/x86: Refuse to attach uprobe to "word-sized" branch insns All branch insns on x86 can be prefixed with the operand-size override prefix, 0x66. It was only ever useful for performing jumps to 32-bit offsets in 16-bit code segments. In 32-bit code, such instructions are useless since they cause IP truncation to 16 bits, and in case of call insns, they save only 16 bits of return address and misalign the stack pointer as a "bonus". In 64-bit code, such instructions are treated differently by Intel and AMD CPUs: Intel ignores the prefix altogether, AMD treats them the same as in 32-bit mode. Before this patch, the emulation code would execute the instructions as if they have no 0x66 prefix. With this patch, we refuse to attach uprobes to such insns. Signed-off-by: Denys Vlasenko Acked-by: Jim Keniston Acked-by: Masami Hiramatsu Signed-off-by: Oleg Nesterov --- arch/x86/kernel/uprobes.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c index ace22916ade3..3cf24a218196 100644 --- a/arch/x86/kernel/uprobes.c +++ b/arch/x86/kernel/uprobes.c @@ -583,6 +583,7 @@ static struct uprobe_xol_ops branch_xol_ops = { static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn) { u8 opc1 = OPCODE1(insn); + int i; /* has the side-effect of processing the entire instruction */ insn_get_length(insn); @@ -612,6 +613,16 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn) return -ENOSYS; } + /* + * 16-bit overrides such as CALLW (66 e8 nn nn) are not supported. + * Intel and AMD behavior differ in 64-bit mode: Intel ignores 66 prefix. + * No one uses these insns, reject any branch insns with such prefix. + */ + for (i = 0; i < insn->prefixes.nbytes; i++) { + if (insn->prefixes.bytes[i] == 0x66) + return -ENOTSUPP; + } + auprobe->branch.opc1 = opc1; auprobe->branch.ilen = insn->length; auprobe->branch.offs = insn->immediate.value; -- 2.30.2