Re: Re: [PATCH] uprobes/x86: Refuse to attach uprobe to "word-sized" branch insns

[Masami Hiramatsu]

(2014/04/25 2:33), Oleg Nesterov wrote:
> Thanks!
> 
> Masami, Jim, any acks?
> 
> On 04/24, Denys Vlasenko wrote:
>>
>> 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 <dvlasenk@xxxxxxxxxx>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>

Thank you :)

>> ---
>>  arch/x86/kernel/uprobes.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
>> index ace2291..29b152d 100644
>> --- a/arch/x86/kernel/uprobes.c
>> +++ b/arch/x86/kernel/uprobes.c
>> @@ -582,6 +582,7 @@ static struct uprobe_xol_ops branch_xol_ops = {
>>  /* Returns -ENOSYS if branch_xol_ops doesn't handle this insn */
>>  static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>>  {
>> +	int i;
>>  	u8 opc1 = OPCODE1(insn);
>>
>>  	/* has the side-effect of processing the entire instruction */
>> @@ -612,6 +613,17 @@ 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;
>> --
>> 1.8.1.4
>>
> 
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-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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