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

[Jim Keniston]

On Thu, 2014-04-24 at 19:33 +0200, Oleg Nesterov wrote:
> Thanks!
> 
> Masami, Jim, any acks?

Acked-by: Jim Keniston <jkenisto@xxxxxxxxxx>

> 
> 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>
> > ---
> >  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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