Re: [PATCH 3.19 1/3] x86, mpx: Check user mode bitness correctly when decoding instructions

[Dave Hansen]

On 12/29/2014 04:52 PM, Andy Lutomirski wrote:
> --- a/arch/x86/mm/mpx.c
> +++ b/arch/x86/mm/mpx.c
> @@ -217,7 +217,7 @@ static int mpx_insn_decode(struct insn *insn,
>  			   struct pt_regs *regs)
>  {
>  	unsigned char buf[MAX_INSN_SIZE];
> -	int x86_64 = !test_thread_flag(TIF_IA32);
> +	int x86_64 = user_64bit_mode(regs);
>  	int not_copied;
>  	int nr_copied;

There are (at least) 3 other uses of the instruction decoder that use
some form of a check on TIF_IA32:

> perf_event_intel_ds.c  intel_pmu_pebs_fixup_ip 785 insn_init(&insn, kaddr, size, is_64bit);
> perf_event_intel_lbr.c branch_type             532 insn_init(&insn, addr, bytes_read, is64);
> uprobes.c              uprobe_init_insn        222 insn_init(insn, auprobe->insn, sizeof(auprobe->insn), x86_64);

Basically doing this:

                is_64bit = kernel_ip(to) || !test_thread_flag(TIF_IA32);

So this method *must* work, at least in practice.

If userspace has MPX on and switches between 32 and 64-bit itself, the
kernel *and* the hardware will suddenly be trying to walk the bounds
tables in the wrong format.  I just don't see an application surviving
very long in that situation.

So I don't have a problem with doing this, long term, as long as we do
it for all of these locations and we do it in a consistent way.  For
MPX, we may even want to enforce that:

	!test_thread_flag(TIF_IA32) == user_64bit_mode(regs)

and freak out if that fails.

But I don't think it's 3.19 material.
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