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