Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

From: Thomas Gleixner
Date: Mon Sep 29 2014 - 15:33:37 EST


On Mon, 29 Sep 2014, Sebastian Lackner wrote:
> On 29.09.2014 19:40, Andy Lutomirski wrote:
> Well, the best documentation I've found is something like
> http://www.fermimn.gov.it/linux/quarta/x86/int.htm
>
> which states:
>
> --- snip ---
> INTERRUPT-TO-INNER-PRIVILEGE:
> [...]
> TF := 0;
> NT := 0;
> --- snip ---
> (Doesn't say anything about HW interrupts though)
>
> This also makes sense at my opinion, since the interrupt handler has
> to know if it should return to the previous task (when NT=1) or to
> the same task (when NT=0).

No, it does not. Simply because Linux does not support nested tasks at
all, because the TSS is not accessible and the TSS.back_link is
sturdily NULL. So even if it would not explode with a #GP in IA-32e
mode it would explode while trying to execute the instruction at NULL.

> That might be possible. It probably makes sense to review other
> parts of the code, for similar issues.

What's the issue? Stupid user space programs segfaulting?

Thanks,

tglx
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