Re: Dealing with the NMI mess

[Maciej W. Rozycki]

On Mon, 7 Sep 2015, Andy Lutomirski wrote:

> >  These are all implementation-specific details, including the INT1
> > instruction, which is why I am not at all surprised that they are omitted
> > from architecture manuals.
> 
> That bit is BS, though.  The INT1 instruction, executed in user mode
> (CPL3) with no hardware debugger attached, will enter the kernel
> through a gate at vector 1, *even if that gate has DPL == 0*.
> 
> If there's an instruction that bypasses hardware protection
> mechanisms, then Intel should document it rather than relying on OS
> writers to know enough folklore to get it right.
> 
> Heck, SDM Volume 3 6.12.1.1 says "The processor checks the DPL of the
> interrupt or trap gate only if an exception or interrupt is generated
> with an INT n, INT 3, or INTO instruction."  It does not say "the
> processor does not check the DPL of the interrupt or trap gate if the
> exception or interrupt is generated with the undocumented ICEBP
> instruction."

 It does not have to be mentioned, because it's implied by how the #DB 
exception is propagated: regardless of its origin it never checks the DPL.  
And user-mode software may well use POPF at any time to set the TF bit in 
the flags register to the same effect, so the OS needs to be prepared for 
a #DB exception it hasn't scheduled itself anyway.

  Maciej
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/