On Thu, Feb 11, 2016 at 05:01:39PM -0800, Andy Lutomirski wrote:
> On Thu, Feb 11, 2016 at 11:49 AM, Borislav Petkov <[email protected]> wrote:
> > On Mon, Jan 25, 2016 at 01:34:14PM -0800, Andy Lutomirski wrote:
> >
> >> + * with a flat 32-bit selector.
>
> How about:
>
> Sigreturn restores SS as follows:
>
> if (saved SS is valid || UC_STRICT_RESTORE_SS is set || saved CS is not
> 64-bit)
> new SS = saved SS
> else
> new SS = a flat 32-bit data segment
Much better!
> How about:
>
> --- cut here ---
>
> This behavior serves three purposes:
>
> - Legacy programs that construct a 64-bit sigcontext from scratch
> with zero or garbage in the SS slot (e.g. old CRIU) and call sigreturn
> will still work.
>
> - Old DOSEMU versions sometimes catch a signal from a segmented
> context, delete the old SS segment (with modify_ldt), and change the
> saved CS to a 64-bit segment. These DOSEMU versions expect sigreturn
> to send them back to 64-bit mode without killing them, despite the
> fact that the SS selector when the signal was raised is no longer
> valid. With UC_STRICT_RESTORE_SS clear, the kernel will fix up SS for
> these DOSEMU versions.
... and with UC_STRICT_RESTORE_SS set, they'll get __USER_DS.
> - Old and new programs that catch a signal and return without
> modifying the saved context will end up in exactly the state they
> started in. Old kernels would lose track of the previous SS value.
>
> --- cut here ---
Yap, definitely better.
> FWIW, I have a DOSEMU patch that makes it use UC_STRICT_RESTORE_SS to
> get the behavior it actually wants on new kernels. It should make it
> faster and more reliable than was possible before.
Cool.
Thanks.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
