Re: [PATCH v3 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context

From: Andy Lutomirski
Date: Thu Feb 11 2016 - 20:02:04 EST

On Thu, Feb 11, 2016 at 11:49 AM, Borislav Petkov <bp@xxxxxxxxx> 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
new SS = a flat 32-bit data segment

>> +
>> + * This behavior serves two purposes. It ensures that older programs
>> + * that are unaware of the signal context's SS slot and either construct
>> + * a signal context from scratch or that catch signals from segmented
>> + * contexts and change CS to a 64-bit selector won't crash due to a bad
>> + * SS value.
> I'm having hard time parsing that sentence and especially placing all
> those "either", "or", "and" connectors at the proper levels. Would it be
> more understandable as pseudocode?
>> It also ensures that signal handlers that do not modify
>> + * the signal context at all return back to the exact CS and SS state
>> + * that they came from.
> So my brain is reliably in a knot after this text.

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fact that the SS selector when the signal was raised is no longer
valid.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.

- 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 ---

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.