From 4c414592a79b82ddca76945c7afb4843684aa9a8 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@amacapital.net>
Date: Sat, 22 Nov 2014 18:00:32 -0800
Subject: [PATCH] x86_64, traps: Stop using IST for #SS

commit 6f442be2fb22be02cafa606f1769fa1e6f894441 upstream.

On a 32-bit kernel, this has no effect, since there are no IST stacks.

On a 64-bit kernel, #SS can only happen in user code, on a failed iret
to user space, a canonical violation on access via RSP or RBP, or a
genuine stack segment violation in 32-bit kernel code.  The first two
cases don't need IST, and the latter two cases are unlikely fatal bugs,
and promoting them to double faults would be fine.

This fixes a bug in which the espfix64 code mishandles a stack segment
violation.

This saves 4k of memory per CPU and a tiny bit of code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
 - No need to define trace_stack_segment
 - Use the errorentry macro to generate #SS asm code
 - Adjust context
 - Checked that this matches Luis's backport for Ubuntu]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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