Patch "x86/fpu: Disable XSAVES* support for now" has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

x86/fpu: Disable XSAVES* support for now

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-fpu-disable-xsaves-support-for-now.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ingo Molnar <[email protected]>
Date: Wed, 20 May 2015 11:45:30 +0200
Subject: x86/fpu: Disable XSAVES* support for now

From: Ingo Molnar <[email protected]>

commit e88221c50cadade0eb4f7f149f4967d760212695 upstream.

The kernel's handling of 'compacted' xsave state layout is buggy:

    http://marc.info/?l=linux-kernel&m=142967852317199

I don't have such a system, and the description there is vague, but from extrapolation I guess that there were two kinds of bugs observed:

- boot crashes, due to size calculations being wrong and the dynamic allocation allocating a too small xstate area. (This is now fixed in the new FPU code - but still present in stable kernels.)

- FPU state corruption and ABI breakage: if signal handlers try to change the FPU state in standard format, which then the kernel tries to restore in the compacted format.

These breakages are scary, but they only occur on a small number of systems that have XSAVES* CPU support. Yet we have had XSAVES support in the upstream kernel for a large number of stable kernel releases, and the fixes are involved and unproven.

So do the safe resolution first: disable XSAVES* support and only use the standard xstate format. This makes the code work and is easy to backport.

On top of this we can work on enabling (and testing!) proper compacted format support, without backporting pressure, on top of the new, cleaned up FPU code.

---
 arch/x86/kernel/i387.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

@@ -169,6 +169,21 @@ static void init_thread_xstate(void)
 		xstate_size = sizeof(struct i387_fxsave_struct);
 	else
 		xstate_size = sizeof(struct i387_fsave_struct);
+
+	/*
+	 * Quirk: we don't yet handle the XSAVES* instructions
+	 * correctly, as we don't correctly convert between
+	 * standard and compacted format when interfacing
+	 * with user-space - so disable it for now.
+	 *
+	 * The difference is small: with recent CPUs the
+	 * compacted format is only marginally smaller than
+	 * the standard FPU state format.
+	 *
+	 * ( This is easy to backport while we are fixing
+	 *   XSAVES* support. )
+	 */
+	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/x86-mce-fix-mce-severity-messages.patch queue-4.0/ktime-fix-ktime_divns-to-do-signed-division.patch queue-4.0/x86-fpu-disable-xsaves-support-for-now.patch