Patch "x86/fpu/xstate: Fix xcomp_bv in XSAVES header" has been added to the 4.9-stable tree

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

x86/fpu/xstate: Fix xcomp_bv in XSAVES header

to the 4.9-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-xstate-fix-xcomp_bv-in-xsaves-header.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Yu-cheng Yu <[email protected]>
Date: Mon, 23 Jan 2017 14:54:44 -0800
Subject: x86/fpu/xstate: Fix xcomp_bv in XSAVES header

From: Yu-cheng Yu <[email protected]>

commit dffba9a31c7769be3231c420d4b364c92ba3f1ac upstream.

The compacted-format XSAVES area is determined at boot time and never changed after. The field xsave.header.xcomp_bv indicates which components are in the fixed XSAVES format.

In fpstate_init() we did not set xcomp_bv to reflect the XSAVES format since at the time there is no valid data.

However, after we do copy_init_fpstate_to_fpregs() in fpu__clear(), as in commit:

b22cbe404a9c x86/fpu: Fix invalid FPU ptrace state after execve()

and when __fpu_restore_sig() does fpu__restore() for a COMPAT-mode app, a #GP occurs. This can be easily triggered by doing valgrind on a COMPAT-mode "Hello World," as reported by Joakim Tjernlund and others:

	https://bugzilla.kernel.org/show_bug.cgi?id=190061

Fix it by setting xcomp_bv correctly.

This patch also moves the xcomp_bv initialization to the proper place, which was in copyin_to_xsaves() as of:

4c833368f0bf x86/fpu: Set the xcomp_bv when we fake up a XSAVES area

which fixed the bug too, but it's more efficient and cleaner to initialize things once per boot, not for every signal handling operation.

---
 arch/x86/kernel/fpu/core.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -236,7 +236,8 @@ void fpstate_init(union fpregs_state *st
 	 * it will #GP. Make sure it is replaced after the memset().
 	 */
 	if (static_cpu_has(X86_FEATURE_XSAVES))
-		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
+		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
+					       xfeatures_mask;
 
 	if (static_cpu_has(X86_FEATURE_FXSR))
 		fpstate_init_fxstate(&state->fxsave);

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

queue-4.9/x86-fpu-xstate-fix-xcomp_bv-in-xsaves-header.patch