[PATCH] x86/fpu/xstate: Fix xcomp_bv in XSAVES header

[Yu-cheng Yu]

This is essentially the same patch as:

  commit dffba9a31c77 ("x86/fpu/xstate: Fix xcomp_bv in XSAVES header")

in the upstream tree, rebased and tested for the stable tree.
This can be applied to both linux-stable-rc/linux-4.9.y and
linux-stable-rc/linux-4.8.y if desired. 

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.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reported-by: Joakim Tjernlund <Joakim.Tjernlund@xxxxxxxxxxxx>
---
 arch/x86/kernel/fpu/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index ebb4e95..96d80df 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -236,7 +236,8 @@ void fpstate_init(union fpregs_state *state)
 	 * 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);
-- 
1.9.1