x86/fpu/xstate: Fix xcomp_bv in XSAVES header

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:

  b22cbe4 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:

  4c83336 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.

Reported-by: Kevin Hao <[email protected]>
Reported-by: Joakim Tjernlund <[email protected]>
Signed-off-by: Yu-cheng Yu <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
[ Combined it with 4c83336. ]
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/kernel/fpu/core.c

@@ -9,6 +9,7 @@
 #include <asm/fpu/regset.h>
 #include <asm/fpu/signal.h>
 #include <asm/fpu/types.h>
+#include <asm/fpu/xstate.h>
 #include <asm/traps.h>
 
 #include <linux/hardirq.h>
@@ -183,7 +184,8 @@ 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);

arch/x86/kernel/fpu/xstate.c

@@ -1070,7 +1070,6 @@ int copyin_to_xsaves(const void *kbuf, const void __user *ubuf,
 	 * Add back in the features that came in from userspace:
 	 */
 	xsave->header.xfeatures |= xfeatures;
-	xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT | xsave->header.xfeatures;
 
 	return 0;
 }