[PATCH] x86/fpu: set the xcomp_bv when we fake up a XSAVES area

[Kevin Hao]

I got the following calltrace on a Apollo Lake SoC with 32bit kernel.
  WARNING: CPU: 2 PID: 261 at arch/x86/include/asm/fpu/internal.h:363 fpu__restore+0x1f5/0x260
  Modules linked in:
  CPU: 2 PID: 261 Comm: check_hostname. Not tainted 4.10.0-rc4-next-20170120 #90
  Hardware name: Intel Corp. Broxton P/NOTEBOOK, BIOS APLIRVPA.X64.0138.B35.1608091058 08/09/2016
  Call Trace:
   dump_stack+0x47/0x5f
   __warn+0xea/0x110
   ? fpu__restore+0x1f5/0x260
   warn_slowpath_null+0x2a/0x30
   fpu__restore+0x1f5/0x260
   __fpu__restore_sig+0x165/0x6b0
   fpu__restore_sig+0x2f/0x50
   restore_sigcontext.isra.9+0xe0/0xf0
   sys_sigreturn+0xaa/0xf0
   do_int80_syscall_32+0x59/0xb0
   entry_INT80_32+0x2a/0x2a
  EIP: 0xb77acc61
  EFLAGS: 00000246 CPU: 2
  EAX: 00000000 EBX: 00000003 ECX: 08151d38 EDX: 00000000
  ESI: bfa9ce20 EDI: 08151d38 EBP: 0000000c ESP: bfa9cdbc
   DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b

The reason is that a #GP occurs when executing XRSTORS. The root cause
is that we forget to set the xcomp_bv when we fake up the XSAVES area
in function copyin_to_xsaves().

Signed-off-by: Kevin Hao <haokexin@xxxxxxxxx>
---
 arch/x86/kernel/fpu/xstate.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 35f7024aace5..2c0df2681481 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1071,6 +1071,8 @@ 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;
 }
-- 
2.9.3