[PATCH v7 12/13] x86/xsaves: Fix fpstate_init() for XRSTORS

[Yu-cheng Yu]

In XSAVES mode if fpstate_init() is used to initialize a
task's extended state area, xsave.header.xcomp_bv[63] must
be set. Otherwise, when the task is scheduled, a warning is
triggered from copy_kernel_to_xregs().

One such test case is: setting an invalid extended state
through PTRACE. When xstateregs_set() rejects the syscall
and re-initializes the task's extended state area. This triggers
the warning mentioned above.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>
---
 arch/x86/include/asm/fpu/types.h | 6 ++++++
 arch/x86/kernel/fpu/core.c       | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 12dd648..48df486 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -232,6 +232,12 @@ struct xstate_header {
 } __attribute__((packed));
 
 /*
+ * xstate_header.xcomp_bv[63] indicates that the extended_state_area
+ * is in compacted format.
+ */
+#define XCOMP_BV_COMPACTED_FORMAT ((u64)1 << 63)
+
+/*
  * This is our most modern FPU state format, as saved by the XSAVE
  * and restored by the XRSTOR instructions.
  *
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index dfd29ac..f7c9be0 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -8,6 +8,7 @@
 #include <asm/fpu/internal.h>
 #include <asm/fpu/regset.h>
 #include <asm/fpu/signal.h>
+#include <asm/fpu/types.h>
 #include <asm/traps.h>
 
 #include <linux/hardirq.h>
@@ -224,6 +225,13 @@ void fpstate_init(union fpregs_state *state)
 
 	memset(state, 0, fpu_kernel_xstate_size);
 
+	/*
+	 * XRSTORS requires that this bit is set in xcomp_bv, or
+	 * it will #GP. Make sure it is replaced after the memset().
+	 */
+	if (cpu_has_xsaves)
+		state->xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT;
+
 	if (cpu_has_fxsr)
 		fpstate_init_fxstate(&state->fxsave);
 	else
-- 
1.9.1