From: Rik van Riel <riel@redhat.com>
Date: Sat, 23 Sep 2017 15:00:04 +0200
Subject: x86/fpu: Add FPU state copying quirk to handle XRSTOR failure on
 Intel Skylake CPUs
Git-commit: 0852b374173bb57f870d78e6c6839c77b339be5f
Patch-mainline: v4.14-rc3
References: bsc#1151955

On Skylake CPUs I noticed that XRSTOR is unable to deal with states
created by copyout_from_xsaves() if the xstate has only SSE/YMM state, and
no FP state. That is, xfeatures had XFEATURE_MASK_SSE set, but not
XFEATURE_MASK_FP.

The reason is that part of the SSE/YMM state lives in the MXCSR and
MXCSR_FLAGS fields of the FP state.

Ensure that whenever we copy SSE or YMM state around, the MXCSR and
MXCSR_FLAGS fields are also copied around.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20170210085445.0f1cc708@annuminas.surriel.com
Link: http://lkml.kernel.org/r/20170923130016.21448-22-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/include/asm/fpu/types.h |    3 +++
 arch/x86/kernel/fpu/xstate.c     |   36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -68,6 +68,9 @@ struct fxregs_state {
 /* Default value for fxregs_state.mxcsr: */
 #define MXCSR_DEFAULT		0x1f80
 
+/* Copy both mxcsr & mxcsr_flags with a single u64 memcpy: */
+#define MXCSR_AND_FLAGS_SIZE sizeof(u64)
+
 /*
  * Software based FPU emulation state. This is arbitrary really,
  * it matches the x87 format to make it easier to understand:
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -921,6 +921,23 @@ int arch_set_user_pkey_access(struct tas
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
 /*
+ * Weird legacy quirk: SSE and YMM states store information in the
+ * MXCSR and MXCSR_FLAGS fields of the FP area. That means if the FP
+ * area is marked as unused in the xfeatures header, we need to copy
+ * MXCSR and MXCSR_FLAGS if either SSE or YMM are in use.
+ */
+static inline bool xfeatures_mxcsr_quirk(u64 xfeatures)
+{
+	if (!(xfeatures & (XFEATURE_MASK_SSE|XFEATURE_MASK_YMM)))
+		return 0;
+
+	if (xfeatures & XFEATURE_MASK_FP)
+		return 0;
+
+	return 1;
+}
+
+/*
  * This is similar to user_regset_copyout(), but will not add offset to
  * the source data pointer or increment pos, count, kbuf, and ubuf.
  */
@@ -1000,7 +1017,15 @@ int copyout_from_xsaves(unsigned int pos
 			if (offset + size >= count)
 				break;
 		}
+	}
 
+	if (xfeatures_mxcsr_quirk(header.xfeatures)) {
+		offset = offsetof(struct fxregs_state, mxcsr);
+		size = MXCSR_AND_FLAGS_SIZE;
+
+		ret = xstate_copyout(offset, size, kbuf, ubuf, &xsave->i387.mxcsr, 0, count);
+		if (ret)
+			return ret;
 	}
 
 	/*
@@ -1067,6 +1092,17 @@ int copyin_to_xsaves(const void *kbuf, c
 		}
 	}
 
+	if (xfeatures_mxcsr_quirk(xfeatures)) {
+		offset = offsetof(struct fxregs_state, mxcsr);
+		size = MXCSR_AND_FLAGS_SIZE;
+
+		if (kbuf)
+			memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
+		else
+			if (__copy_from_user(&xsave->i387.mxcsr, ubuf + offset, size))
+				return -EFAULT;
+	}
+
 	/*
 	 * The state that came in from userspace was user-state only.
 	 * Mask all the user states out of 'xfeatures':