linux-kernel - [PATCH 5.10 119/199] x86/mmx: Use KFPU

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Date:   Mon, 25 Jan 2021 19:39:01 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Krzysztof Mazur <krzysiek@...lesie.net>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Krzysztof Piotr Olędzki <ole@....pl>
Subject: [PATCH 5.10 119/199] x86/mmx: Use KFPU_387 for MMX string operations

From: Andy Lutomirski <luto@...nel.org>

commit 67de8dca50c027ca0fa3b62a488ee5035036a0da upstream.

The default kernel_fpu_begin() doesn't work on systems that support XMM but
haven't yet enabled CR4.OSFXSR.  This causes crashes when _mmx_memcpy() is
called too early because LDMXCSR generates #UD when the aforementioned bit
is clear.

Fix it by using kernel_fpu_begin_mask(KFPU_387) explicitly.

Fixes: 7ad816762f9b ("x86/fpu: Reset MXCSR to default in kernel_fpu_begin()")
Reported-by: Krzysztof Mazur <krzysiek@...lesie.net>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Tested-by: Krzysztof Piotr Olędzki <ole@....pl>
Tested-by: Krzysztof Mazur <krzysiek@...lesie.net>
Cc: <stable@...r.kernel.org>
Link: https://lkml.kernel.org/r/e7bf21855fe99e5f3baa27446e32623358f69e8d.1611205691.git.luto@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/lib/mmx_32.c |   20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

--- a/arch/x86/lib/mmx_32.c
+++ b/arch/x86/lib/mmx_32.c
@@ -26,6 +26,16 @@
 #include <asm/fpu/api.h>
 #include <asm/asm.h>
 
+/*
+ * Use KFPU_387.  MMX instructions are not affected by MXCSR,
+ * but both AMD and Intel documentation states that even integer MMX
+ * operations will result in #MF if an exception is pending in FCW.
+ *
+ * EMMS is not needed afterwards because, after calling kernel_fpu_end(),
+ * any subsequent user of the 387 stack will reinitialize it using
+ * KFPU_387.
+ */
+
 void *_mmx_memcpy(void *to, const void *from, size_t len)
 {
 	void *p;
@@ -37,7 +47,7 @@ void *_mmx_memcpy(void *to, const void *
 	p = to;
 	i = len >> 6; /* len/64 */
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"1: prefetch (%0)\n"		/* This set is 28 bytes */
@@ -127,7 +137,7 @@ static void fast_clear_page(void *page)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"  pxor %%mm0, %%mm0\n" : :
@@ -160,7 +170,7 @@ static void fast_copy_page(void *to, voi
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	/*
 	 * maybe the prefetch stuff can go before the expensive fnsave...
@@ -247,7 +257,7 @@ static void fast_clear_page(void *page)
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"  pxor %%mm0, %%mm0\n" : :
@@ -282,7 +292,7 @@ static void fast_copy_page(void *to, voi
 {
 	int i;
 
-	kernel_fpu_begin();
+	kernel_fpu_begin_mask(KFPU_387);
 
 	__asm__ __volatile__ (
 		"1: prefetch (%0)\n"