Re: [PATCH 5.10 119/199] x86/mmx: Use KFPU_387 for MMX string operations

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Hi,

I think for both 5.4-stable and 5.10-stable we also need https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e45122893a9870813f9bd7b4add4f613e6f29008 - "x86/fpu: Add kernel_fpu_begin_mask() to selectively initialize state"

Without this, there is no kernel_fpu_begin_mask().

Thanks,
 Krzysztof


On 2021-01-25 at 10:39, Greg Kroah-Hartman wrote:
From: Andy Lutomirski <luto@xxxxxxxxxx>

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@xxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Krzysztof Piotr Olędzki <ole@xxxxxx>
Tested-by: Krzysztof Mazur <krzysiek@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/e7bf21855fe99e5f3baa27446e32623358f69e8d.1611205691.git.luto@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
  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"







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