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    Subject[PATCH 5.10 119/199] x86/mmx: Use KFPU_387 for MMX string operations
    Date
    From: Andy Lutomirski <luto@kernel.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@podlesie.net>
    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Tested-by: Krzysztof Piotr Olędzki <ole@ans.pl>
    Tested-by: Krzysztof Mazur <krzysiek@podlesie.net>
    Cc: <stable@vger.kernel.org>
    Link: https://lkml.kernel.org/r/e7bf21855fe99e5f3baa27446e32623358f69e8d.1611205691.git.luto@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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"

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