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    Subject[PATCH] Re: SSE related security hole
    H. Peter Anvin wrote:
    >>> Ummm...last I knew, fxrstor is *expensive*. The fninit/xor regs
    >>> setup is likely *very* much faster. Someone should check this
    >>> before we sacrifice 100 cycles needlessly or something.
    >>
    >> most probably yes, fxrestor needs to read ram, pxor also takes
    >> some icache and bytecode ram but it sounds like it will be faster.
    >>
    >> Maybe we could also interleave the pxor with the xorps, since they
    >> uses different parts of the cpu, Honza?
    >>
    >
    >
    > You almost certainly should. The reason I suggested FXRSTOR is that
    > it would initialize the entire FPU, including any state that future
    > processors may add, thus reducing the likelihood of any funnies in
    > the future.

    Here's a patch to do just that. It initializes the saved state in the
    task struct and falls through to restore_fpu().

    --

    Brian Gerst
    diff -urN linux-2.5.8/arch/i386/kernel/i387.c linux/arch/i386/kernel/i387.c
    --- linux-2.5.8/arch/i386/kernel/i387.c Thu Mar 7 21:18:32 2002
    +++ linux/arch/i386/kernel/i387.c Fri Apr 19 18:23:53 2002
    @@ -31,13 +31,20 @@
    * value at reset if we support XMM instructions and then
    * remeber the current task has used the FPU.
    */
    -void init_fpu(void)
    +void init_fpu(struct task_struct *tsk)
    {
    - __asm__("fninit");
    - if ( cpu_has_xmm )
    - load_mxcsr(0x1f80);
    -
    - current->used_math = 1;
    + if ( cpu_has_xmm ) {
    + memset(&tsk->thread.i387.fxsave, 0, sizeof(struct i387_fxsave_struct));
    + tsk->thread.i387.fxsave.cwd = 0x37f;
    + tsk->thread.i387.fxsave.mxcsr = 0x1f80;
    + } else {
    + memset(&tsk->thread.i387.fsave, 0, sizeof(struct i387_fsave_struct));
    + tsk->thread.i387.fsave.cwd = 0xffff037f;
    + tsk->thread.i387.fsave.swd = 0xffff0000;
    + tsk->thread.i387.fsave.twd = 0xffffffff;
    + tsk->thread.i387.fsave.fos = 0xffff0000;
    + }
    + tsk->used_math = 1;
    }

    /*
    diff -urN linux-2.5.8/arch/i386/kernel/traps.c linux/arch/i386/kernel/traps.c
    --- linux-2.5.8/arch/i386/kernel/traps.c Sun Apr 14 23:48:18 2002
    +++ linux/arch/i386/kernel/traps.c Fri Apr 19 18:22:12 2002
    @@ -757,13 +757,12 @@
    */
    asmlinkage void math_state_restore(struct pt_regs regs)
    {
    + struct task_struct *tsk = current;
    clts(); /* Allow maths ops (or we recurse) */

    - if (current->used_math) {
    - restore_fpu(current);
    - } else {
    - init_fpu();
    - }
    + if (!tsk->used_math)
    + init_fpu(tsk);
    + restore_fpu(tsk);
    set_thread_flag(TIF_USEDFPU); /* So we fnsave on switch_to() */
    }

    diff -urN linux-2.5.8/include/asm-i386/i387.h linux/include/asm-i386/i387.h
    --- linux-2.5.8/include/asm-i386/i387.h Fri Apr 19 18:44:48 2002
    +++ linux/include/asm-i386/i387.h Fri Apr 19 18:46:12 2002
    @@ -17,7 +17,7 @@
    #include <asm/sigcontext.h>
    #include <asm/user.h>

    -extern void init_fpu(void);
    +extern void init_fpu(struct task_struct *);
    /*
    * FPU lazy state save handling...
    */
    \
     
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