Messages in this thread | | | Date | Sat, 20 Apr 2002 06:21:49 +0200 | From | Andrea Arcangeli <> | Subject | Re: [PATCH] Re: SSE related security hole |
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On Fri, Apr 19, 2002 at 08:08:15PM -0400, Brian Gerst wrote: > 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 19:35:14 2002 > @@ -31,13 +31,21 @@ > * 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_fxsr) { > + memset(&tsk->thread.i387.fxsave, 0, sizeof(struct i387_fxsave_struct)); > + tsk->thread.i387.fxsave.cwd = 0x37f; > + if (cpu_has_xmm) > + 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() */ > } >
I don't think it's good enough for merging yet. If you really want to do the fxrestor, you should at least do the init_fpu only once during bootup. The fxrestor is probably just overkill, but the memset + the initializations is completly superflous in a fast path, I'd also use the proper set_fpu_cwd and friends instead of doing it by hand. Even better is to merge the:
/* Simulate an empty FPU. */ set_fpu_cwd(child, 0x037f); set_fpu_swd(child, 0x0000); set_fpu_twd(child, 0xffff); set_fpu_mxcsr(child, 0x1f80);
/* Simulate an empty FPU. */ set_fpu_cwd(child, 0x037f); set_fpu_swd(child, 0x0000); set_fpu_twd(child, 0xffff);
in ptrace.c in a single function instead of duplicating functionality by hand.
I still think the xor will be faster, no dcache pollution at all and less I/O to ram. Future features can require change to the "empty FPU" state anyways.
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