Messages in this thread | | | Date | Thu, 24 Jul 2008 14:54:31 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [patch] x64, fpu: fix possible FPU leakage in error conditions |
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On Thu, 24 Jul 2008, Suresh Siddha wrote: > > Meanwhile, I wanted to keep this patch simple, so that it can be easily > applied to -stable series aswell.
Hmm. There's somethign more fundamentally wrong, it really shouldn't be this ugly.
For example, the signal handler path right now does
if (!access_ok(VERIFY_READ, buf, sizeof(*buf))) goto badframe; err |= restore_i387(buf);
but the thing is, the only really valid reason for "restore_i387()" to fail is because the read failed.
Which in turn implies that if it fails, it should just do the same thing as that access_ok() failure did!
So why doesn't it just do
if (!access_ok(VERIFY_READ, buf, sizeof(*buf))) goto badframe; if (restore_i387(buf)) goto badframe:
because I don't see why that path should even _care_ about the i387 details? Especially since it doesn't even try to do that if the buffer pointer is totally bogus..
What am I missing? This code looks unnecessarily complex, and your patch makes it even harder to follow. Is this complexity really needed and worth it?
Also, looking at that "math_state_restore()" thing some more, I can't for the life of me convince myself that even just initializing the state is enough. We've used math before, and if we cannot restore it from the fxsave area, why would we _ever_ say that it's ok to try to continue with some _other_ state?
IOW, rather than resetting it, shouldn't we force a SIGFPE or something?
Sorry for being difficult, but I'd much rather get the x87 state handling _right_ and make it logically consistent than paper over yet another mistake we've done in this area. For example, regular 32-bit x86 doesn't do any of this crap. It just does "restore_fpu()" in math_state_restore().
Why does x86-64 need to do anythign else? It's not even a user address, it cannot take page faults. So exactly what are we protecting against?
I may well be missing something here, so please fill me in..
Linus
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