Messages in this thread | | | Date | Thu, 24 Jul 2008 11:50:53 -0700 | From | Suresh Siddha <> | Subject | Re: [patch] x64, fpu: fix possible FPU leakage in error conditions |
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On Thu, Jul 24, 2008 at 11:31:42AM -0700, Linus Torvalds wrote: > > > On Thu, 24 Jul 2008, Suresh Siddha wrote: > > > > In the error condition for restore_fpu_checking() (especially during > > the 64bit signal return), we are doing init_fpu(), which saves the live > > FPU register state (possibly belonging to some other process context) into the > > thread struct (through unlazy_fpu() in init_fpu()). This is wrong and can leak > > the FPU data. > > > > Remove the unlazy_fpu() from the init_fpu(). init_fpu() will now always > > init the FPU data in the thread struct. For the error conditions in > > restore_fpu_checking(), restore the initialized FPU data from the thread > > struct. > > Why? The thread struct is guaranteed to contain pointless data.
init_fpu() will set it to sane init state, from where we can restore.
> If we cannot restore the FP state from the signal stack, we should not try > to restore it from anywhere _else_ either, since nowhere else will have > any better results. > > I suspect we should just reset the x87 state (which was the _intention_ of > the code), possibly by just doing "stts + used_math = 0". The signal > handling code already checks for errors, and will force a SIGSEGV if this > ever happens.
Yes, this was what I had in mind earlier and should be ok for signal handling case. But as you also noted below:
> (Yes, there is also a restore_fpu_checking() in math_state_restore(), but > that one _already_ uses ¤t->thread.xstate->fxsave as the buffer to > restore from, so trying to do that _again_ when it fails seems to be > really really wrong - we already _did_ that, and that was what failed to > begin with)
We are doing init_fpu(), which should make the data sane again.
This is a paranoid case, just to make sure that the next math_state_restore() doesn't cause #GP, after someone sets illegal values through ptrace() or 32bit signal handling (which modifies fpu state in thread struct). I say paranoid, because we already do the necessary checks in the corresponding locations like ptrace/32-bit signal handling.
If we don't do init_fpu() + restore from the sane init state, process has to be killed, in the paranoid failing scenario of math_state_restore()
thanks, suresh
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