Messages in this thread | | | Date | Thu, 24 Jul 2008 11:31:42 -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: > > 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.
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, 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)
Linus
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