linux-kernel - Re: [patch] x64, fpu: fix possible FPU leakage in error conditions

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Date:	Thu, 24 Jul 2008 11:31:42 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
cc:	x86@...nel.org, andi@...stfloor.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...nel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch] x64, fpu: fix possible FPU leakage in error conditions



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 &current->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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