linux-kernel - Re: PATCH? process_32.c:__switch_to() calls __math_state_restore() before updating current

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Date:	Sat, 6 Feb 2010 13:08:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PATCH? process_32.c:__switch_to() calls __math_state_restore()
	before updating current_task

On 02/06, Oleg Nesterov wrote:
>
> On 02/05, Oleg Nesterov wrote:
> >
> > In short: why restore_i387_xstate() does init_fpu() when !used_math(),
> > can't (or shouldn't) it merely do set_used_math() ?
> >
> > 	restore_i387_xstate:
> > 		
> > 		if (!used_math()) {
> > 			err = init_fpu(tsk);
> > 			if (err)
> > 				return err;
> > 		}
> >
> > note that buf != NULL. This means that used_math() was true when
> > get_sigframe() was called, otherwise buf == sigcontext->fpstate
> > would be NULL, right?
> >
> > So, the task must have the valid ->thread.xstate,
>
> This is probably correct. But a malicious user can set sc->fpsate
> and fool the kernel, so we must assume buf != NULL implies the
                             ^^^^
                             can't
> valid ->xstate.

Oleg.

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