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SubjectRe: 2.6.26-git: NULL pointer deref in __switch_to
On Sat, Jun 14 2008, Ingo Molnar wrote:
>
> * Suresh Siddha <suresh.b.siddha@intel.com> wrote:
>
> > Somehow (as described below?) TS_USEDFPU is set but the fpu is not
> > allocated or freed.
> >
> > Please try the appended patch.
>
> i've queued up your fix in tip/x86/urgent. (Git access coordinates:
> http://people.redhat.com/mingo/tip.git/README)
>
> i'm wondering why this problem was not hit more frequently. Does it need
> some special FPU use to trigger? Or does it need an exec() with the FPU
> stack still active? (normally the FPU stack is empty at exec() time)
>
> Ingo
>
> -------------------->
> commit fade25af0c80b9caed83beb0b961559f4c33a49f
> Author: Suresh Siddha <suresh.b.siddha@intel.com>
> Date: Fri Jun 13 15:47:12 2008 -0700
>
> x86: fix NULL pointer deref in __switch_to
>
> Patrick McHardy reported a crash:
>
> > > I get this oops once a day, its apparently triggered by something
> > > run by cron, but the process is a different one each time.
> > >
> > > Kernel is -git from yesterday shortly before the -rc6 release
> > > (last commit is the usb-2.6 merge, the x86 patches are missing),
> > > .config is attached.
> > >
> > > I'll retry with current -git, but the patches that have gone in
> > > since I last updated don't look related.
> > >
> > > [62060.043009] BUG: unable to handle kernel NULL pointer dereference at
> > > 000001ff
> > > [62060.043009] IP: [<c0102a9b>] __switch_to+0x2f/0x118
> > > [62060.043009] *pde = 00000000
> > > [62060.043009] Oops: 0002 [#1] PREEMPT
>
> Vegard Nossum analyzed it:
>
> > This decodes to
> >
> > 0: 0f ae 00 fxsave (%eax)
> >
> > so it's related to the floating-point context. This is the exact
> > location of the crash:
> >
> > $ addr2line -e arch/x86/kernel/process_32.o -i ab0
> > include/asm/i387.h:232
> > include/asm/i387.h:262
> > arch/x86/kernel/process_32.c:595
> >
> > ...so it looks like prev_task->thread.xstate->fxsave has become NULL.
> > Or maybe it never had any other value.
>
> Somehow (as described below) TS_USEDFPU is set but the fpu is not
> allocated or freed.
>
> Another possible FPU pre-emption issue with the sleazy FPU optimization
> which was benign before but not so anymore, with the dynamic FPU allocation
> patch.
>
> New task is getting exec'd and it is prempted at the below point.
>
> flush_thread() {
> ...
> /*
> * Forget coprocessor state..
> */
> clear_fpu(tsk);
> <----- Preemption point
> clear_used_math();
> ...
> }
>
> Now when it context switches in again, as the used_math() is still set
> and fpu_counter can be > 5, we will do a math_state_restore() which sets
> the task's TS_USEDFPU. After it continues from the above preemption point
> it does clear_used_math() and much later free_thread_xstate().
>
> Now, at the next context switch, it is quite possible that xstate is
> null, used_math() is not set and TS_USEDFPU is still set. This will
> trigger unlazy_fpu() causing kernel oops.
>
> Fix this by clearing tsk's fpu_counter before clearing task's fpu.
>
> Reported-by: Patrick McHardy <kaber@trash.net>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 6d54833..e2db9ac 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -333,6 +333,7 @@ void flush_thread(void)
> /*
> * Forget coprocessor state..
> */
> + tsk->fpu_counter = 0;
> clear_fpu(tsk);
> clear_used_math();
> }
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index ac54ff5..c6eb5c9 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -294,6 +294,7 @@ void flush_thread(void)
> /*
> * Forget coprocessor state..
> */
> + tsk->fpu_counter = 0;
> clear_fpu(tsk);
> clear_used_math();
> }

Don't you need an smp_wmb() between that fpu_counter clear and
clear_fpu(), since your fix seems to rely on strict ordering between
the two?

--
Jens Axboe



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