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SubjectRe: 2.6.26-git: NULL pointer deref in __switch_to

* 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();
}

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