• Suresh Siddha's avatar
    x86: fix NULL pointer deref in __switch_to · 75118a82
    Suresh Siddha authored
    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: default avatarPatrick McHardy <kaber@trash.net>
    Signed-off-by: default avatarSuresh Siddha <suresh.b.siddha@intel.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    75118a82
process_32.c 18.5 KB