linux-kernel - Re: [PATCH] perf: Fix oops when kthread execs user process

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Date:   Wed, 29 May 2019 11:44:02 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Peter Zijlstra <peterz@...radead.org>,
        Young Xiao <92siuyang@...il.com>
Cc:     will.deacon@....com, linux@...linux.org.uk, mark.rutland@....com,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kan.liang@...ux.intel.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, ravi.bangoria@...ux.vnet.ibm.com
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process

Peter Zijlstra <peterz@...radead.org> writes:
> On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote:
>> When a kthread calls call_usermodehelper() the steps are:
>>   1. allocate current->mm
>>   2. load_elf_binary()
>>   3. populate current->thread.regs
>> 
>> While doing this, interrupts are not disabled. If there is a perf
>> interrupt in the middle of this process (i.e. step 1 has completed
>> but not yet reached to step 3) and if perf tries to read userspace
>> regs, kernel oops.
>> 
>> Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
>> pt_regs are not set.
>> 
>> See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs
>> user process") for details.
>
> Why the hell do we set current->mm before it is complete? Note that
> normally exec() builds the new mm before attaching it, see exec_mmap()
> in flush_old_exec().
>
> Also, why did those PPC folks 'fix' this in isolation? And why didn't
> you Cc them?

We just assumed it was our bug, 'cause we have plenty of those :)

cheers