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Date:   Tue, 28 May 2019 16:32:24 +0100
From:   Will Deacon <will.deacon@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk,
        mark.rutland@arm.com, mingo@redhat.com, bp@alien8.de,
        hpa@zytor.com, x86@kernel.org, kan.liang@linux.intel.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        ravi.bangoria@linux.vnet.ibm.com, mpe@ellerman.id.au
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
Message-ID: <20190528153224.GE20758@fuggles.cambridge.arm.com>
References: <1559046689-24091-1-git-send-email-92siuyang@gmail.com>
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On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote:
> 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.

This seems to be because pt_regs(current) gives NULL for kthreads on Power.

> > 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().

From the initial report [1], it doesn't look like the mm isn't initialised,
but rather than we're dereferencing a NULL pt_regs pointer somehow for the
current task (see previous comment). I don't see how that can happen on
arm64, given that we put the pt_regs on the kernel stack which is allocated
during fork.

Will

[1] https://git.kernel.org/linus/bf05fc25f268