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

From: Will Deacon
Date: Tue May 28 2019 - 11:35:51 EST

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.