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Date:   Tue, 28 May 2019 19:32:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Will Deacon <will.deacon@arm.com>
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: <20190528173228.GW2623@hirez.programming.kicks-ass.net>
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On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote:
> 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.

'funny' thing that, perf_sample_regs_user() seems to assume that
anything with current->mm is in fact a user task, and that assumption is
just plain wrong, consider use_mm().

So I'm thinking the right thing to do here is something like the below;
umh should get PF_KTHREAD cleared when it passes exec(). And this should
also fix the power splat I'm thinking.

---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index abbd4b3b96c2..9929404b6eb9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
 	if (user_mode(regs)) {
 		regs_user->abi = perf_reg_abi(current);
 		regs_user->regs = regs;
-	} else if (current->mm) {
+	} else if (!(current->flags & PF_KTHREAD) && current->mm) {
 		perf_get_regs_user(regs_user, regs, regs_user_copy);
 	} else {
 		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;