linux-kernel - Re: [PATCH v2 28/28] cfi: Use RCU while invoking __module

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Message-ID: <20250103101357180-0800.eberman@hu-eberman-lv.qualcomm.com>
Date: Mon, 6 Jan 2025 10:00:47 -0800
From: Elliot Berman <elliot.berman@....qualcomm.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
        Daniel Gomez <da.gomez@...sung.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Pavlu <petr.pavlu@...e.com>, Thomas Gleixner <tglx@...utronix.de>,
        Elliot Berman <quic_eberman@...cinc.com>, Kees Cook <kees@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 28/28] cfi: Use RCU while invoking __module_address().

On Thu, Jan 02, 2025 at 04:24:22PM -0800, Sami Tolvanen wrote:
> Hi Elliot,
> 
> On Mon, Dec 30, 2024 at 7:33 PM Elliot Berman
> <elliot.berman@....qualcomm.com> wrote:
> >
> > On Fri, Dec 20, 2024 at 06:41:42PM +0100, Sebastian Andrzej Siewior wrote:
> > > __module_address() can be invoked within a RCU section, there is no
> > > requirement to have preemption disabled.
> > >
> > > I'm not sure if using rcu_read_lock() will introduce the regression that
> > > has been fixed in commit 14c4c8e41511a ("cfi: Use
> > > rcu_read_{un}lock_sched_notrace").
> > >
> >
> > You can replace the rcu_read_lock_sched_notrace() with guard(rcu)().
> > Regular rcu lock doesn't generate function traces, so the recursive loop
> > isn't possible.
> >
> > I've tested:
> >  - the current kernel (no recursive loop)
> >  - Revert back to rcu_read_lock_sched() (fails)
> 
> Which kernel version did you test? I assume something pre-KCFI as
> arm64 doesn't use this code since v6.1.
> 

Ah, thanks for calling me out. I dug a bit more, I thought I was looking
at a recursive loop in the ftrace buffers, but was actually the expected
behavior. When I tested on the other configurations, the stm dummy
driver hadn't kicked in yet by the time I looked at the ftrace. Indeed,
this function code is not used on arm64.

I experimented with an x86 build as well and I was able to get the hang
I remember seeing after some tweaks to force a CFI failure. Still,
guard(rcu)() is okay by me :)

> >  - Your series as-is (no recurisve loop)
> 
> Note that this patch only adds a comment to is_module_cfi_trap(), so I
> wouldn't expect a functional change.
> 

Agreed I wouldn't expect it to make any issues; I mentioned it for
completeness sake.

Regards,
Elliot