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

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Message-ID: <CABCJKudYd9HiOX=d9ghVB+v37JEPJoKX_XHyopF=S7FYFabQSQ@mail.gmail.com>
Date: Thu, 2 Jan 2025 15:59:18 -0800
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Petr Pavlu <petr.pavlu@...e.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>, 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().

Hi Petr,

On Mon, Dec 30, 2024 at 1:13 PM Petr Pavlu <petr.pavlu@...e.com> wrote:
>
> On 12/20/24 18:41, 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").
> >
> > Cc: Elliot Berman <quic_eberman@...cinc.com>
> > Cc: Kees Cook <kees@...nel.org>
> > Cc: Nathan Chancellor <nathan@...nel.org>
> > Cc: Sami Tolvanen <samitolvanen@...gle.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: llvm@...ts.linux.dev
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  kernel/cfi.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/cfi.c b/kernel/cfi.c
> > index 08caad7767176..c8f2b5a51b2e6 100644
> > --- a/kernel/cfi.c
> > +++ b/kernel/cfi.c
> > @@ -71,6 +71,10 @@ static bool is_module_cfi_trap(unsigned long addr)
> >       struct module *mod;
> >       bool found = false;
> >
> > +     /*
> > +      * XXX this could be RCU protected but would it introcude the regression
> > +      * fixed in 14c4c8e41511a ("cfi: Use rcu_read_{un}lock_sched_notrace")
> > +      */
> >       rcu_read_lock_sched_notrace();
> >
> >       mod = __module_address(addr);
>
> I think that since 89245600941e ("cfi: Switch to -fsanitize=kcfi"), this
> can be a call to rcu_read_lock_sched(), or in your case rcu_read_lock().
> The recursive case where __cfi_slowpath_diag() could end up calling
> itself is no longer present, as all that logic is gone. I then don't see
> another reason this should use the notrace variant.
>
> @Sami, could you please confirm this?

Switching is_module_cfi_trap() to use rcu_read_lock() in this series
should be fine. KCFI checks don't perform function calls, so there's
no risk of recursion, and this function is only called during the
error handling path.

Sami