hi
On Tue, Apr 25, 2023 at 6:13 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Apr 24, 2023 at 02:55:11PM -0400, Joel Fernandes wrote:
> > This is amazing debugging Boqun, like a boss! One comment below:
> >
> > > > > Or something simple I haven't thought of? :)
> > > >
> > > > At what points can r13 change? Only when some particular functions are
> > > > called?
> > > >
> > >
> > > r13 is the local paca:
> > >
> > > register struct paca_struct *local_paca asm("r13");
> > >
> > > , which is a pointer to percpu data.
> > >
> > > So if a task schedule from one CPU to anotehr CPU, the value gets
> > > changed.
> >
> > It appears the whole issue, per your analysis, is that the stack
> > checking code in gcc should not cache or alias r13, and must read its
> > most up-to-date value during stack checking, as its value may have
> > changed during a migration to a new CPU.
> >
> > Did I get that right?
> >
> > IMO, even without a reproducer, gcc on PPC should just not do that,
> > that feels terribly broken for the kernel. I wonder what clang does,
> > I'll go poke around with compilerexplorer after lunch.
> >
> > Adding +Peter Zijlstra as well to join the party as I have a feeling
> > he'll be interested. ;-)
>
> I'm a little confused; the way I understand the whole stack protector
> thing to work is that we push a canary on the stack at call and on
> return check it is still valid. Since in general tasks randomly migrate,
> the per-cpu validation canary should be the same on all CPUs.
>
> Additionally, the 'new' __srcu_read_{,un}lock_nmisafe() functions use
> raw_cpu_ptr() to get 'a' percpu sdp, preferably that of the local cpu,
> but no guarantees.
>
> Both cases use r13 (paca) in a racy manner, and in both cases it should
> be safe.
New test results today: both gcc build from git (git clone
git://gcc.gnu.org/git/gcc.git) and Ubuntu 22.04 gcc-12.1.0
are immune from the above issue. We can see the assembly code on
http://140.211.169.189/0425/srcu_gp_start_if_needed-gcc-12.txt
while
Both native gcc on PPC vm (gcc version 9.4.0), and gcc cross compiler
on my x86 laptop (gcc version 10.4.0) will reproduce the bug.
