Re: [PATCH v2 28/28] cfi: Use RCU while invoking __module_address().

[Elliot Berman]

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)
 - Your series as-is (no recurisve loop)
 - Replace with guard(rcu)() (no recursive loop)

Whether you'd like to stick with the current patch or replace with
guard(rcu)():

Tested-by: Elliot Berman <elliot.berman@xxxxxxxxxxxxxxxx>  # sm8650-qrd

-

I don't know why I didn't mention steps to reproduce, even for my own
benefit. Lesson learned :)

Here are the steps to reproduce; you'll need a system with support for
CFI: qemu arm64 probably does the trick and you'll need clang>=16. I'm
happy to help test future revisions of this series since I have the
setup all done.

```
modprobe -a dummy_stm stm_ftrace stm_p_basic
mkdir -p /sys/kernel/config/stp-policy/dummy_stm.0.my-policy/default
echo function > /sys/kernel/tracing/current_tracer
echo 1 > /sys/kernel/tracing/tracing_on
echo dummy_stm.0 > /sys/class/stm_source/ftrace/stm_source_link
```

The trace buffer should not be full of stm calls due to the recursive
loop as mentioned in my original commit.


Regards,
Elliot Berman

> Cc: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: llvm@xxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  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);
> -- 
> 2.45.2
>