Re: [PATCH] ftrace: Fix use-after-free issue in ftrace_location()

[Zheng Yejian]

On 2024/4/10 23:28, Steven Rostedt wrote:
> On Mon, 1 Apr 2024 20:55:43 +0800
> Zheng Yejian <zhengyejian1@xxxxxxxxxx> wrote:
>
> > KASAN reports a bug:
> >
> >    BUG: KASAN: use-after-free in ftrace_location+0x90/0x120
> >    Read of size 8 at addr ffff888141d40010 by task insmod/424
> >    CPU: 8 PID: 424 Comm: insmod Tainted: G        W          6.9.0-rc2+ #213
> >    [...]
> >    Call Trace:
> >     <TASK>
> >     dump_stack_lvl+0x68/0xa0
> >     print_report+0xcf/0x610
> >     kasan_report+0xb5/0xe0
> >     ftrace_location+0x90/0x120
> >     register_kprobe+0x14b/0xa40
> >     kprobe_init+0x2d/0xff0 [kprobe_example]
> >     do_one_initcall+0x8f/0x2d0
> >     do_init_module+0x13a/0x3c0
> >     load_module+0x3082/0x33d0
> >     init_module_from_file+0xd2/0x130
> >     __x64_sys_finit_module+0x306/0x440
> >     do_syscall_64+0x68/0x140
> >     entry_SYSCALL_64_after_hwframe+0x71/0x79
> >
> > The root cause is that when lookup_rec() is lookuping ftrace record of
> > an address in some module, and at the same time in ftrace_release_mod(),
> > the memory that saving ftrace records has been freed as that module is
> > being deleted.
> >
> >    register_kprobes() {
> >      check_kprobe_address_safe() {
> >        arch_check_ftrace_location() {
> >          ftrace_location() {
> >            lookup_rec()  // access memory that has been freed by
> >                          // ftrace_release_mod() !!!
> >
> > It seems that the ftrace_lock is required when lookuping records in
> > ftrace_location(), so is ftrace_location_range().
> >
> > Fixes: ae6aa16fdc16 ("kprobes: introduce ftrace based optimization")
> > Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
> > ---
> >   kernel/trace/ftrace.c | 28 ++++++++++++++++++----------
> >   1 file changed, 18 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index da1710499698..838d175709c1 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -1581,7 +1581,7 @@ static struct dyn_ftrace *lookup_rec(unsigned long start, unsigned long end)
> >   }
> >   
> >   /**
> > - * ftrace_location_range - return the first address of a traced location
> > + * ftrace_location_range_locked - return the first address of a traced location
> >    *	if it touches the given ip range
> >    * @start: start of range to search.
> >    * @end: end of range to search (inclusive). @end points to the last byte
> > @@ -1592,7 +1592,7 @@ static struct dyn_ftrace *lookup_rec(unsigned long start, unsigned long end)
> >    * that is either a NOP or call to the function tracer. It checks the ftrace
> >    * internal tables to determine if the address belongs or not.
> >    */
> > -unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> > +static unsigned long ftrace_location_range_locked(unsigned long start, unsigned long end)
> >   {
> >   	struct dyn_ftrace *rec;
> >   
> > @@ -1603,6 +1603,17 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> >   	return 0;
> >   }
> >   
> > +unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> > +{
> > +	unsigned long loc;
> > +
> > +	mutex_lock(&ftrace_lock);
> > +	loc = ftrace_location_range_locked(start, end);
> > +	mutex_unlock(&ftrace_lock);
>
> I'm not so sure we can take a mutex in all places that call this function.
>
> What about using RCU?
>
> 	rcu_read_lock();
> 	loc = ftrace_location_range_rcu(start, end);
> 	rcu_read_unlock();
>
> Then in ftrace_release_mod() we can have:
>
>   out_unlock:
> 	mutex_unlock();
>
> 	/* Need to synchronize with ftrace_location() */
> 	if (tmp_pages)
> 		synchronize_rcu();
>
> -- Steve

Yes, it is better to use RCU, I'll do it in v2.

--
Thanks
Zheng Yejian

> > +
> > +	return loc;
> > +}
> > +
> >   /**
> >    * ftrace_location - return the ftrace location
> >    * @ip: the instruction pointer to check
> > @@ -1614,25 +1625,22 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> >    */
> >   unsigned long ftrace_location(unsigned long ip)
> >   {
> > -	struct dyn_ftrace *rec;
> > +	unsigned long loc;
> >   	unsigned long offset;
> >   	unsigned long size;
> >   
> > -	rec = lookup_rec(ip, ip);
> > -	if (!rec) {
> > +	loc = ftrace_location_range(ip, ip);
> > +	if (!loc) {
> >   		if (!kallsyms_lookup_size_offset(ip, &size, &offset))
> >   			goto out;
> >   
> >   		/* map sym+0 to __fentry__ */
> >   		if (!offset)
> > -			rec = lookup_rec(ip, ip + size - 1);
> > +			loc = ftrace_location_range(ip, ip + size - 1);
> >   	}
> >   
> > -	if (rec)
> > -		return rec->ip;
> > -
> >   out:
> > -	return 0;
> > +	return loc;
> >   }
> >   
> >   /**