Re: [PATCH 1/2] ftrace: Fix NULL pointer dereference in t_probe_next()

[Steven Rostedt <rostedt@goodmis.org>]

On Thu,  4 Jul 2019 20:04:41 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> LTP testsuite on powerpc results in the below crash:
> 
>   Unable to handle kernel paging request for data at address 0x00000000
>   Faulting instruction address: 0xc00000000029d800
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   LE SMP NR_CPUS=2048 NUMA PowerNV
>   ...
>   CPU: 68 PID: 96584 Comm: cat Kdump: loaded Tainted: G        W
>   NIP:  c00000000029d800 LR: c00000000029dac4 CTR: c0000000001e6ad0
>   REGS: c0002017fae8ba10 TRAP: 0300   Tainted: G        W
>   MSR:  9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 28022422  XER: 20040000
>   CFAR: c00000000029d90c DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0
>   ...
>   NIP [c00000000029d800] t_probe_next+0x60/0x180
>   LR [c00000000029dac4] t_mod_start+0x1a4/0x1f0
>   Call Trace:
>   [c0002017fae8bc90] [c000000000cdbc40] _cond_resched+0x10/0xb0 (unreliable)
>   [c0002017fae8bce0] [c0000000002a15b0] t_start+0xf0/0x1c0
>   [c0002017fae8bd30] [c0000000004ec2b4] seq_read+0x184/0x640
>   [c0002017fae8bdd0] [c0000000004a57bc] sys_read+0x10c/0x300
>   [c0002017fae8be30] [c00000000000b388] system_call+0x5c/0x70
> 
> The test (ftrace_set_ftrace_filter.sh) is part of ftrace stress tests
> and the crash happens when the test does 'cat
> $TRACING_PATH/set_ftrace_filter'.
> 
> The address points to the second line below, in t_probe_next(), where
> filter_hash is dereferenced:
>   hash = iter->probe->ops.func_hash->filter_hash;
>   size = 1 << hash->size_bits;
> 
> This happens due to a race with register_ftrace_function_probe(). A new
> ftrace_func_probe is created and added into the func_probes list in
> trace_array under ftrace_lock. However, before initializing the filter,
> we drop ftrace_lock, and re-acquire it after acquiring regex_lock. If
> another process is trying to read set_ftrace_filter, it will be able to
> acquire ftrace_lock during this window and it will end up seeing a NULL
> filter_hash.
> 
> Fix this by just checking for a NULL filter_hash in t_probe_next(). If
> the filter_hash is NULL, then this probe is just being added and we can
> simply return from here.

Hmm, this is very subtle. I'll take a deeper look at this to see if we
can keep the race from happening.

Thanks!

-- Steve

> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
>  kernel/trace/ftrace.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 7b037295a1f1..0791eafb693d 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3093,6 +3093,10 @@ t_probe_next(struct seq_file *m, loff_t *pos)
>  		hnd = &iter->probe_entry->hlist;
>  
>  	hash = iter->probe->ops.func_hash->filter_hash;
> +
> +	if (!hash)
> +		return NULL;
> +
>  	size = 1 << hash->size_bits;
>  
>   retry: