From: "Naveen N. Rao" naveen.n.rao@linux.vnet.ibm.com

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.

Link: http://lkml.kernel.org/r/05e021f757625cbbb006fad41380323dbe4e3b43.1562249521...

Cc: stable@vger.kernel.org Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array") Signed-off-by: Naveen N. Rao naveen.n.rao@linux.vnet.ibm.com Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/ftrace.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index eca34503f178..80beed2cf0da 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -3095,6 +3095,10 @@ 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: