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Subject[PATCH 5.2 075/143] ftrace: Fix NULL pointer dereference in t_probe_next()
From: Naveen N. Rao <>

commit 7bd46644ea0f6021dc396a39a8bfd3a58f6f1f9f upstream.

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]
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 ( is part of ftrace stress tests
and the crash happens when the test does 'cat

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

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.


Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
Signed-off-by: Naveen N. Rao <>
Signed-off-by: Steven Rostedt (VMware) <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/trace/ftrace.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3095,6 +3095,10 @@ t_probe_next(struct seq_file *m, loff_t
hnd = &iter->probe_entry->hlist;

hash = iter->probe->ops.func_hash->filter_hash;
+ if (!hash)
+ return NULL;
size = 1 << hash->size_bits;


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