[PATCH 0/2] ftrace: two fixes with func_probes handling

[PATCH 0/2] ftrace: two fixes with func_probes handling

[Naveen N. Rao]

Two patches addressing bugs in ftrace function probe handling. The first 
patch addresses a NULL pointer dereference reported by LTP tests, while 
the second one is a trivial patch to address a missing check for return 
value, found by code inspection.

- Naveen


Naveen N. Rao (2):
  ftrace: Fix NULL pointer dereference in t_probe_next()
  ftrace: Check for successful allocation of hash

 kernel/trace/ftrace.c | 9 +++++++++
 1 file changed, 9 insertions(+)

-- 
2.22.0

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

[Naveen N. Rao]

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.

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:
-- 
2.22.0

[PATCH 2/2] ftrace: Check for successful allocation of hash

[Naveen N. Rao]

In register_ftrace_function_probe(), we are not checking the return
value of alloc_and_copy_ftrace_hash(). The subsequent call to
ftrace_match_records() may end up dereferencing the same. Add a check to
ensure this doesn't happen.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 kernel/trace/ftrace.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 0791eafb693d..0d5f7d4a4936 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4328,6 +4328,11 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
 	old_hash = *orig_hash;
 	hash = alloc_and_copy_ftrace_hash(FTRACE_HASH_DEFAULT_BITS, old_hash);
 
+	if (!hash) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	ret = ftrace_match_records(hash, glob, strlen(glob));
 
 	/* Nothing found? */
-- 
2.22.0

Re: [PATCH 0/2] ftrace: two fixes with func_probes handling

[Naveen N. Rao]

Naveen N. Rao wrote:
> Two patches addressing bugs in ftrace function probe handling. The first 
> patch addresses a NULL pointer dereference reported by LTP tests, while 
> the second one is a trivial patch to address a missing check for return 
> value, found by code inspection.

Steven,
Can you please take a look at these patches?

Thanks,
Naveen

Re: [PATCH 0/2] ftrace: two fixes with func_probes handling

[Steven Rostedt]

On Thu, 08 Aug 2019 20:45:04 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> Naveen N. Rao wrote:
> > Two patches addressing bugs in ftrace function probe handling. The first 
> > patch addresses a NULL pointer dereference reported by LTP tests, while 
> > the second one is a trivial patch to address a missing check for return 
> > value, found by code inspection.  
> 
> Steven,
> Can you please take a look at these patches?

Thanks for the ping. Yes I will.

-- Steve

Re: [PATCH 0/2] ftrace: two fixes with func_probes handling

[Steven Rostedt]

On Thu, 08 Aug 2019 20:45:04 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> Naveen N. Rao wrote:
> > Two patches addressing bugs in ftrace function probe handling. The first 
> > patch addresses a NULL pointer dereference reported by LTP tests, while 
> > the second one is a trivial patch to address a missing check for return 
> > value, found by code inspection.  
> 
> Steven,
> Can you please take a look at these patches?
>

Sorry for the late reply, I've been traveling a lot lately. I'm looking
at these now. I'm trying to see how they triggered a bug.

-- Steve

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

[Steven Rostedt]

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:

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

[Steven Rostedt]

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


>  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:

OK, I added this, but I'm also adding this on top:

-- Steve

From 372e0d01da71c84dcecf7028598a33813b0d5256 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Date: Fri, 30 Aug 2019 16:30:01 -0400
Subject: [PATCH] ftrace: Check for empty hash and comment the race with
 registering probes

The race between adding a function probe and reading the probes that exist
is very subtle. It needs a comment. Also, the issue can also happen if the
probe has has the EMPTY_HASH as its func_hash.

Cc: stable@vger.kernel.org
Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/trace/ftrace.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 80beed2cf0da..6200a6fe10e3 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3096,7 +3096,11 @@ t_probe_next(struct seq_file *m, loff_t *pos)
 
 	hash = iter->probe->ops.func_hash->filter_hash;
 
-	if (!hash)
+	/*
+	 * A probe being registered may temporarily have an empty hash
+	 * and it's at the end of the func_probes list.
+	 */
+	if (!hash || hash == EMPTY_HASH)
 		return NULL;
 
 	size = 1 << hash->size_bits;
@@ -4324,6 +4328,10 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
 
 	mutex_unlock(&ftrace_lock);
 
+	/*
+	 * Note, there's a small window here that the func_hash->filter_hash
+	 * may be NULL or empty. Need to be carefule when reading the loop.
+	 */
 	mutex_lock(&probe->ops.func_hash->regex_lock);
 
 	orig_hash = &probe->ops.func_hash->filter_hash;
-- 
2.20.1

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

[Naveen N. Rao]

Steven Rostedt wrote:
> On Thu,  4 Jul 2019 20:04:41 +0530
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> 
> 
>>  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:
> 
> OK, I added this, but I'm also adding this on top:

Thanks, the additional comments do make this much clearer.

Regards,
Naveen