[PATCH 3.17-rc4 v2 1/2] trace: kdb: Fix kernel panic during ftdump

From: Daniel Thompson
Date: Wed Sep 24 2014 - 06:33:30 EST

Currently kdb's ftdump command unconditionally crashes due to a null
pointer de-reference whenever the command is run. This in turn causes
the kernel to panic.

The abridged stacktrace (gathered with ARCH=arm) is:
--- cut here ---
[<c09535ac>] (panic) from [<c02132dc>] (die+0x264/0x440)
[<c02132dc>] (die) from [<c0952eb8>]
[<c0952eb8>] (__do_kernel_fault.part.11) from [<c021f954>]
[<c021f954>] (do_page_fault) from [<c020846c>] (do_DataAbort+0x48/0xac)

[<c020846c>] (do_DataAbort) from [<c0213c58>] (__dabt_svc+0x38/0x60)
Exception stack(0xc0deba88 to 0xc0debad0)
ba80: e8c29180 00000001 e9854304 e9854300 c0f567d8
baa0: 00000000 00000000 00000000 c0f117b8 c0e3a3c0 c0debb0c 00000000
bac0: 0000672e c02f4d60 60000193 ffffffff
[<c0213c58>] (__dabt_svc) from [<c02f4d60>] (kdb_ftdump+0x1e4/0x3d8)
[<c02f4d60>] (kdb_ftdump) from [<c02ce328>] (kdb_parse+0x2b8/0x698)
[<c02ce328>] (kdb_parse) from [<c02ceef0>] (kdb_main_loop+0x52c/0x784)
[<c02ceef0>] (kdb_main_loop) from [<c02d1b0c>] (kdb_stub+0x238/0x490)
--- cut here ---

The NULL deref occurs due to the initialized use of struct trace_iter's
buffer_iter member.

This patch solves this by providing a collection of ring_buffer_iter(s)
and using this to initialize buffer_iter. Note that static allocation
is used solely because the trace_iter itself is also static allocated.
Static allocation also means that we have to NULL-ify the pointer during
cleanup to avoid use-after-free problems.

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/trace/trace_kdb.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index bd90e1b..8faa7ce 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -20,10 +20,12 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
/* use static because iter can be a bit big for the stack */
static struct trace_iterator iter;
+ static struct ring_buffer_iter *buffer_iter[CONFIG_NR_CPUS];
unsigned int old_userobj;
int cnt = 0, cpu;

+ iter.buffer_iter = buffer_iter;

for_each_tracing_cpu(cpu) {
atomic_inc(&per_cpu_ptr(iter.trace_buffer->data, cpu)->disabled);
@@ -86,9 +88,12 @@ out:
atomic_dec(&per_cpu_ptr(iter.trace_buffer->data, cpu)->disabled);

- for_each_tracing_cpu(cpu)
- if (iter.buffer_iter[cpu])
+ for_each_tracing_cpu(cpu) {
+ if (iter.buffer_iter[cpu]) {
+ iter.buffer_iter[cpu] = NULL;
+ }
+ }


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