[PATCH v2] trace: fix null pointer dereference in tracing_err_log_open()

[Mateusz Stachyra]

Fix an issue in function 'tracing_err_log_open'.
The function doesn't call 'seq_open' if file is opened only with
write permissions, which results in 'file->private_data' being left at null.
If we then use 'lseek' on that opened file, 'seq_lseek' dereferences
'file->private_data' in 'mutex_lock(&m->lock)', resulting in a Kernel panic.
Writing to this node requires root privilages, therefore this bug
has very little security impact.

Tracefs node: /sys/kernel/tracing/error_log

Example Kernel panic:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038
Call trace:
 mutex_lock+0x30/0x110
 seq_lseek+0x34/0xb8
 __arm64_sys_lseek+0x6c/0xb8
 invoke_syscall+0x58/0x13c
 el0_svc_common+0xc4/0x10c
 do_el0_svc+0x24/0x98
 el0_svc+0x24/0x88
 el0t_64_sync_handler+0x84/0xe4
 el0t_64_sync+0x1b4/0x1b8
Code: d503201f aa0803e0 aa1f03e1 aa0103e9 (c8e97d02)
---[ end trace 561d1b49c12cf8a5 ]---
Kernel panic - not syncing: Oops: Fatal exception

Signed-off-by: Mateusz Stachyra <m.stachyra@xxxxxxxxxxx>
Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b04f52e7c..4529e264c 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8146,7 +8146,7 @@ static const struct file_operations tracing_err_log_fops = {
 	.open           = tracing_err_log_open,
 	.write		= tracing_err_log_write,
 	.read           = seq_read,
-	.llseek         = seq_lseek,
+	.llseek         = tracing_lseek,
 	.release        = tracing_err_log_release,
 };
 
-- 
2.25.1