[PATCH] 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>
---
 kernel/trace/trace.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5d2c5678b..bfa8e2d01 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8097,8 +8097,16 @@ static int tracing_err_log_open(struct inode *inode, struct file *file)
 		return ret;
 
 	/* If this file was opened for write, then erase contents */
-	if ((file->f_mode & FMODE_WRITE) && (file->f_flags & O_TRUNC))
+	if ((file->f_mode & FMODE_WRITE) && (file->f_flags & O_TRUNC)) {
 		clear_tracing_err_log(tr);
+		ret = seq_open(file, &tracing_err_log_seq_ops);
+		if (!ret) {
+			struct seq_file *m = file->private_data;
+			m->private = tr;
+		} else {
+			trace_array_put(tr);
+		}
+	}
 
 	if (file->f_mode & FMODE_READ) {
 		ret = seq_open(file, &tracing_err_log_seq_ops);

base-commit: 1ef6663a587ba3e57dc5065a477db1c64481eedd
-- 
2.25.1