tracing: Fix null pointer dereference in tracing_err_log_open()

In the Linux kernel, the following vulnerability has been resolved:

tracing: Fix null pointer dereference in tracing_err_log_open()

Fix an issue in function 'tracing_err_log_open'.

The function doesn't call 'seq_open' if the file is opened only with

write permissions, which results in 'file->private_data' being left as 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 privileges, 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