ftrace: Fix null pointer dereference in ftrace_add_mod()

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

ftrace: Fix null pointer dereference in ftrace_add_mod()

The @ftrace_mod is allocated by kzalloc(), so both the members {prev,next}

of @ftrace_mode->list are NULL, it's not a valid state to call list_del().

If kstrdup() for @ftrace_mod->{func|module} fails, it goes to @out_free

tag and calls free_ftrace_mod() to destroy @ftrace_mod, then list_del()

will write prev->next and next->prev, where null pointer dereference

happens.

BUG: kernel NULL pointer dereference, address: 0000000000000008

Oops: 0002 [#1] PREEMPT SMP NOPTI

Call Trace:

<TASK>

ftrace_mod_callback+0x20d/0x220

? do_filp_open+0xd9/0x140

ftrace_process_regex.isra.51+0xbf/0x130

ftrace_regex_write.isra.52.part.53+0x6e/0x90

vfs_write+0xee/0x3a0

? __audit_filter_op+0xb1/0x100

? auditd_test_task+0x38/0x50

ksys_write+0xa5/0xe0

do_syscall_64+0x3a/0x90

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Kernel panic - not syncing: Fatal exception

So call INIT_LIST_HEAD() to initialize the list member to fix this issue.