tracing: Limit access to parser->buffer when trace_get_user failed

tracing: Limit access to parser->buffer when trace_get_user failed

When the length of the string written to set_ftrace_filter exceeds
FTRACE_BUFF_MAX, the following KASAN alarm will be triggered:

BUG: KASAN: slab-out-of-bounds in strsep+0x18c/0x1b0
Read of size 1 at addr ffff0000d00bd5ba by task ash/165

CPU: 1 UID: 0 PID: 165 Comm: ash Not tainted 6.16.0-g6bcdbd62bd56-dirty
Hardware name: linux,dummy-virt (DT)
Call trace:
 show_stack+0x34/0x50 (C)
 dump_stack_lvl+0xa0/0x158
 print_address_description.constprop.0+0x88/0x398
 print_report+0xb0/0x280
 kasan_report+0xa4/0xf0
 __asan_report_load1_noabort+0x20/0x30
 strsep+0x18c/0x1b0
 ftrace_process_regex.isra.0+0x100/0x2d8
 ftrace_regex_release+0x484/0x618
 __fput+0x364/0xa58
 ____fput+0x28/0x40
 task_work_run+0x154/0x278
 do_notify_resume+0x1f0/0x220
 el0_svc+0xec/0xf0
 el0t_64_sync_handler+0xa0/0xe8
 el0t_64_sync+0x1ac/0x1b0

The reason is that trace_get_user will fail when processing a string
longer than FTRACE_BUFF_MAX, but not set the end of parser->buffer to 0.
Then an OOB access will be triggered in ftrace_regex_release->
ftrace_process_regex->strsep->strpbrk. We can solve this problem by
limiting access to parser->buffer when trace_get_user failed.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/20250813040232.1344527-1-pulehui@huaweicloud.com
Fixes: 8c9af478c06b ("ftrace: Handle commands when closing set_ftrace_filter file")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>