Commit f9574568 authored by Snild Dolkow's avatar Snild Dolkow Committed by Greg Kroah-Hartman

kthread, tracing: Don't expose half-written comm when creating kthreads

commit 3e536e22 upstream.

There is a window for racing when printing directly to task->comm,
allowing other threads to see a non-terminated string. The vsnprintf
function fills the buffer, counts the truncated chars, then finally
writes the \0 at the end.

	creator                     other
	vsnprintf:
	  fill (not terminated)
	  count the rest            trace_sched_waking(p):
	  ...                         memcpy(comm, p->comm, TASK_COMM_LEN)
	  write \0

The consequences depend on how 'other' uses the string. In our case,
it was copied into the tracing system's saved cmdlines, a buffer of
adjacent TASK_COMM_LEN-byte buffers (note the 'n' where 0 should be):

	crash-arm64> x/1024s savedcmd->saved_cmdlines | grep 'evenk'
	0xffffffd5b3818640:     "irq/497-pwr_evenkworker/u16:12"

...and a strcpy out of there would cause stack corruption:

	[224761.522292] Kernel panic - not syncing: stack-protector:
	    Kernel stack is corrupted in: ffffff9bf9783c78

	crash-arm64> kbt | grep 'comm\|trace_print_context'
	#6  0xffffff9bf9783c78 in trace_print_context+0x18c(+396)
	      comm (char [16]) =  "irq/497-pwr_even"

	crash-arm64> rd 0xffffffd4d0e17d14 8
	ffffffd4d0e17d14:  2f71726900000000 5f7277702d373934   ....irq/497-pwr_
	ffffffd4d0e17d24:  726f776b6e657665 3a3631752f72656b   evenkworker/u16:
	ffffffd4d0e17d34:  f9780248ff003231 cede60e0ffffff9b   12..H.x......`..
	ffffffd4d0e17d44:  cede60c8ffffffd4 00000fffffffffd4   .....`..........

The workaround in e09e2867 (use strlcpy in __trace_find_cmdline) was
likely needed because of this same bug.

Solved by vsnprintf:ing to a local buffer, then using set_task_comm().
This way, there won't be a window where comm is not terminated.

Link: http://lkml.kernel.org/r/20180726071539.188015-1-snild@sony.com

Cc: stable@vger.kernel.org
Fixes: bc0c38d1 ("ftrace: latency tracer infrastructure")
Reviewed-by: 's avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: 's avatarSnild Dolkow <snild@sony.com>
Signed-off-by: 's avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 10419b0c
......@@ -311,8 +311,14 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
task = create->result;
if (!IS_ERR(task)) {
static const struct sched_param param = { .sched_priority = 0 };
char name[TASK_COMM_LEN];
vsnprintf(task->comm, sizeof(task->comm), namefmt, args);
/*
* task is already visible to other tasks, so updating
* COMM must be protected.
*/
vsnprintf(name, sizeof(name), namefmt, args);
set_task_comm(task, name);
/*
* root may have changed our (kthreadd's) priority or CPU mask.
* The kernel thread should not inherit these properties.
......
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