I didn't receive any replies on this, and it doesn't seem to have made it into the latest 3.18 or 4.4 releases. Previously sent on Aug 23rd: https://lists.linaro.org/pipermail/linux-stable-mirror/2018-August/056347.ht...

I assume I sent it wrong; maybe missing keywords or recipients? If anyone can tell me what I missed, I'd appreciate it.

Trivial backport of commit 3e536e222f293053; newer kernels have simply moved the vararg macros.

Testing: 3.18 and 4.4 booted OK in qemu.

8------------------------------------------------------8<

[backport of commit 3e536e222f293053 from mainline]

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 e09e28671 (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.

Cc: stable@vger.kernel.org Fixes: bc0c38d139ec7 ("ftrace: latency tracer infrastructure") Reviewed-by: Steven Rostedt (VMware) rostedt@goodmis.org [backported to 3.18 / 4.4 by Snild] Signed-off-by: Snild Dolkow snild@sony.com --- kernel/kthread.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c index 850b255..ac6849e 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -313,10 +313,16 @@ 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]; va_list args;

va_start(args, namefmt); - 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); va_end(args); /* * root may have changed our (kthreadd's) priority or CPU mask.