On 11/11/18 8:49 PM, Ben Hutchings wrote: > 3.16.61-rc1 review patch. If anyone has any objections, please let me know. > > ------------------ > > From: Snild Dolkow <snild@xxxxxxxx> > > commit 3e536e222f2930534c252c1cc7ae799c725c5ff9 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 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. > > Link: http://lkml.kernel.org/r/20180726071539.188015-1-snild@xxxxxxxx > > Fixes: bc0c38d139ec7 ("ftrace: latency tracer infrastructure") > Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > Signed-off-by: Snild Dolkow <snild@xxxxxxxx> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > [bwh: Backported to 3.16: adjust context] > Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> > --- > kernel/kthread.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -309,10 +309,16 @@ struct task_struct *kthread_create_on_no > if (!IS_ERR(task)) { > static const struct sched_param param = { .sched_priority = 0 }; > va_list args; > + char name[TASK_COMM_LEN]; > > + /* > + * task is already visible to other tasks, so updating > + * COMM must be protected. > + */ > va_start(args, namefmt); > - vsnprintf(task->comm, sizeof(task->comm), namefmt, args); > + vsnprintf(name, sizeof(name), namefmt, args); > va_end(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. > Reviewed-by: Snild Dolkow <snild@xxxxxxxx>
