Re: [PATCH RESEND] kthread, tracing: Don't expose half-written comm when creating kthreads

[Steven Rostedt]

On Tue, 24 Jul 2018 10:17:37 +0200
Snild Dolkow <snild@xxxxxxxx> wrote:

> On 07/23/2018 06:41 PM, Steven Rostedt wrote:
> > On Mon, 23 Jul 2018 17:49:36 +0200
> > Snild Dolkow <snild@xxxxxxxx> wrote:  
> >> Any issues with the commit message? Reading it back again now, it doesn't
> >> seem quite as clear as when I wrote it.  
> > 
> > Yeah, I think it does need some updates:
> >   
> >> There was a window for racing when task->comm was being written. The  
> > 
> > It would be nice to explain this race window in more detail.
> >   
> I hope the following is more clear:
> 
> kthread, tracing: Don't expose half-written comm when creating kthreads
> 
> 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                read/use comm

I think it would be better to state what was reading the comm. Like

					trace_sched_waking(p)
					  memcpy(comm, p->comm, TASK_COMM_LEN)

But the rest looks fine.

-- Steve


> 	  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.
> 
> 
> //Snild