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SubjectRe: [PATCH RESEND] kthread, tracing: Don't expose half-written comm when creating kthreads
On Tue, 24 Jul 2018 10:17:37 +0200
Snild Dolkow <snild@sony.com> wrote:

> On 07/23/2018 06:41 PM, Steven Rostedt wrote:
> > On Mon, 23 Jul 2018 17:49:36 +0200
> > Snild Dolkow <snild@sony.com> 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

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