]> git.proxmox.com Git - mirror_ubuntu-eoan-kernel.git/commit
kthread, tracing: Don't expose half-written comm when creating kthreads
authorSnild Dolkow <snild@sony.com>
Thu, 26 Jul 2018 07:15:39 +0000 (09:15 +0200)
committerSteven Rostedt (VMware) <rostedt@goodmis.org>
Thu, 26 Jul 2018 13:59:33 +0000 (09:59 -0400)
commit3e536e222f2930534c252c1cc7ae799c725c5ff9
treea6b1cdba75d191d08974f90c88d3abd1ab99fe75
parent2519c1bbe38d7acacc9aacba303ca6f97482ed53
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            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@sony.com
Cc: stable@vger.kernel.org
Fixes: bc0c38d139ec7 ("ftrace: latency tracer infrastructure")
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Snild Dolkow <snild@sony.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
kernel/kthread.c