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    SubjectRe: [PATCH 3.16 298/366] kthread, tracing: Don't expose half-written comm when creating kthreads
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    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@sony.com>
    >
    > 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@sony.com
    >
    > 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>
    > [bwh: Backported to 3.16: adjust context]
    > Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    > ---
    > 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@sony.com>

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