Messages in this thread | | | Date | Thu, 24 Jan 2019 12:03:43 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH v2 1/3] tracing: uprobes: Re-enable $comm support for uprobe events |
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On Wed, 23 Jan 2019 21:09:56 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, 24 Jan 2019 10:43:22 +0900 > Masami Hiramatsu <mhiramat@kernel.org> wrote: > > > > > kernel/trace/trace_uprobe.c | 15 +++++++++++++-- > > > > 1 file changed, 13 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c > > > > index 3a1d5ab6b4ba..b07e498ccbc6 100644 > > > > --- a/kernel/trace/trace_uprobe.c > > > > +++ b/kernel/trace/trace_uprobe.c > > > > @@ -156,7 +156,10 @@ fetch_store_string(unsigned long addr, void *dest, void *base) > > > > if (unlikely(!maxlen)) > > > > return -ENOMEM; > > > > > > > > - ret = strncpy_from_user(dst, src, maxlen); > > > > + if (addr == (unsigned long)current->comm) > > > > + ret = strlcpy(dst, current->comm, maxlen); > > > > > > As user space (although only root) defines the size of the event being > > > stored, and we could trick addr to be current->comm (although > > > difficult), we could possibly leak data if maxlen is > TASK_COMM_LEN. I > > > would feel better if we tested maxlen against TASK_COMM_LEN in this > > > case. > > > > > > if (maxlen > TASK_COMM_LEN) > > > maxlen = TASK_COMM_LEN; > > > > > > Or if we don't think it can happen, add a WARN_ON(maxlen > > > > TASK_COMM_LEN). > > > > Hmm, I thought current->comm is null terminated, isn't it? > > Yes it is. I was thinking it was a memcpy (I blame conference fatigue ;-) > > > Anyway, if user can specify current->comm, he must be able to specify > > current->comm + TASK_COMM_LEN too by kprobe_events. > > Moreover, it can leak any data in kernel... > > But this is for uprobes, which I why I was concerned.
OK, what we will leak by this, is the address of current->comm to root user for a specific application running instance. Since they can try to write a data on a register and trace it like "+0(%ax):string" and if rax register has hit the current->comm, uprobe event will record comm, but other case, it will show "(fault)". So they can trial and error on that (but leaking just a temporary task instance address).
If you concern this, I can change it to store a special fixed value, like (unsigned long)-EINVAL, instead of current->comm address. :-)
> > And also, maxlen is calculated by fetch_store_strlen, right before > > this has been called. > > > > I rather concern the case that if we have shorter size of maxlen than > > current->comm. Would we better show "(fault)" or tail-cut name ? > > (of course this is very difficult to happen, since the length is > > already checked.) > > Actually, it would still be OK, as strlcpy does guarantee to be nul > terminated as long as it's greater than zero.
Ah, I meant that we can record a shorter name, like "shutdown" -> "sh".
> Hmm, strlcpy doesn't pad the rest if what is written is shorter than > what is allocated. Could that leak data?
Even though, there should be a data that had been recorded on the ring buffer.
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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