linux-kernel - Re: [PATCH 0/2] tracing: Detect unsafe dereferencing of pointers from trace events

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Message-ID: <20210302082355.GA8633@nchen>
Date:   Tue, 2 Mar 2021 16:23:55 +0800
From:   Peter Chen <peter.chen@...nel.org>
To:     Pawel Laszczak <pawell@...ence.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jacob Wen <jian.w.wen@...cle.com>,
        Felipe Balbi <balbi@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 0/2] tracing: Detect unsafe dereferencing of pointers
 from trace events

On 21-03-01 05:27:04, Pawel Laszczak wrote:
> 
> + Peter Chen - Maintainer of CDNS3 driver
> 
> >
> >[ Resending with an address that should work for Felipe ]
> >
> >On Sat, 27 Feb 2021 14:18:02 -0500
> >Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> >> On Fri, 26 Feb 2021 14:21:00 -0800
> >> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >>
> >> > On Fri, Feb 26, 2021 at 11:07 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >> > >
> >> > > The first patch scans the print fmts of the trace events looking for
> >> > > dereferencing pointers from %p*, and making sure that they refer back
> >> > > to the trace event itself.
> >> > >
> >> > > The second patch handles strings "%s" [..]
> >> >
> >> > Doing this at runtime really feels like the wrong thing to do.
> >> >
> >> > It won't even protect us from what happened - people like me and
> >> > Andrew won't even run those tracepoints in the first place, so we
> >> > won't notice.
> >> >
> >> > It really would be much better in every respect to have this done by
> >> > checkpatch, I think.
> >>
> >> And after fixing the parsing to not trigger false positives, an
> >> allyesconfig boot found this:
> >>
> >> event cdns3_gadget_giveback has unsafe dereference of argument 11
> >> print_fmt: "%s: req: %p, req buff %p, length: %u/%u %s%s%s, status: %d, trb: [start:%d, end:%d: virt addr %pa], flags:%x SID: %u",
> >__get_str(name), REC->req, REC->buf,
> >>  REC->actual, REC->length, REC->zero ? "Z" : "z", REC->short_not_ok ? "S" : "s", REC->no_interrupt ? "I" : "i", REC->status, REC-
> >>start_trb, REC->end_trb, REC->start_trb_addr, REC->flags, RE
> >> C->stream_id
> >>
> >> (as the above is from a trace event class, it triggered for every event
> >> in that class).
> >>
> >> As it looks like it uses %pa which IIUC from the printk code, it
> >> dereferences the pointer to find it's virtual address. The event has
> >> this as the field:
> >>
> >>                 __field(struct cdns3_trb *, start_trb_addr)
> >>
> >> Assigns it with:
> >>
> >>                 __entry->start_trb_addr = req->trb;
> >>
> >> And prints that with %pa, which will dereference pointer at the time of
> >> reading, where the address in question may no longer be around. That
> >> looks to me as a potential bug.

Steven, thanks for reporting. Do you mind sending patch to fix it?
If you have no time to do it, I will do it later.

-- 

Thanks,
Peter Chen