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

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Message-ID: <BYAPR07MB5381637CFA12C3988CA06550DD9A9@BYAPR07MB5381.namprd07.prod.outlook.com>
Date:   Mon, 1 Mar 2021 05:27:04 +0000
From:   Pawel Laszczak <pawell@...ence.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     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>,
        "peter.chen@...nel.org" <peter.chen@...nel.org>
Subject: RE: [PATCH 0/2] tracing: Detect unsafe dereferencing of pointers from
 trace events


+ 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.
>>
>> [ Cc'd the people responsible for that code. ]
>>
>> -- Steve