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

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Date:   Sat, 27 Feb 2021 19:08:31 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     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>,
        Pawel Laszczak <pawell@...ence.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


[ 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