linux-kernel - Re: [PATCH v5 12/12] user_events: Validate user payloads for size and null termination

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Date:   Thu, 18 Nov 2021 11:45:26 -0800
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, linux-trace-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 12/12] user_events: Validate user payloads for size
 and null termination

On Wed, Nov 17, 2021 at 08:52:20PM -0500, Steven Rostedt wrote:
> On Tue, 16 Nov 2021 13:11:50 -0800
> Beau Belgrave <beaub@...ux.microsoft.com> wrote:
> 
> > On Mon, Nov 15, 2021 at 04:50:47PM -0800, Beau Belgrave wrote:
> > > +static int user_event_validate(struct user_event *user, void *data, int len)
> > > +{
> > > +	struct list_head *head = &user->validators;
> > > +	struct user_event_validator *validator;
> > > +	void *pos, *end = data + len;
> > > +	u16 *val, offset, size;
> > > +
> > > +	list_for_each_entry(validator, head, link) {
> > > +		pos = data + validator->offset;
> > > +		val = pos;
> > > +
> > > +		/* Already done min_size check, no bounds check here */
> > > +		offset = *val++;
> > > +		size = *val++;  
> > 
> > I believe I have these backwards, size should come first for both dyn
> > and rel data. Is this correct?
> 
> it's size << 16 | offset;
> 
> 
> > 
> > > diff --git a/tools/testing/selftests/user_events/ftrace_test.c b/tools/testing/selftests/user_events/ftrace_test.c
> > > index 9d53717139e6..bea694e9df8c 100644
> > > --- a/tools/testing/selftests/user_events/ftrace_test.c
> > > +++ b/tools/testing/selftests/user_events/ftrace_test.c
> > > @@ -21,6 +21,11 @@ const char *status_file = "/sys/kernel/debug/tracing/user_events_status";
> > >  const char *enable_file = "/sys/kernel/debug/tracing/events/user_events/__test_event/enable";
> > >  const char *trace_file = "/sys/kernel/debug/tracing/trace";
> > >  
> > > +struct rel_loc {
> > > +	__u16 offset;
> > > +	__u16 size;
> > > +} __packed;
> > > +  
> > 
> > Same here.
> 
> I would not use pointer arithmetic or the above structure, as I'm not sure
> they work the same for both big and little endian. It's best to just use
> u32 and '|' (or) the two unsigned shorts into one integer.
> 
> -- Steve

Got it, totally makes sense now, thank you!

-Beau