linux-kernel - Re: [PATCH 4/6] hw_breakpoint: Factor out __modify_user_hw

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Date:   Mon, 27 Nov 2017 22:20:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Milind Chabbi <chabbi.milind@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Oleg Nesterov <onestero@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 4/6] hw_breakpoint: Factor out
 __modify_user_hw_breakpoint function

On Mon, Nov 27, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 27, 2017 at 06:25:32PM +0100, Jiri Olsa wrote:
> > On Mon, Nov 27, 2017 at 06:12:03PM +0100, Peter Zijlstra wrote:
> > > But what validates the input attr is the same as the event attr, aside
> > > from those fields?
> > 
> > we don't.. the attr serves as a holder to carry those fields
> > into the function
> 
> Then that's a straight up bug.
> 
> > the current kernel interface does not check anything else
> 
> Not enough, if the new attr would fail perf_event_open() it should also
> fail this modify thing.


On IRC you asked:

<jolsa> peterz, I dont follow.. why should we check fields that we dont use?

Suppose someone does:

	attr = malloc(sizeof(*attr)); // uninitialized memory
	attr->type = BP;
	attr->bp_addr = new_addr;
	attr->bp_type = bp_type;
	attr->bp_len = bp_len;
	ioctl(fd, PERF_IOC_MOD_ATTR, &attr);

And feeds absolute shite for the rest of the fields.

Then we later want to extend IOC_MOD_ATTR to allow changing
attr::sample_type but we can't, because that would break the above
application.

Therefore we must be very strict to check only the fields we can change
have changed.