linux-kernel - Re: [PATCH RFC 3/8] openat2: explicitly return -E2BIG for (usize > PAGE

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Message-Id: <0455ebf7-3f84-44c7-84b3-9ed6e218cdc0@app.fastmail.com>
Date: Mon, 02 Sep 2024 19:23:47 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Aleksa Sarai" <cyphar@...har.com>
Cc: "Ingo Molnar" <mingo@...hat.com>, "Peter Zijlstra" <peterz@...radead.org>,
 "Juri Lelli" <juri.lelli@...hat.com>,
 "Vincent Guittot" <vincent.guittot@...aro.org>,
 "Dietmar Eggemann" <dietmar.eggemann@....com>,
 "Steven Rostedt" <rostedt@...dmis.org>,
 "Benjamin Segall" <bsegall@...gle.com>, "Mel Gorman" <mgorman@...e.de>,
 "Valentin Schneider" <vschneid@...hat.com>,
 "Alexander Viro" <viro@...iv.linux.org.uk>,
 "Christian Brauner" <brauner@...nel.org>, "Jan Kara" <jack@...e.cz>,
 shuah <shuah@...nel.org>, "Kees Cook" <kees@...nel.org>,
 "Florian Weimer" <fweimer@...hat.com>, "Mark Rutland" <mark.rutland@....com>,
 linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 linux-kselftest@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH RFC 3/8] openat2: explicitly return -E2BIG for (usize > PAGE_SIZE)

On Mon, Sep 2, 2024, at 16:08, Aleksa Sarai wrote:
>> >  	if (unlikely(usize < OPEN_HOW_SIZE_VER0))
>> >  		return -EINVAL;
>> > +	if (unlikely(usize > PAGE_SIZE))
>> > +		return -E2BIG;
>> > 
>> 
>> Is PAGE_SIZE significant here? If there is a need to enforce a limit,
>> I would expect this to be the same regardless of kernel configuration,
>> since the structure layout is also independent of the configuration.
>
> PAGE_SIZE is what clone3, perf_event_open, sched_setattr, bpf, etc all
> use. The idea was that PAGE_SIZE is the absolute limit of any reasonable
> extensible structure size because we are never going to have argument
> structures that are larger than a page (I think this was discussed in
> the original copy_struct_from_user() patchset thread in late 2019, but I
> can't find the reference at the moment.)
>
> I simply forgot to add this when I first submitted openat2, the original
> intention was to just match the other syscalls.

Ok, I see. I guess it makes sense to keep this one consistent with the
other ones, but we may want to revisit this in the future and
come up with something that is independent of CONFIG_PAGE_SIZE.

>> Where is the current -EFAULT for users passing more than a page?
>> I only see it for reads beyond the VMA, but not e.g. when checking
>> terabytes of zero pages from an anonymous mapping.
>
> I meant that we in practice return -EFAULT if you pass a really large
> size (because you end up running off the end of mapped memory). There is
> no explicit -EFAULT for large sizes, which is exactly the problem. :P

Got it, thanks.

     Arnd