linux-kernel - Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}

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Message-ID: <20190905134017.rstiqa6v6roslzlu@yavin.dot.cyphar.com>
Date:   Thu, 5 Sep 2019 23:40:17 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers

On 2019-09-05, Christian Brauner <christian.brauner@...ntu.com> wrote:
> On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> > A common pattern for syscall extensions is increasing the size of a
> > struct passed from userspace, such that the zero-value of the new fields
> > result in the old kernel behaviour (allowing for a mix of userspace and
> > kernel vintages to operate on one another in most cases). This is done
> > in both directions -- hence two helpers -- though it's more common to
> > have to copy user space structs into kernel space.
> > 
> > Previously there was no common lib/ function that implemented
> > the necessary extension-checking semantics (and different syscalls
> > implemented them slightly differently or incompletely[1]). A future
> > patch replaces all of the common uses of this pattern to use the new
> > copy_struct_{to,from}_user() helpers.
> > 
> > [1]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
> >      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
> >      always rejects differently-sized struct arguments.
> > 
> > Suggested-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> > Signed-off-by: Aleksa Sarai <cyphar@...har.com>

[...]

> > +	if (unlikely(!access_ok(src, usize)))
> > +		return -EFAULT;
> > +
> > +	/* Deal with trailing bytes. */
> > +	if (usize < ksize)
> > +		memset(dst + size, 0, rest);
[...]
> That's a change in behavior for clone3() and sched at least, no? Unless
> - which I guess you might have done - you have moved the "error out when
> the struct is too small" part before the call to copy_struct_from_user()
> for them.

Yes, I've put the minimum size check to the callers in all of the
cases (in the case of clone3() I've #define'd a CLONE_ARGS_SIZE_VER0 to
match the others -- see patch 2 of the series).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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