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From: Aleksa Sarai <cyphar@cyphar.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Christian Brauner <christian@brauner.io>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Eric Biederman <ebiederm@xmission.com>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Tycho Andersen <tycho@tycho.ws>,
	David Drysdale <drysdale@google.com>,
	Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Aleksa Sarai <asarai@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
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	patrick.bellasi@arm.com
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers
Date: Wed, 11 Sep 2019 20:37:30 +1000
Message-ID: <20190911103730.72unmfp7lsvvafxo@yavin> (raw)
In-Reply-To: <20190905105749.GW2386@hirez.programming.kicks-ass.net>

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On 2019-09-05, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 05, 2019 at 11:43:05AM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote:
> > > On 2019-09-05, Peter Zijlstra <peterz@infradead.org> wrote:
> > > > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> > > > > +/**
> > > > > + * copy_struct_to_user: copy a struct to user space
> > > > > + * @dst:   Destination address, in user space.
> > > > > + * @usize: Size of @dst struct.
> > > > > + * @src:   Source address, in kernel space.
> > > > > + * @ksize: Size of @src struct.
> > > > > + *
> > > > > + * Copies a struct from kernel space to user space, in a way that guarantees
> > > > > + * backwards-compatibility for struct syscall arguments (as long as future
> > > > > + * struct extensions are made such that all new fields are *appended* to the
> > > > > + * old struct, and zeroed-out new fields have the same meaning as the old
> > > > > + * struct).
> > > > > + *
> > > > > + * @ksize is just sizeof(*dst), and @usize should've been passed by user space.
> > > > > + * The recommended usage is something like the following:
> > > > > + *
> > > > > + *   SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize)
> > > > > + *   {
> > > > > + *      int err;
> > > > > + *      struct foo karg = {};
> > > > > + *
> > > > > + *      // do something with karg
> > > > > + *
> > > > > + *      err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg));
> > > > > + *      if (err)
> > > > > + *        return err;
> > > > > + *
> > > > > + *      // ...
> > > > > + *   }
> > > > > + *
> > > > > + * There are three cases to consider:
> > > > > + *  * If @usize == @ksize, then it's copied verbatim.
> > > > > + *  * If @usize < @ksize, then kernel space is "returning" a newer struct to an
> > > > > + *    older user space. In order to avoid user space getting incomplete
> > > > > + *    information (new fields might be important), all trailing bytes in @src
> > > > > + *    (@ksize - @usize) must be zerored
> > > > 
> > > > s/zerored/zero/, right?
> > > 
> > > It should've been "zeroed".
> > 
> > That reads wrong to me; that way it reads like this function must take
> > that action and zero out the 'rest'; which is just wrong.
> > 
> > This function must verify those bytes are zero, not make them zero.
> > 
> > > > >                                          , otherwise -EFBIG is returned.
> > > > 
> > > > 'Funny' that, copy_struct_from_user() below seems to use E2BIG.
> > > 
> > > This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for
> > > a "too big" struct passed to the kernel, and EFBIG for a "too big"
> > > struct passed to user-space. I would personally have preferred EMSGSIZE
> > > instead of EFBIG, but felt using the existing error codes would be less
> > > confusing.
> > 
> > Sadly a recent commit:
> > 
> >   1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and code")
> > 
> > Made the situation even 'worse'.
> 
> And thinking more about things; I'm not convinced the above patch is
> actually right.
> 
> Do we really want to simply truncate all the attributes of the task?
> 
> And should we not at least set sched_flags when there are non-default
> clamp values applied?
> 
> See; that is I think the primary bug that had chrt failing; we tried to
> publish the default clamp values as !0.

I just saw this patch in -rc8 -- should I even attempt to port
sched_getattr(2) to copy_struct_to_user()? I agree that publishing a
default non-zero value is a mistake -- once you do that, old user space
will either get confused or lose information.

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

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  reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 20:19 [PATCH v12 00/12] namei: openat2(2) path resolution restrictions Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Aleksa Sarai
2019-09-04 20:48   ` Linus Torvalds
2019-09-04 21:00   ` Randy Dunlap
2019-09-05  7:32   ` Peter Zijlstra
2019-09-05  9:26     ` Aleksa Sarai
2019-09-05  9:43       ` Peter Zijlstra
2019-09-05 10:57         ` Peter Zijlstra
2019-09-11 10:37           ` Aleksa Sarai [this message]
2019-09-05 13:35         ` Aleksa Sarai
2019-09-05 17:01         ` Aleksa Sarai
2019-09-05  8:43   ` Rasmus Villemoes
2019-09-05  9:50     ` Aleksa Sarai
2019-09-05 10:45       ` Christian Brauner
2019-09-05  9:09   ` Andreas Schwab
2019-09-05 10:13     ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Gabriel Paubert
2019-09-05 11:05   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner
2019-09-05 11:17     ` Rasmus Villemoes
2019-09-05 11:29       ` Christian Brauner
2019-09-05 13:40     ` Aleksa Sarai
2019-09-05 11:09   ` Christian Brauner
2019-09-05 11:27     ` Aleksa Sarai
2019-09-05 11:40       ` Christian Brauner
2019-09-05 18:07   ` Al Viro
2019-09-05 18:23     ` Christian Brauner
2019-09-05 18:28       ` Al Viro
2019-09-05 18:35         ` Christian Brauner
2019-09-05 19:56         ` Aleksa Sarai
2019-09-05 22:31           ` Al Viro
2019-09-06  7:00           ` Christian Brauner
2019-09-05 23:00     ` Aleksa Sarai
2019-09-05 23:49       ` Al Viro
2019-09-06  0:09         ` Aleksa Sarai
2019-09-06  0:14         ` Al Viro
2019-09-04 20:19 ` [PATCH v12 02/12] clone3: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 03/12] sched_setattr: switch to copy_struct_{to,from}_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 04/12] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 06/12] procfs: switch magic-link modes to be more sane Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 08/12] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 09/12] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-09-04 21:09   ` Linus Torvalds
2019-09-04 21:35     ` Linus Torvalds
2019-09-04 21:36       ` Linus Torvalds
2019-09-04 21:48     ` Aleksa Sarai
2019-09-04 22:16       ` Linus Torvalds
2019-09-04 22:31       ` David Howells
2019-09-04 22:38         ` Linus Torvalds
2019-09-04 23:29           ` Al Viro
2019-09-04 23:44             ` Linus Torvalds
2019-09-04 20:19 ` [PATCH v12 11/12] open: openat2(2) syscall Aleksa Sarai
2019-09-04 21:00   ` Randy Dunlap
2019-09-07 12:40   ` Jeff Layton
2019-09-07 16:58     ` Linus Torvalds
2019-09-07 17:42       ` Andy Lutomirski
2019-09-07 17:45         ` Linus Torvalds
2019-09-07 18:15           ` Andy Lutomirski
2019-09-10  6:35           ` Ingo Molnar
2019-09-08 16:24     ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 12/12] selftests: add openat2(2) selftests Aleksa Sarai

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