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Message-ID: <20190905094305.GJ2349@hirez.programming.kicks-ass.net>
Date: Thu, 5 Sep 2019 11:43:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Aleksa Sarai <cyphar@...har.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>,
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 Thu, Sep 05, 2019 at 07:26:22PM +1000, Aleksa Sarai wrote:
> On 2019-09-05, Peter Zijlstra <peterz@...radead.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'.
> > > + if (unlikely(!access_ok(src, usize)))
> > > + return -EFAULT;
> > > +
> > > + /* Deal with trailing bytes. */
> > > + if (usize < ksize)
> > > + memset(dst + size, 0, rest);
> > > + else if (usize > ksize) {
> > > + const void __user *addr = src + size;
> > > + char buffer[BUFFER_SIZE] = {};
> >
> > Isn't that too big for on-stack?
>
> Is a 64-byte buffer too big? I picked the number "at random" to be the
> size of a cache line, but I could shrink it down to 32 bytes if the size
> is an issue (I wanted to avoid needless allocations -- hence it being
> on-stack).
Ah, my ctags gave me a definition of BUFFER_SIZE that was 512. I suppose
64 should be OK.
> > > +
> > > + while (rest > 0) {
> > > + size_t bufsize = min(rest, sizeof(buffer));
> > > +
> > > + if (__copy_from_user(buffer, addr, bufsize))
> > > + return -EFAULT;
> > > + if (memchr_inv(buffer, 0, bufsize))
> > > + return -E2BIG;
> > > +
> > > + addr += bufsize;
> > > + rest -= bufsize;
> > > + }
> >
> > The perf implementation uses get_user(); but if that is too slow, surely
> > we can do something with uaccess_try() here?
>
> Is there a non-x86-specific way to do that (unless I'm mistaken only x86
> has uaccess_try() or the other *_try() wrappers)? The main "performance
> improvement" (if you can even call it that) is that we use memchr_inv()
> which finds non-matching characters more efficiently than just doing a
> loop.
Oh, you're right, that's x86 only :/
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