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Message-ID: <20190905230003.bek7vqdvruzi4ybx@yavin.dot.cyphar.com>
Date: Fri, 6 Sep 2019 09:00:03 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: 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, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> > +/*
> > + * "memset(p, 0, size)" but for user space buffers. Caller must have already
> > + * checked access_ok(p, size).
> > + */
> > +static int __memzero_user(void __user *p, size_t s)
> > +{
> > + const char zeros[BUFFER_SIZE] = {};
> > + while (s > 0) {
> > + size_t n = min(s, sizeof(zeros));
> > +
> > + if (__copy_to_user(p, zeros, n))
> > + return -EFAULT;
> > +
> > + p += n;
> > + s -= n;
> > + }
> > + return 0;
> > +}
>
> That's called clear_user().
Already switched, I didn't know about clear_user() -- I assumed it
would've been called bzero_user() or memzero_user() and didn't find it
when looking.
> > +int copy_struct_to_user(void __user *dst, size_t usize,
> > + const void *src, size_t ksize)
> > +{
> > + size_t size = min(ksize, usize);
> > + size_t rest = abs(ksize - usize);
> > +
> > + if (unlikely(usize > PAGE_SIZE))
> > + return -EFAULT;
>
> Why?
>
> > + } else if (usize > ksize) {
> > + if (__memzero_user(dst + size, rest))
> > + return -EFAULT;
> > + }
> > + /* Copy the interoperable parts of the struct. */
> > + if (__copy_to_user(dst, src, size))
> > + return -EFAULT;
>
> Why not simply clear_user() and copy_to_user()?
I'm not sure I understand what you mean -- are you asking why we need to
do memchr_inv(src + size, 0, rest) earlier?
>
> > +int copy_struct_from_user(void *dst, size_t ksize,
> > + const void __user *src, size_t usize)
> > +{
> > + size_t size = min(ksize, usize);
> > + size_t rest = abs(ksize - usize);
>
> Cute, but... you would be just as well without that 'rest' thing.
I would argue it's harder to mess up using "rest" compared to getting
"ksize - usize" and "usize - ksize" mixed up (and it's a bit more
readable).
> > +
> > + if (unlikely(usize > PAGE_SIZE))
> > + return -EFAULT;
>
> Again, why?
As discussed in a sister thread, I will leave this in the callers
(though I would argue callers should always do some kind of sanity check
like this).
>
> > + if (unlikely(!access_ok(src, usize)))
> > + return -EFAULT;
>
> Why not simply copy_from_user() here?
>
> > + /* 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] = {};
> > +
> > + 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;
>
> Frankly, that looks like a candidate for is_all_zeroes_user().
> With the loop like above serving as a dumb default. And on
> badly alighed address it _will_ be dumb. Probably too much
> so - something like
> if ((unsigned long)addr & 1) {
> u8 v;
> if (get_user(v, (__u8 __user *)addr))
> return -EFAULT;
> if (v)
> return -E2BIG;
> addr++;
> }
> if ((unsigned long)addr & 2) {
> u16 v;
> if (get_user(v, (__u16 __user *)addr))
> return -EFAULT;
> if (v)
> return -E2BIG;
> addr +=2;
> }
> if ((unsigned long)addr & 4) {
> u32 v;
> if (get_user(v, (__u32 __user *)addr))
> return -EFAULT;
> if (v)
> return -E2BIG;
> }
> <read the rest like you currently do>
> would be saner, and things like x86 could trivially add an
> asm variant - it's not hard. Incidentally, memchr_inv() is
> an overkill in this case...
Why is memchr_inv() overkill?
But yes, breaking this out to an asm-generic is_all_zeroes_user()
wouldn't hurt -- and I'll put a cleaned-up version of the alignment
handling there too. Should I drop it in asm-generic/uaccess.h, or
somewhere else?
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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