From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Christian Brauner <christian@brauner.io>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
GNU C Library <libc-alpha@sourceware.org>,
Linux API <linux-api@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper
Date: Wed, 25 Sep 2019 18:04:00 -0000 [thread overview]
Message-ID: <20190925180412.GK26530@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wjagt257WHiOr2v1Bx_3q7tuzogabw_1EnodKm0vt+-WQ@mail.gmail.com>
On Wed, Sep 25, 2019 at 10:48:31AM -0700, Linus Torvalds wrote:
> On Wed, Sep 25, 2019 at 10:21 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > Just to make sure I understand, the following diff would this solve the
> > problem? If so, I'll apply it, and re-send in a few hours.
>
> Actually, looking at it more, it's still buggy.
>
> That final "size smaller than unsigned long" doesn't correctly handle
> the case of (say) a single byte in the middle of a 8-byte word.
>
> So you need to do something like this:
>
> int is_zeroed_user(const void __user *from, size_t size)
> {
> unsigned long val, mask, align;
>
> if (unlikely(!size))
> return true;
>
> if (!user_access_begin(from, size))
> return -EFAULT;
>
> align = (uintptr_t) from % sizeof(unsigned long);
> from -= align;
> size += align;
>
> mask = ~aligned_byte_mask(align);
>
> while (size >= sizeof(unsigned long)) {
> unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> val &= mask;
> if (unlikely(val))
> goto done;
> mask = ~0ul;
> from += sizeof(unsigned long);
> size -= sizeof(unsigned long);
> }
>
> if (size) {
> /* (@from + @size) is unaligned. */
> unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> mask &= aligned_byte_mask(size);
> val &= mask;
> }
IMO it's better to lift reading the first word out of the loop, like this:
align = (uintptr_t) from % sizeof(unsigned long);
from -= align;
unsafe_get_user(val, (unsigned long __user *) from, err_fault);
if (align) {
size += align;
val &= ~aligned_byte_mask(align);
}
while (size > sizeof(unsigned long)) {
if (unlikely(val))
goto done;
from += sizeof(unsigned long);
size -= sizeof(unsigned long);
unsafe_get_user(val, (unsigned long __user *) from, err_fault);
}
if (size != size(unsigned long))
val &= aligned_byte_mask(size);
done:
Do you see any problems with that variant?
next prev parent reply other threads:[~2019-09-25 18:04 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-25 17:00 [PATCH v1 0/4] " Aleksa Sarai
2019-09-25 17:00 ` [PATCH v1 4/4] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai
2019-09-25 17:00 ` [PATCH v1 3/4] sched_setattr: " Aleksa Sarai
2019-09-25 17:00 ` [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper Aleksa Sarai
2019-09-25 17:11 ` Linus Torvalds
2019-09-25 17:21 ` Aleksa Sarai
2019-09-25 17:48 ` Linus Torvalds
2019-09-25 18:04 ` Al Viro [this message]
2019-09-25 18:13 ` Linus Torvalds
2019-09-25 19:43 ` Al Viro
2019-09-25 20:23 ` Linus Torvalds
2019-09-25 17:18 ` Christian Brauner
2019-09-25 17:20 ` Christian Brauner
2019-09-25 19:16 ` kbuild test robot
2019-09-25 20:47 ` kbuild test robot
2019-09-25 17:00 ` [PATCH v1 2/4] clone3: switch to copy_struct_from_user() Aleksa Sarai
2019-09-25 17:22 ` Christian Brauner
2019-09-25 18:59 ` kbuild test robot
2019-09-25 19:08 ` kbuild test robot
2019-09-25 17:09 ` [PATCH v1 0/4] lib: introduce copy_struct_from_user() helper Christian Brauner
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