From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 709C4C432C2 for ; Wed, 25 Sep 2019 17:18:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 48345214D9 for ; Wed, 25 Sep 2019 17:18:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2442925AbfIYRS1 (ORCPT ); Wed, 25 Sep 2019 13:18:27 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:50885 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730327AbfIYRS0 (ORCPT ); Wed, 25 Sep 2019 13:18:26 -0400 Received: from [185.81.138.21] (helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1iDAvd-0007Tv-1p; Wed, 25 Sep 2019 17:18:13 +0000 Date: Wed, 25 Sep 2019 19:18:11 +0200 From: Christian Brauner To: Aleksa Sarai Cc: Ingo Molnar , Peter Zijlstra , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Rasmus Villemoes , Al Viro , Linus Torvalds , libc-alpha@sourceware.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper Message-ID: <20190925171810.ajfx4zlmj5scct4m@wittgenstein> References: <20190925165915.8135-1-cyphar@cyphar.com> <20190925165915.8135-2-cyphar@cyphar.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190925165915.8135-2-cyphar@cyphar.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 25, 2019 at 06:59:12PM +0200, 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). > > While this interface exists for communication in both directions, only > one interface is straightforward to have reasonable semantics for > (userspace passing a struct to the kernel). For kernel returns to > userspace, what the correct semantics are (whether there should be an > error if userspace is unaware of a new extension) is very > syscall-dependent and thus probably cannot be unified between syscalls > (a good example of this problem is [1]). > > Previously there was no common lib/ function that implemented > the necessary extension-checking semantics (and different syscalls > implemented them slightly differently or incompletely[2]). Future > patches replace common uses of this pattern to make use of > copy_struct_from_user(). > > [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and > robustify sched_read_attr() ABI logic and code") > > [2]: 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 > Signed-off-by: Aleksa Sarai > --- > include/linux/uaccess.h | 4 +++ > lib/Makefile | 2 +- > lib/strnlen_user.c | 52 +++++++++++++++++++++++++++++ > lib/struct_user.c | 73 +++++++++++++++++++++++++++++++++++++++++ > 4 files changed, 130 insertions(+), 1 deletion(-) > create mode 100644 lib/struct_user.c Hm, why the new file? Couldn't this just live in usercopy.c? > > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h > index 34a038563d97..824569e309e4 100644 > --- a/include/linux/uaccess.h > +++ b/include/linux/uaccess.h > @@ -230,6 +230,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to, > > #endif /* ARCH_HAS_NOCACHE_UACCESS */ > > +extern int is_zeroed_user(const void __user *from, size_t count); > +extern int copy_struct_from_user(void *dst, size_t ksize, > + const void __user *src, size_t usize); > + > /* > * probe_kernel_read(): safely attempt to read from a location > * @dst: pointer to the buffer that shall take the data > diff --git a/lib/Makefile b/lib/Makefile > index 29c02a924973..d86c71feaf0a 100644 > --- a/lib/Makefile > +++ b/lib/Makefile > @@ -28,7 +28,7 @@ endif > CFLAGS_string.o := $(call cc-option, -fno-stack-protector) > endif > > -lib-y := ctype.o string.o vsprintf.o cmdline.o \ > +lib-y := ctype.o string.o struct_user.o vsprintf.o cmdline.o \ > rbtree.o radix-tree.o timerqueue.o xarray.o \ > idr.o extable.o \ > sha1.o chacha.o irq_regs.o argv_split.o \ > diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c > index 7f2db3fe311f..7eb665732954 100644 > --- a/lib/strnlen_user.c > +++ b/lib/strnlen_user.c > @@ -123,3 +123,55 @@ long strnlen_user(const char __user *str, long count) > return 0; > } > EXPORT_SYMBOL(strnlen_user); > + > +/** > + * is_zeroed_user: check if a userspace buffer is full of zeros > + * @from: Source address, in userspace. > + * @size: Size of buffer. > + * > + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for > + * userspace addresses. If there are non-zero bytes present then false is > + * returned, otherwise true is returned. > + * > + * Returns: > + * * -EFAULT: access to userspace failed. > + */ > +int is_zeroed_user(const void __user *from, size_t size) *sigh*, I'm probably going to get yelled at because of this but: does this really provide any _performance_ benefits over the dumb get_user() loop that we currently have that we care about right now? My point being, that the loop - imho - is much easier to understand than what is going on here with all the masking, and aligning etc. that we have here. But I'm not going to fight it. > +{ > + u64 val; > + uintptr_t align = (uintptr_t) from % 8; > + > + if (unlikely(!size)) > + return true; Nit: I'd prefer int variables be checked with if (size != 0) :) > + > + from -= align; > + size += align; > + > + if (!user_access_begin(from, size)) > + return -EFAULT; > + > + while (size >= 8) { > + unsafe_get_user(val, (u64 __user *) from, err_fault); > + if (align) { > + /* @from is unaligned. */ > + val &= ~aligned_byte_mask(align); > + align = 0; > + } > + if (val) > + goto done; > + from += 8; > + size -= 8; > + } > + if (size) { > + /* (@from + @size) is unaligned. */ > + unsafe_get_user(val, (u64 __user *) from, err_fault); > + val &= aligned_byte_mask(size); > + } > + > +done: > + user_access_end(); > + return (val == 0); > +err_fault: > + user_access_end(); > + return -EFAULT; > +} > diff --git a/lib/struct_user.c b/lib/struct_user.c > new file mode 100644 > index 000000000000..57d79eb53bfa > --- /dev/null > +++ b/lib/struct_user.c > @@ -0,0 +1,73 @@ > +// SPDX-License-Identifier: GPL-2.0-or-later > +/* > + * Copyright (C) 2019 SUSE LLC > + * Copyright (C) 2019 Aleksa Sarai > + */ > + > +#include > +#include > +#include > +#include > +#include > + > +/** > + * copy_struct_from_user: copy a struct from userspace > + * @dst: Destination address, in kernel space. This buffer must be @ksize > + * bytes long. > + * @ksize: Size of @dst struct. > + * @src: Source address, in userspace. > + * @usize: (Alleged) size of @src struct. > + * > + * Copies a struct from userspace to kernel 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 userspace. > + * The recommended usage is something like the following: > + * > + * SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize) > + * { > + * int err; > + * struct foo karg = {}; > + * > + * err = copy_struct_from_user(&karg, sizeof(karg), uarg, size); > + * if (err) > + * return err; > + * > + * // ... > + * } > + * > + * There are three cases to consider: > + * * If @usize == @ksize, then it's copied verbatim. > + * * If @usize < @ksize, then the userspace has passed an old struct to a > + * newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize) > + * are to be zero-filled. > + * * If @usize > @ksize, then the userspace has passed a new struct to an > + * older kernel. The trailing bytes unknown to the kernel (@usize - @ksize) > + * are checked to ensure they are zeroed, otherwise -E2BIG is returned. > + * > + * Returns (in all cases, some data may have been copied): > + * * -E2BIG: (@usize > @ksize) and there are non-zero trailing bytes in @src. > + * * -EFAULT: access to userspace failed. > + */ > +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 = max(ksize, usize) - size; > + > + /* Deal with trailing bytes. */ > + if (usize < ksize) { > + memset(dst + size, 0, rest); > + } else if (usize > ksize) { > + int ret = is_zeroed_user(src + size, rest); > + if (ret <= 0) > + return ret ?: -E2BIG; > + } > + /* Copy the interoperable parts of the struct. */ > + if (copy_from_user(dst, src, size)) > + return -EFAULT; > + return 0; > +} > -- > 2.23.0 >