From: Christian Brauner <christian.brauner@ubuntu.com>
To: Aleksa Sarai <cyphar@cyphar.com>
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>,
Peter Zijlstra <peterz@infradead.org>,
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>,
containers@lists.linux-foundation.org,
linux-alpha@vger.kernel.org, linux-api@vger.kernel.org,
linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers
Date: Thu, 5 Sep 2019 13:05:45 +0200 [thread overview]
Message-ID: <20190905110544.d6c5t7rx25kvywmi@wittgenstein> (raw)
In-Reply-To: <20190904201933.10736-2-cyphar@cyphar.com>
On Thu, Sep 05, 2019 at 06:19:22AM +1000, 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). This is done
> in both directions -- hence two helpers -- though it's more common to
> have to copy user space structs into kernel space.
>
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[1]). A future
> patch replaces all of the common uses of this pattern to use the new
> copy_struct_{to,from}_user() helpers.
>
> [1]: 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 <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> include/linux/uaccess.h | 5 ++
> lib/Makefile | 2 +-
> lib/struct_user.c | 182 ++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 188 insertions(+), 1 deletion(-)
> create mode 100644 lib/struct_user.c
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 34a038563d97..0ad9544a1aee 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -230,6 +230,11 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>
> #endif /* ARCH_HAS_NOCACHE_UACCESS */
>
> +extern int copy_struct_to_user(void __user *dst, size_t usize,
> + const void *src, size_t ksize);
> +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/struct_user.c b/lib/struct_user.c
> new file mode 100644
> index 000000000000..7301ab1bbe98
> --- /dev/null
> +++ b/lib/struct_user.c
> @@ -0,0 +1,182 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (C) 2019 SUSE LLC
> + * Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
> + */
> +
> +#include <linux/types.h>
> +#include <linux/export.h>
> +#include <linux/uaccess.h>
> +#include <linux/kernel.h>
> +#include <linux/string.h>
> +
> +#define BUFFER_SIZE 64
> +
> +/*
> + * "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;
> +}
> +
> +/**
> + * 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, otherwise -EFBIG is returned.
> + * * If @usize > @ksize, then the kernel is "returning" an older struct to a
> + * newer user space. The trailing bytes in @dst (@usize - @ksize) will be
> + * zero-filled.
> + *
> + * Returns (in all cases, some data may have been copied):
> + * * -EFBIG: (@usize < @ksize) and there are non-zero trailing bytes in @src.
> + * * -EFAULT: access to user space failed.
> + */
> +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;
Looks like this should be -EFBIG.
> + if (unlikely(!access_ok(dst, usize)))
> + return -EFAULT;
> +
> + /* Deal with trailing bytes. */
> + if (usize < ksize) {
> + if (memchr_inv(src + size, 0, rest))
> + return -EFBIG;
> + } else if (usize > ksize) {
> + if (__memzero_user(dst + size, rest))
> + return -EFAULT;
Is zeroing that memory really our job? Seems to me we should just check
it is zeroed.
> + }
> + /* Copy the interoperable parts of the struct. */
> + if (__copy_to_user(dst, src, size))
> + return -EFAULT;
> + return 0;
> +}
> +EXPORT_SYMBOL(copy_struct_to_user);
> +
> +/**
> + * copy_struct_from_user: copy a struct from user space
> + * @dst: Destination address, in kernel space. This buffer must be @ksize
> + * bytes long.
> + * @ksize: Size of @dst struct.
> + * @src: Source address, in user space.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from user space 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 user space.
> + * 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 user space 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 user space 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.
> + * * -E2BIG: @usize is "too big" (at time of writing, >PAGE_SIZE).
> + * * -EFAULT: access to user space 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 = abs(ksize - usize);
> +
> + if (unlikely(usize > PAGE_SIZE))
> + return -EFAULT;
That should be -E2BIG.
> + if (unlikely(!access_ok(src, usize)))
> + return -EFAULT;
> +
> + /* Deal with trailing bytes. */
> + if (usize < ksize)
> + memset(dst + size, 0, rest);
I think kernel style mandates that if one branch in an if-else ladder
requires {} all other must use {} as well. So this should be:
if () {
// one line
} else {
// one line
// another line
}
That's a change in behavior for clone3() and sched at least, no? Unless
- which I guess you might have done - you have moved the "error out when
the struct is too small" part before the call to copy_struct_from_user()
for them.
> + 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;
> +
> + addr += bufsize;
> + rest -= bufsize;
> + }
> + }
> + /* Copy the interoperable parts of the struct. */
> + if (__copy_from_user(dst, src, size))
> + return -EFAULT;
> + return 0;
> +}
> +EXPORT_SYMBOL(copy_struct_from_user);
> --
> 2.23.0
>
next prev parent reply other threads:[~2019-09-05 11:06 UTC|newest]
Thread overview: 65+ 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
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 ` Christian Brauner [this message]
2019-09-05 11:17 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers 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
2019-09-17 21:30 ` Jann Horn
2019-09-18 13:51 ` Aleksa Sarai
2019-09-18 15:46 ` 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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