From: Aleksa Sarai <cyphar@cyphar.com> To: Peter Zijlstra <peterz@infradead.org> 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>, 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, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, 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 19:26:22 +1000 Message-ID: <20190905092622.tlb6nn3uisssdfbu@yavin.dot.cyphar.com> (raw) In-Reply-To: <20190905073205.GY2332@hirez.programming.kicks-ass.net> [-- Attachment #1: Type: text/plain, Size: 7846 bytes --] On 2019-09-05, Peter Zijlstra <peterz@infradead.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". > > , 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. > > > + * * 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; > > Not documented above. Implementation consistent with *from*, but see > below. Will update the kernel-doc. > > + 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; > > + } > > + /* 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; > > Documented above as returning -E2BIG. I will switch this (and to) back to -E2BIG -- I must've had a brain-fart when doing some refactoring. > > > + 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). > > + > > + 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. > > + } > > + /* Copy the interoperable parts of the struct. */ > > + if (__copy_from_user(dst, src, size)) > > + return -EFAULT; > > + return 0; > > +} > > +EXPORT_SYMBOL(copy_struct_from_user); > > And personally I'm not a big fan of EXPORT_SYMBOL(). I don't have much of an opinion (after all, it only really makes sense a lot of sense for syscalls) -- though out-of-tree modules that define ioctl()s wouldn't be able to make use of them. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/> [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 228 bytes --]
next prev parent reply index 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 [this message] 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 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner 2019-09-05 11:17 ` 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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