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; + 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; + 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] = {}; + + 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);