(Damn, I forgot to add Kees to Cc.)
On 2019-09-26, Aleksa Sarai <cyphar@xxxxxxxxxx> 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().
>
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
>
> [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 <linux@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> ---
> include/linux/bitops.h | 7 +++
> include/linux/uaccess.h | 4 ++
> lib/strnlen_user.c | 8 +--
> lib/test_user_copy.c | 59 ++++++++++++++++++---
> lib/usercopy.c | 115 ++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 180 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..a23f4c054768 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
> #include <asm/types.h>
> #include <linux/bits.h>
>
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> +#else
> +# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif
> +
> #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
> #define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>
> 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/strnlen_user.c b/lib/strnlen_user.c
> index 7f2db3fe311f..39d588aaa8cd 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -2,16 +2,10 @@
> #include <linux/kernel.h>
> #include <linux/export.h>
> #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>
> #include <asm/word-at-a-time.h>
>
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
> /*
> * Do a strnlen, return length of string *with* final '\0'.
> * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..f7cde3845ccc 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -31,14 +31,58 @@
> # define TEST_U64
> #endif
>
> -#define test(condition, msg) \
> -({ \
> - int cond = (condition); \
> - if (cond) \
> - pr_warn("%s\n", msg); \
> - cond; \
> +#define test(condition, msg, ...) \
> +({ \
> + int cond = (condition); \
> + if (cond) \
> + pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__); \
> + cond; \
> })
>
> +static int test_is_zeroed_user(char *kmem, char __user *umem, size_t size)
> +{
> + int ret = 0;
> + size_t start, end, i;
> + size_t zero_start = size / 4;
> + size_t zero_end = size - zero_start;
> +
> + /*
> + * We conduct a series of is_zeroed_user() tests on a block of memory
> + * with the following byte-pattern (trying every possible [start,end]
> + * pair):
> + *
> + * [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> + *
> + * And we verify that is_zeroed_user() acts identically to memchr_inv().
> + */
> +
> + for (i = 0; i < zero_start; i += 2)
> + kmem[i] = 0x00;
> + for (i = 1; i < zero_start; i += 2)
> + kmem[i] = 0xff;
> +
> + for (i = zero_end; i < size; i += 2)
> + kmem[i] = 0xff;
> + for (i = zero_end + 1; i < size; i += 2)
> + kmem[i] = 0x00;
> +
> + ret |= test(copy_to_user(umem, kmem, size),
> + "legitimate copy_to_user failed");
> +
> + for (start = 0; start <= size; start++) {
> + for (end = start; end <= size; end++) {
> + int retval = is_zeroed_user(umem + start, end - start);
> + int expected = memchr_inv(kmem + start, 0, end - start) == NULL;
> +
> + ret |= test(retval != expected,
> + "is_zeroed_user(=%d) != memchr_inv(=%d) mismatch (start=%lu, end=%lu)",
> + retval, expected, start, end);
> + }
> + }
> +
> + return ret;
> +}
> +
> static int __init test_user_copy_init(void)
> {
> int ret = 0;
> @@ -106,6 +150,9 @@ static int __init test_user_copy_init(void)
> #endif
> #undef test_legit
>
> + /* Test usage of is_zeroed_user(). */
> + ret |= test_is_zeroed_user(kmem, usermem, PAGE_SIZE);
> +
> /*
> * Invalid usage: none of these copies should succeed.
> */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..f795cf0946ad 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>
> /* out-of-line parts */
>
> @@ -31,3 +32,117 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
> }
> EXPORT_SYMBOL(_copy_to_user);
> #endif
> +
> +/**
> + * 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)
> +{
> + unsigned long val;
> + uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> + if (unlikely(!size))
> + return true;
> +
> + from -= align;
> + size += align;
> +
> + if (!user_access_begin(from, size))
> + return -EFAULT;
> +
> + unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> + if (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 < sizeof(unsigned long))
> + val &= aligned_byte_mask(size);
> +
> +done:
> + user_access_end();
> + return (val == 0);
> +err_fault:
> + user_access_end();
> + return -EFAULT;
> +}
> +EXPORT_SYMBOL(is_zeroed_user);
> +
> +/**
> + * 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
>
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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