Re: [PATCH v4 1/4] lib: introduce copy_struct_from_user() helper

[Kees Cook <keescook@chromium.org>]

On Tue, Oct 01, 2019 at 11:10:52AM +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).
> 
> 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@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/bitops.h  |   7 +++
>  include/linux/uaccess.h |  70 +++++++++++++++++++++
>  lib/strnlen_user.c      |   8 +--
>  lib/test_user_copy.c    | 136 ++++++++++++++++++++++++++++++++++++++--
>  lib/usercopy.c          |  55 ++++++++++++++++
>  5 files changed, 263 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..c94a9ff9f082 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 70bbdc38dc37..8abbc713f7fb 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -231,6 +231,76 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
>  
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>  
> +extern int check_zeroed_user(const void __user *from, size_t size);
> +
> +/**
> + * 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 = {};
> + *
> + *      if (usize > PAGE_SIZE)
> + *        return -E2BIG;
> + *      if (usize < FOO_SIZE_VER0)
> + *        return -EINVAL;
> + *
> + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, usize);
> + *      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.
> + */
> +static __always_inline
> +int copy_struct_from_user(void *dst, size_t ksize,
> +			  const void __user *src, size_t usize)

And of course I forgot to realize both this and check_zeroed_user()
should also have the __must_check attribute. Sorry for forgetting that
earlier!

With that, please consider it:

Reviewed-by: Kees Cook <keescook@chromium.org>

Thanks for working on this!

-- 
Kees Cook