Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper

[Christian Brauner]

On Wed, Sep 25, 2019 at 06:59:12PM +0200, 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().
> 
> [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/uaccess.h |  4 +++
>  lib/Makefile            |  2 +-
>  lib/strnlen_user.c      | 52 +++++++++++++++++++++++++++++
>  lib/struct_user.c       | 73 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 130 insertions(+), 1 deletion(-)
>  create mode 100644 lib/struct_user.c

Hm, why the new file?
Couldn't this just live in usercopy.c?

> 
> 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/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/strnlen_user.c b/lib/strnlen_user.c
> index 7f2db3fe311f..7eb665732954 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -123,3 +123,55 @@ long strnlen_user(const char __user *str, long count)
>  	return 0;
>  }
>  EXPORT_SYMBOL(strnlen_user);
> +
> +/**
> + * 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)

*sigh*, I'm probably going to get yelled at because of this but: does
this really provide any _performance_ benefits over the dumb get_user()
loop that we currently have that we care about right now? My point
being, that the loop - imho - is much easier to understand than what is
going on here with all the masking, and aligning etc. that we have here.
But I'm not going to fight it.

> +{
> +	u64 val;
> +	uintptr_t align = (uintptr_t) from % 8;
> +
> +	if (unlikely(!size))
> +		return true;

Nit: I'd prefer int variables be checked with if (size != 0) :)

> +
> +	from -= align;
> +	size += align;
> +
> +	if (!user_access_begin(from, size))
> +		return -EFAULT;
> +
> +	while (size >= 8) {
> +		unsafe_get_user(val, (u64 __user *) from, err_fault);
> +		if (align) {
> +			/* @from is unaligned. */
> +			val &= ~aligned_byte_mask(align);
> +			align = 0;
> +		}
> +		if (val)
> +			goto done;
> +		from += 8;
> +		size -= 8;
> +	}
> +	if (size) {
> +		/* (@from + @size) is unaligned. */
> +		unsafe_get_user(val, (u64 __user *) from, err_fault);
> +		val &= aligned_byte_mask(size);
> +	}
> +
> +done:
> +	user_access_end();
> +	return (val == 0);
> +err_fault:
> +	user_access_end();
> +	return -EFAULT;
> +}
> diff --git a/lib/struct_user.c b/lib/struct_user.c
> new file mode 100644
> index 000000000000..57d79eb53bfa
> --- /dev/null
> +++ b/lib/struct_user.c
> @@ -0,0 +1,73 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (C) 2019 SUSE LLC
> + * Copyright (C) 2019 Aleksa Sarai <cyphar@xxxxxxxxxx>
> + */
> +
> +#include <linux/types.h>
> +#include <linux/export.h>
> +#include <linux/uaccess.h>
> +#include <linux/kernel.h>
> +#include <linux/string.h>
> +
> +/**
> + * 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
>