Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

From: Peter Zijlstra <[email-masked]>
Date: Thu, 5 Sep 2019 09:32:05 +0200

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?

>                                          , otherwise -EFBIG is returned.

'Funny' that, copy_struct_from_user() below seems to use E2BIG.

> + *  * 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.

> +	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.

> +	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?

> +
> +		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?

> +	}
> +	/* 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().