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

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-09-05, Peter Zijlstra <peterz@infradead.org> wrote:
> 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?

It should've been "zeroed".

> >                                          , otherwise -EFBIG is returned.
> 
> 'Funny' that, copy_struct_from_user() below seems to use E2BIG.

This is a copy of the semantics that sched_[sg]etattr(2) uses -- E2BIG for
a "too big" struct passed to the kernel, and EFBIG for a "too big"
struct passed to user-space. I would personally have preferred EMSGSIZE
instead of EFBIG, but felt using the existing error codes would be less
confusing.

> 
> > + *  * 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.

Will update the kernel-doc.

> > +	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.

I will switch this (and to) back to -E2BIG -- I must've had a brain-fart
when doing some refactoring.

> 
> > +	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?

Is a 64-byte buffer too big? I picked the number "at random" to be the
size of a cache line, but I could shrink it down to 32 bytes if the size
is an issue (I wanted to avoid needless allocations -- hence it being
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?

Is there a non-x86-specific way to do that (unless I'm mistaken only x86
has uaccess_try() or the other *_try() wrappers)? The main "performance
improvement" (if you can even call it that) is that we use memchr_inv()
which finds non-matching characters more efficiently than just doing a
loop.

> > +	}
> > +	/* 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().

I don't have much of an opinion (after all, it only really makes sense a
lot of sense for syscalls) -- though out-of-tree modules that define
ioctl()s wouldn't be able to make use of them.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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