On 2019-09-05, Al Viro [off-list ref] wrote:

On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:

quoted

+/*
+ * "memset(p, 0, size)" but for user space buffers. Caller must have already
+ * checked access_ok(p, size).
+ */
+static int __memzero_user(void __user *p, size_t s)
+{
+	const char zeros[BUFFER_SIZE] = {};
+	while (s > 0) {
+		size_t n = min(s, sizeof(zeros));
+
+		if (__copy_to_user(p, zeros, n))
+			return -EFAULT;
+
+		p += n;
+		s -= n;
+	}
+	return 0;
+}

That's called clear_user().

Already switched, I didn't know about clear_user() -- I assumed it
would've been called bzero_user() or memzero_user() and didn't find it
when looking.

quoted

+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;

Why?

quoted

+	} 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;

Why not simply clear_user() and copy_to_user()?

I'm not sure I understand what you mean -- are you asking why we need to
do memchr_inv(src + size, 0, rest) earlier?

quoted

+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);

Cute, but... you would be just as well without that 'rest' thing.

I would argue it's harder to mess up using "rest" compared to getting
"ksize - usize" and "usize - ksize" mixed up (and it's a bit more
readable).

quoted

+
+	if (unlikely(usize > PAGE_SIZE))
+		return -EFAULT;

Again, why?

As discussed in a sister thread, I will leave this in the callers
(though I would argue callers should always do some kind of sanity check
like this).

quoted

+	if (unlikely(!access_ok(src, usize)))
+		return -EFAULT;

Why not simply copy_from_user() here?

quoted

+	/* 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] = {};
+
+		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;

Frankly, that looks like a candidate for is_all_zeroes_user().
With the loop like above serving as a dumb default.  And on
badly alighed address it _will_ be dumb.  Probably too much
so - something like
	if ((unsigned long)addr & 1) {
		u8 v;
		if (get_user(v, (__u8 __user *)addr))
			return -EFAULT;
		if (v)
			return -E2BIG;
		addr++;
	}
	if ((unsigned long)addr & 2) {
		u16 v;
		if (get_user(v, (__u16 __user *)addr))
			return -EFAULT;
		if (v)
			return -E2BIG;
		addr +=2;
	}
	if ((unsigned long)addr & 4) {
		u32 v;
		if (get_user(v, (__u32 __user *)addr))
			return -EFAULT;
		if (v)
			return -E2BIG;
	}
	<read the rest like you currently do>
would be saner, and things like x86 could trivially add an
asm variant - it's not hard.  Incidentally, memchr_inv() is
an overkill in this case...

Why is memchr_inv() overkill?

But yes, breaking this out to an asm-generic is_all_zeroes_user()
wouldn't hurt -- and I'll put a cleaned-up version of the alignment
handling there too. Should I drop it in asm-generic/uaccess.h, or
somewhere else?

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