exec: open code copy_string_kernel

Currently copy_string_kernel is just a wrapper around copy_strings that
simplifies the calling conventions and uses set_fs to allow passing a
kernel pointer.  But due to the fact the we only need to handle a single
kernel argument pointer, the logic can be sigificantly simplified while
getting rid of the set_fs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Link: http://lkml.kernel.org/r/20200501104105.2621149-3-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -592,17 +592,42 @@ out:
  */
 int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
 {
-	int r;
-	mm_segment_t oldfs = get_fs();
-	struct user_arg_ptr argv = {
-		.ptr.native = (const char __user *const  __user *)&arg,
-	};
+	int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */;
+	unsigned long pos = bprm->p;
 
-	set_fs(KERNEL_DS);
-	r = copy_strings(1, argv, bprm);
-	set_fs(oldfs);
+	if (len == 0)
+		return -EFAULT;
+	if (!valid_arg_len(bprm, len))
+		return -E2BIG;
 
-	return r;
+	/* We're going to work our way backwards. */
+	arg += len;
+	bprm->p -= len;
+	if (IS_ENABLED(CONFIG_MMU) && bprm->p < bprm->argmin)
+		return -E2BIG;
+
+	while (len > 0) {
+		unsigned int bytes_to_copy = min_t(unsigned int, len,
+				min_not_zero(offset_in_page(pos), PAGE_SIZE));
+		struct page *page;
+		char *kaddr;
+
+		pos -= bytes_to_copy;
+		arg -= bytes_to_copy;
+		len -= bytes_to_copy;
+
+		page = get_arg_page(bprm, pos, 1);
+		if (!page)
+			return -E2BIG;
+		kaddr = kmap_atomic(page);
+		flush_arg_page(bprm, pos & PAGE_MASK, page);
+		memcpy(kaddr + offset_in_page(pos), arg, bytes_to_copy);
+		flush_kernel_dcache_page(page);
+		kunmap_atomic(kaddr);
+		put_arg_page(page);
+	}
+
+	return 0;
 }
 EXPORT_SYMBOL(copy_string_kernel);