mm: mremap: validate input before taking lock

This patch is very similar to commit 84d96d8976 ("mm: madvise:
complete input validation before taking lock"): perform some basic
validation of the input to mremap() before taking the
&current->mm->mmap_sem lock.

This also makes the MREMAP_FIXED => MREMAP_MAYMOVE dependency slightly
more explicit.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mremap.c

@@ -456,13 +456,14 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	unsigned long charged = 0;
 	bool locked = false;
 
-	down_write(&current->mm->mmap_sem);
-
 	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
-		goto out;
+		return ret;
+
+	if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
+		return ret;
 
 	if (addr & ~PAGE_MASK)
-		goto out;
+		return ret;
 
 	old_len = PAGE_ALIGN(old_len);
 	new_len = PAGE_ALIGN(new_len);
@@ -473,12 +474,13 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	 * a zero new-len is nonsensical.
 	 */
 	if (!new_len)
-		goto out;
+		return ret;
+
+	down_write(&current->mm->mmap_sem);
 
 	if (flags & MREMAP_FIXED) {
-		if (flags & MREMAP_MAYMOVE)
-			ret = mremap_to(addr, old_len, new_addr, new_len,
-					&locked);
+		ret = mremap_to(addr, old_len, new_addr, new_len,
+				&locked);
 		goto out;
 	}