[PATCH] mm: mremap: validate input before taking lock

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

This patch is very similar to 84d96d897671: 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>
---
 mm/mremap.c |   18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 463a257..00b6905 100644
--- a/mm/mremap.c
+++ b/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;
 	}
 
-- 
1.7.9.5

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