Subject: [PATCH v2] slab: kmalloc_size_roundup() must not return 0 for non-zero size

[David Laight]

The typical use of kmalloc_size_roundup() is:
	ptr = kmalloc(sz = kmalloc_size_roundup(size), ...);
	if (!ptr) return -ENOMEM.
This means it is vitally important that the returned value isn't
less than the argument even if the argument is insane.
In particular if kmalloc_slab() fails or the value is above
(MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return
it's single zero-length buffer.

Fix by returning the input size on error or if the size exceeds
a 'sanity' limit.
kmalloc() will then return NULL is the size really is too big.


Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
Fixes: 05a940656e1eb ("slab: Introduce kmalloc_size_roundup()")
---
v2:
    - Use KMALLOC_MAX_SIZE for upper limit.
      (KMALLOC_MAX_SIZE + 1 may give better code on some archs!)
    - Invert test for overlarge for consistency.
    - Put a likely() on result of kmalloc_slab().

 mm/slab_common.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index cd71f9581e67..0fb7c7e19bad 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -747,22 +747,22 @@ size_t kmalloc_size_roundup(size_t size)
 {
 	struct kmem_cache *c;
 
-	/* Short-circuit the 0 size case. */
-	if (unlikely(size == 0))
-		return 0;
-	/* Short-circuit saturated "too-large" case. */
-	if (unlikely(size == SIZE_MAX))
-		return SIZE_MAX;
+	if (size && size <= KMALLOC_MAX_CACHE_SIZE) {
+		/*
+		 * The flags don't matter since size_index is common to all.
+		 * Neither does the caller for just getting ->object_size.
+		 */
+		c = kmalloc_slab(size, GFP_KERNEL, 0);
+		return likely(c) ? c->object_size : size;
+	}
+
 	/* Above the smaller buckets, size is a multiple of page size. */
-	if (size > KMALLOC_MAX_CACHE_SIZE)
+	if (size && size <= KMALLOC_MAX_SIZE)
 		return PAGE_SIZE << get_order(size);
 
-	/*
-	 * The flags don't matter since size_index is common to all.
-	 * Neither does the caller for just getting ->object_size.
-	 */
-	c = kmalloc_slab(size, GFP_KERNEL, 0);
-	return c ? c->object_size : 0;
+	/* Return 'size' for 0 and very large - kmalloc() may fail. */
+	return size;
+
 }
 EXPORT_SYMBOL(kmalloc_size_roundup);
 
-- 
2.17.1

-
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