[PATCH] 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()")
---
The 'sanity limit' value doesn't really matter (even if too small)
It could be 'MAX_ORDER + PAGE_SHIFT' but one ppc64 has MAX_ORDER 16
and I don't know if that also has large pages.
Maybe it could be 1ul << 30 on 64bit, but it really doesn't matter
if it is too big.

The original patch also added kmalloc_size_roundup() to mm/slob.c
that can also round up a value to zero - but has since been removed.

 mm/slab_common.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index cd71f9581e67..8418eccda8cf 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -747,22 +747,21 @@ 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;
-	/* Above the smaller buckets, size is a multiple of page size. */
-	if (size > KMALLOC_MAX_CACHE_SIZE)
-		return PAGE_SIZE << get_order(size);
+	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;
+	}
 
-	/*
-	 * 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. */
+	if (unlikely((size - 1) >> (sizeof (long) == 8 ? 34 : 30)))
+		return size;
+
+	/* Above the smaller buckets, size is a multiple of page size. */
+	return PAGE_SIZE << get_order(size);
 }
 EXPORT_SYMBOL(kmalloc_size_roundup);
 
-- 
2.17.1

-
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