[PATCH 1/7] genalloc: track beginning of allocations

On 05/03/2018 21:00, J Freyensee wrote:

.
.


On 2/28/18 12:06 PM, Igor Stoppa wrote:

quoted

+
+/**
+ * gen_pool_dma_alloc() - allocate special memory from the pool for DMA usage
+ * @pool: pool to allocate from
+ * @size: number of bytes to allocate from the pool
+ * @dma: dma-view physical address return value.  Use NULL if unneeded.
+ *
+ * Allocate the requested number of bytes from the specified pool.
+ * Uses the pool allocation function (with first-fit algorithm by default).
+ * Can not be used in NMI handler on architectures without
+ * NMI-safe cmpxchg implementation.
+ *
+ * Return:
+ * * address of the memory allocated	- success
+ * * NULL				- error
+ */
+void *gen_pool_dma_alloc(struct gen_pool *pool, size_t size, dma_addr_t *dma);
+

OK, so gen_pool_dma_alloc() is defined here, which believe is the API 
line being drawn for this series.

so,
.
.
.

quoted

  
  /**
- * gen_pool_dma_alloc - allocate special memory from the pool for DMA usage
+ * gen_pool_dma_alloc() - allocate special memory from the pool for DMA usage
   * @pool: pool to allocate from
   * @size: number of bytes to allocate from the pool
   * @dma: dma-view physical address return value.  Use NULL if unneeded.

@@ -342,14 +566,15 @@ EXPORT_SYMBOL(gen_pool_alloc_algo);
   * Uses the pool allocation function (with first-fit algorithm by default).
   * Can not be used in NMI handler on architectures without
   * NMI-safe cmpxchg implementation.
+ *
+ * Return:
+ * * address of the memory allocated	- success
+ * * NULL				- error
   */
  void *gen_pool_dma_alloc(struct gen_pool *pool, size_t size, dma_addr_t *dma)
  {
  	unsigned long vaddr;
  
-	if (!pool)
-		return NULL;
-

why is this being removed?? I don't believe this code was getting 
removed from your v17 series patches.

Because, as Matthew Wilcox pointed out [1] (well, that's how I
understood it) de-referencing a NULL pointer will cause the kernel to
complain loudly.

Where is the NULL pointer coming from?

a) from a bug in the user of the API - in that case it will be noticed,
reported and fixed, that is how also other in-kernel APIs work

b) from an attacker - it will still trigger an error from the kernel,
but it cannot really do much else, besides crashing repeatedly and
causing a DOS. However, there are so many other places that could be
under similar attack, that it doesn't seem to make a difference having a
check here only.

If the value was coming from userspace, that would be a completely
different case and some sort of sanitation would be mandatory.

Otherwise, looks good,

Reviewed-by: Jay Freyensee <redacted>

thanks


[1] http://www.openwall.com/lists/kernel-hardening/2018/02/26/16


--
igor

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