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SubjectRe: [POSSIBLE BUG] iommu/io-pgtable-arm: possible dereferencing of NULL pointer
On Mon, Jul 18, 2022 at 12:20:06PM +0300, Subkhankulov Rustam wrote:
> Version: 5-19-rc6
>
> In function '__arm_lpae_alloc_pages' pointer 'dev' is compared with
> NULL at [drivers/iommu/io-pgtable-arm.c: 203]. This means that the
> pointer can be NULL.
>
> -----------------------------------------------------------------------
> 203 p = alloc_pages_node(dev ? dev_to_node(dev) : NUMA_NO_NODE,
> 204 gfp | __GFP_ZERO, order);
> -----------------------------------------------------------------------
>
> Then, if cfg->coherent_walk == 0 at [drivers/iommu/io-pgtable-arm.c:
> 209], function 'dma_map_single', which is defined as
> 'dma_map_single_attrs', is called and pointer dev is passed as
> first parameter.
>
> -----------------------------------------------------------------------
> 209 if (!cfg->coherent_walk) {
> 208 dma = dma_map_single(dev, pages, size, DMA_TO_DEVICE);
> -----------------------------------------------------------------------
>
> Therefore, pointer 'dev' passed to function 'dev_driver_string'
> in macro 'dev_WARN_ONCE' at [include/linux/dma-mapping.h: 326],
> where it is dereferenced at [drivers/base/core.c: 2091].
>
> -----------------------------------------------------------------------
> 2083 const char *dev_driver_string(const struct device *dev)
> 2084 {
> 2085 struct device_driver *drv;
> 2086
> ---
> 2091 drv = READ_ONCE(dev->driver);
> -----------------------------------------------------------------------
>
> Thus, if it is possible that 'dev' is null at the same time
> that flag 'coherent_walk' is 0, then NULL pointer will be
> dereferenced.
>
> Should we somehow avoid NULL pointer dereference or is this
> situation impossible and we should remove comparison with NULL?

I think 'dev' is only null in the case of the selftest initcall
(see arm_lpae_do_selftests()), and 'coherent_walk' is always true there.

Will

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