linux-kernel - Re: [POSSIBLE BUG] iommu/io-pgtable-arm: possible dereferencing of NULL pointer

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Date:   Tue, 19 Jul 2022 18:36:10 +0100
From:   Will Deacon <will@...nel.org>
To:     Subkhankulov Rustam <subkhankulov@...ras.ru>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        ldv-project@...uxtesting.org
Subject: Re: [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