On Tue, Mar 23, 2021 at 02:14:13PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann
>
> Dereferencing a zero-length array is always a bug, and we get a warning
> with 'make W=1' here:
>
> arch/arm/mm/dma-mapping.c: In function 'dma_contiguous_early_fixup':
> arch/arm/mm/dma-mapping.c:395:15: error: array subscript is outside
> array bounds of 'struct dma_contig_early_reserve[0]' [-Werror=array-bounds]
> 395 | dma_mmu_remap[dma_mmu_remap_num].base = base;
> | ~^~~
> arch/arm/mm/dma-mapping.c:389:40: note: while referencing 'dma_mmu_remap'
> 389 | static struct dma_contig_early_reserve dma_mmu_remap[MAX_CMA_AREAS]
> __initdata;
> |^
> arch/arm/mm/dma-mapping.c:396:15: error: array subscript is outside
> array bounds of 'struct dma_contig_early_reserve[0]' [-Werror=array-bounds]
>
> Add a runtime check to prevent this from happening, while also
> avoiding the compile-time warning.
>
> Fixes: c79095092834 ("ARM: integrate CMA with DMA-mapping subsystem")
> Signed-off-by: Arnd Bergmann
> ---
> arch/arm/mm/dma-mapping.c | 9 +
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index c4b8df2ad328..af29344fb150 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -392,6 +392,11 @@ static int dma_mmu_remap_num __initdata;
>
> void __init dma_contiguous_early_fixup(phys_addr_t base, unsigned long size)
> {
> + if (!MAX_CMA_AREAS || dma_mmu_remap_num >= MAX_CMA_AREAS) {
> + WARN_ONCE(1, "number of CMA areas\n");
> + return;
> + }
> +
What if dma_mmu_remap_num were negative - that condition is not checked
and will also result in an overflow of the array. If we're being fussy
enough to bounds check, we ought to do it properly.
So, I think a better solution would be to make dma_mmu_remap_num an
unsigned int, and then to use:
if (dma_mmu_remap_num >= ARRAY_SIZE(dma_mmu_remap)) {
...
}
which is really the condition we're after here.
--
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