Re: [PATCH] ARM: dma-mapping: fix out of bounds access in CMA
From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Date: 2021-03-23 13:30:01
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On Tue, Mar 23, 2021 at 02:14:13PM +0100, Arnd Bergmann wrote:
quoted hunk ↗ jump to hunk
From: Arnd Bergmann <arnd@arndb.de> 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 <unknown> 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 <unknown> 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 <arnd@arndb.de> --- 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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