iommu/io-pgtable-arm: Avoid dereferencing bogus PTEs

In the case of corrupted page tables, or when an invalid size is given,
__arm_lpae_unmap() may recurse beyond the maximum number of levels.
Unfortunately the detection of this error condition only happens *after*
calculating a nonsense offset from something which might not be a valid
table pointer and dereferencing that to see if it is a valid PTE.

Make things a little more robust by checking the level is valid before
doing anything which depends on it being so.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index 7df97777662d4d8a9284a8f2cae4cc0e8891210a..366a354c689d5ccfd06a981eb4b398966b2951da 100644 (file)
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -486,11 +486,13 @@ static int __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
        void *cookie = data->iop.cookie;
        size_t blk_size = ARM_LPAE_BLOCK_SIZE(lvl, data);
 
+       /* Something went horribly wrong and we ran out of page table */
+       if (WARN_ON(lvl == ARM_LPAE_MAX_LEVELS))
+               return 0;
+
        ptep += ARM_LPAE_LVL_IDX(iova, lvl, data);
        pte = *ptep;
-
-       /* Something went horribly wrong and we ran out of page table */
-       if (WARN_ON(!pte || (lvl == ARM_LPAE_MAX_LEVELS)))
+       if (WARN_ON(!pte))
                return 0;
 
        /* If the size matches this level, we're in the right place */