On 13/07/2026 10:58 am, Krzysztof Karas wrote:
It is possible, when a very large mapping uses only one
scatterlist, that padding overflows scatterlist's length field.
This results in:
1) silently wrapping the value
2) smaller than desired mappings produced by iommu_map_sg
3) leaving mapped bytes in memory (no iommu_unmap)
Address this issue by adding overflow detection for scatterlist
length field.
Fixes: 809eac54cdd6 ("iommu/dma: Implement scatterlist segment merging")
Signed-off-by: Krzysztof Karas <[email protected]>
---
v3:
* Used check_add_overflow suggested by Jason.
I decided not to include previous r-bs due to the change in the
core of this patch: overflows_type -> check_add_overflow and I
obvserved some folks have heavy preference for one or the other.
drivers/iommu/dma-iommu.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 381b60d9e7ce..1a36fd9bf10b 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -1493,7 +1493,18 @@ int iommu_dma_map_sg(struct device *dev, struct
scatterlist *sg, int nents,
* time through here (i.e. before it has a meaningful value).
*/
Sorry to be a pain, but since we *have* now spent the time reasoning
through the details, for completeness I think it would be worth also
explicitly checking "s_length & UINT_MAX != 0" (or equivalent) after the
the prior iova_align() - since that's only rounding up an unsigned int
value to a power of 2 it can't arbitrarily overflow like the addition
below, but it could still wrap to zero in the assignment to s->length,
or indeed in the size_t itself on 32-bit.
if (pad_len && pad_len < s_length - 1) {
- prev->length += pad_len;
+ unsigned int new_pad_len;
Super-nit: "pad_len" is the amount of potential padding from the end of
the previous segment up to the next segment boundary; the value we're
calculating here is the updated *total* segment length, so
"new_prev_len" would be more accurate and a bit clearer - the logic here
is fiddly enough as it is, so every little helps :)
With those tweaks (since it sounds like there's still another respin or
two to go for the rest of the series anyway),
Reviewed-by: Robin Murphy <[email protected]>
Cheers,
Robin.
+ /*
+ * For large mappings spanning multiple GBs we
+ * may not be able to fit all needed padding into
+ * sg->length.
+ */
+ if (check_add_overflow(prev->length, pad_len,
&new_pad_len)) {
+ ret = -EOVERFLOW;
+ goto out_restore_sg;
+ }
+
+ prev->length = new_pad_len;
iova_len += pad_len;
}
