nvme: Discard workaround for non-conformant devices

Users observe IOMMU related errors when performing discard on nvme from
non-compliant nvme devices reading beyond the end of the DMA mapped
ranges to discard.

Two different variants of this behavior have been observed: SM22XX
controllers round up the read size to a multiple of 512 bytes, and Phison
E12 unconditionally reads the maximum discard size allowed by the spec
(256 segments or 4kB).

Make nvme_setup_discard unconditionally allocate the maximum DSM buffer
so the driver DMA maps a memory range that will always succeed.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=202665 many
Signed-off-by: Eduard Hasenleithner <eduard@hasenleithner.at>
[changelog, use existing define, kernel coding style]
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/core.c

@@ -574,8 +574,14 @@ static blk_status_t nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 	struct nvme_dsm_range *range;
 	struct bio *bio;
 
-	range = kmalloc_array(segments, sizeof(*range),
-				GFP_ATOMIC | __GFP_NOWARN);
+	/*
+	 * Some devices do not consider the DSM 'Number of Ranges' field when
+	 * determining how much data to DMA. Always allocate memory for maximum
+	 * number of segments to prevent device reading beyond end of buffer.
+	 */
+	static const size_t alloc_size = sizeof(*range) * NVME_DSM_MAX_RANGES;
+
+	range = kzalloc(alloc_size, GFP_ATOMIC | __GFP_NOWARN);
 	if (!range) {
 		/*
 		 * If we fail allocation our range, fallback to the controller
@@ -615,7 +621,7 @@ static blk_status_t nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 
 	req->special_vec.bv_page = virt_to_page(range);
 	req->special_vec.bv_offset = offset_in_page(range);
-	req->special_vec.bv_len = sizeof(*range) * segments;
+	req->special_vec.bv_len = alloc_size;
 	req->rq_flags |= RQF_SPECIAL_PAYLOAD;
 
 	return BLK_STS_OK;