[RFC PATCH 1/1] dma-debug: fix check_for_illegal_area() in debug_dma_map_sg()

[Gerald Schaefer]

The following warning occurred sporadically on s390:
DMA-API: nvme 0006:00:00.0: device driver maps memory from kernel text or rodata [addr=0000000048cc5e2f] [len=131072]
WARNING: CPU: 4 PID: 825 at kernel/dma/debug.c:1083 check_for_illegal_area+0xa8/0x138

It is a false-positive warning, due to a broken logic in debug_dma_map_sg().
check_for_illegal_area() should check for overlay of sg elements with kernel
text or rodata. It is called with sg_dma_len(s) instead of s->length as
parameter. After the call to ->map_sg(), sg_dma_len() contains the length
of possibly combined sg elements in the DMA address space, and not the
individual sg element length, which would be s->length.

The check will then use the kernel start address of an sg element, and add
the DMA length for overlap check, which can result in the false-positive
warning because the DMA length can be larger than the actual single sg
element length in kernel address space.

In addition, the call to check_for_illegal_area() happens in the iteration
over mapped_ents, which will not include all individual sg elements if
any of them were combined in ->map_sg().

Fix this by using s->length instead of sg_dma_len(s). Also put the call to
check_for_illegal_area() in a separate loop, iterating over all the
individual sg elements ("nents" instead of "mapped_ents").

Fixes: 884d05970bfb ("dma-debug: use sg_dma_len accessor")
Tested-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
---
 kernel/dma/debug.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index 14de1271463f..d7d44b7fe7e2 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -1299,6 +1299,12 @@ void debug_dma_map_sg(struct device *dev, struct scatterlist *sg,
 	if (unlikely(dma_debug_disabled()))
 		return;
 
+	for_each_sg(sg, s, nents, i) {
+		if (!PageHighMem(sg_page(s))) {
+			check_for_illegal_area(dev, sg_virt(s), s->length);
+		}
+	}
+
 	for_each_sg(sg, s, mapped_ents, i) {
 		entry = dma_entry_alloc();
 		if (!entry)
@@ -1316,10 +1322,6 @@ void debug_dma_map_sg(struct device *dev, struct scatterlist *sg,
 
 		check_for_stack(dev, sg_page(s), s->offset);
 
-		if (!PageHighMem(sg_page(s))) {
-			check_for_illegal_area(dev, sg_virt(s), sg_dma_len(s));
-		}
-
 		check_sg_segment(dev, s);
 
 		add_dma_entry(entry);
-- 
2.25.1