dma-debug: fix check_for_illegal_area() in debug_dma_map_sg()

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 <[email protected]>

Signed-off-by Gerald Schaefer <[email protected]>

---
  kernel/dma/debug.c | 10 ++++++----
  1 file changed, 6 insertions(+), 4 deletions(-)

kernel/dma/debug.c +6 -0

index 14de1271463f..d7d44b7fe7e2 100644
@@ -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)

kernel/dma/debug.c +0 -4

-		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);
 

Quoted 50 lines · 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 <[email protected]>

Signed-off-by Gerald Schaefer <[email protected]>

---
  kernel/dma/debug.c | 10 ++++++----
  1 file changed, 6 insertions(+), 4 deletions(-)

kernel/dma/debug.c +6 -0

index 14de1271463f..d7d44b7fe7e2 100644
@@ -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,
 

kernel/dma/debug.c void debug_dma_map_sg(struct device *dev, struct scatterlist *sg, · line 1323

Quoted 1 line

kernel/dma/debug.c +0 -0

  		check_for_stack(dev, sg_page(s), s->offset);

Strictly this should probably be moved to the new loop as well, as it is similarly concerned with validating the source segments rather than the DMA mappings - I think with virtually-mapped stacks it might technically be possible for a stack page to be physically adjacent to a "valid" page such that it could get merged and overlooked if it were near the end of the list, although in fairness that would probably be indicative of something having gone far more fundamentally wrong. Otherwise, the overall reasoning looks sound to me.

Quoted 51 lines · On Tue, 6 Jul 2021 10:22:40 +0100 Robin Murphy <[email protected]> wrote:

Quoted 50 lines · 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 <[email protected]>

Signed-off-by Gerald Schaefer <[email protected]>

---
   kernel/dma/debug.c | 10 ++++++----
   1 file changed, 6 insertions(+), 4 deletions(-)

kernel/dma/debug.c +6 -0

index 14de1271463f..d7d44b7fe7e2 100644
@@ -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,
 

Quoted 10 lines

kernel/dma/debug.c void debug_dma_map_sg(struct device *dev, struct scatterlist *sg, · line 1323

Quoted 1 line

kernel/dma/debug.c +0 -0

   		check_for_stack(dev, sg_page(s), s->offset);

Strictly this should probably be moved to the new loop as well, as it is similarly concerned with validating the source segments rather than the DMA mappings - I think with virtually-mapped stacks it might technically be possible for a stack page to be physically adjacent to a "valid" page such that it could get merged and overlooked if it were near the end of the list, although in fairness that would probably be indicative of something having gone far more fundamentally wrong. Otherwise, the overall reasoning looks sound to me.

I see, good point. I think I can add this to my patch, and a different subject like "dma-debug: fix sg checks in debug_dma_map_sg()".

However, I do not quite understand why check_for_stack() does not also consider s->length. It seems to check only the first page of an sg element.

So, shouldn't check_for_stack() behave similar to check_for_illegal_area(), i.e. check all source sg elements for overlap with the task stack area?

If yes, then this probably should be a separate patch, but I can try to come up with something and send a new RFC with two patches. Maybe check_for_stack() can also be integrated into check_for_illegal_area(), they are both called at the same places. And mapping memory from the stack also sounds rather illegal.