[PATCH v3] mm: fix div by zero in bdi_ratio_from_pages

[Stefan Roesch <shr@devkernel.io>]

During testing it has been detected, that it is possible to get div by
zero error in bdi_set_min_bytes. The error is caused by the function
bdi_ratio_from_pages(). bdi_ratio_from_pages() calls
global_dirty_limits. If the dirty threshold is 0, the div by zero is
raised. This can happen if the root user is setting:

echo 0 > /proc/sys/vm/dirty_ratio

The following is a test case:

echo 0 > /proc/sys/vm/dirty_ratio
cd /sys/class/bdi/<device>
echo 1 > strict_limit
echo 8192 > min_bytes

==> error is raised.

The problem is addressed by returning -EINVAL if dirty_ratio or
dirty_bytes is set to 0.

Reported-by: cheung wall <zzqq0103.hey@gmail.com>
Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@devkernel.io/T/#t
Signed-off-by: Stefan Roesch <shr@devkernel.io>

---
Changes in V3:
- Used long instead of unsigned long for min_ratio / max_ratio

Changes in V2:
- check for -EINVAL in bdi_set_min_bytes()
- check for -EINVAL in bdi_set_max_bytes()
---
 mm/page-writeback.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d213ead95675..d9861e42b2bd 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -692,6 +692,8 @@ static unsigned long bdi_ratio_from_pages(unsigned long pages)
 	unsigned long ratio;
 
 	global_dirty_limits(&background_thresh, &dirty_thresh);
+	if (!dirty_thresh)
+		return -EINVAL;
 	ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
 
 	return ratio;
@@ -790,13 +792,15 @@ int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes)
 {
 	int ret;
 	unsigned long pages = min_bytes >> PAGE_SHIFT;
-	unsigned long min_ratio;
+	long min_ratio;
 
 	ret = bdi_check_pages_limit(pages);
 	if (ret)
 		return ret;
 
 	min_ratio = bdi_ratio_from_pages(pages);
+	if (min_ratio < 0)
+		return min_ratio;
 	return __bdi_set_min_ratio(bdi, min_ratio);
 }
 
@@ -809,13 +813,15 @@ int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes)
 {
 	int ret;
 	unsigned long pages = max_bytes >> PAGE_SHIFT;
-	unsigned long max_ratio;
+	long max_ratio;
 
 	ret = bdi_check_pages_limit(pages);
 	if (ret)
 		return ret;
 
 	max_ratio = bdi_ratio_from_pages(pages);
+	if (max_ratio < 0)
+		return max_ratio;
 	return __bdi_set_max_ratio(bdi, max_ratio);
 }
 

base-commit: fbfd64d25c7af3b8695201ebc85efe90be28c5a3
-- 
2.47.1