From 4ce718f39776b5e162ea6b8010da7a6f02b13dc0 Mon Sep 17 00:00:00 2001 From: Stefan Roesch Date: Fri, 3 Jan 2025 17:20:37 -0800 Subject: mm: fix div by zero in bdi_ratio_from_pages 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/ 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. [shr@devkernel.io: check for -EINVAL in bdi_set_min_bytes() and bdi_set_max_bytes()] Link: https://lkml.kernel.org/r/20250108014723.166637-1-shr@devkernel.io [shr@devkernel.io: v3] Link: https://lkml.kernel.org/r/20250109063411.6591-1-shr@devkernel.io Link: https://lkml.kernel.org/r/20250104012037.159386-1-shr@devkernel.io Signed-off-by: Stefan Roesch Reported-by: cheung wall Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@devkernel.io/T/#t Acked-by: David Hildenbrand Cc: Matthew Wilcox (Oracle) Cc: Qiang Zhang Signed-off-by: Andrew Morton --- 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); } -- cgit v1.2.3