[PATCH 2/2] mm: Avoid overflows in dirty throttling logic

[Jan Kara <jack@suse.cz>]

The dirty throttling logic is interspersed with assumptions that dirty
limits in PAGE_SIZE units fit into 32-bit (so that various
multiplications fit into 64-bits). If limits end up being larger, we
will hit overflows, possible divisions by 0 etc. Fix these problems by
never allowing so large dirty limits as they have dubious practical
value anyway. For dirty_bytes / dirty_background_bytes interfaces we can
just refuse to set so large limits. For dirty_ratio /
dirty_background_ratio it isn't so simple as the dirty limit is computed
from the amount of available memory which can change due to memory
hotplug etc. So when converting dirty limits from ratios to numbers of
pages, we just don't allow the result to exceed UINT_MAX.

Reported-by: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/page-writeback.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2573e2d504af..8a1c92090129 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -415,13 +415,20 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	else
 		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
 
-	if (bg_thresh >= thresh)
-		bg_thresh = thresh / 2;
 	tsk = current;
 	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
+	/*
+	 * Dirty throttling logic assumes the limits in page units fit into
+	 * 32-bits. This gives 16TB dirty limits max which is hopefully enough.
+	 */
+	if (thresh > UINT_MAX)
+		thresh = UINT_MAX;
+	/* This makes sure bg_thresh is within 32-bits as well */
+	if (bg_thresh >= thresh)
+		bg_thresh = thresh / 2;
 	dtc->thresh = thresh;
 	dtc->bg_thresh = bg_thresh;
 
@@ -471,7 +478,11 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	if (rt_task(tsk))
 		dirty += dirty / 4;
 
-	return dirty;
+	/*
+	 * Dirty throttling logic assumes the limits in page units fit into
+	 * 32-bits. This gives 16TB dirty limits max which is hopefully enough.
+	 */
+	return min_t(unsigned long, dirty, UINT_MAX);
 }
 
 /**
@@ -508,10 +519,17 @@ static int dirty_background_bytes_handler(struct ctl_table *table, int write,
 		void *buffer, size_t *lenp, loff_t *ppos)
 {
 	int ret;
+	unsigned long old_bytes = dirty_background_bytes;
 
 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
-	if (ret == 0 && write)
+	if (ret == 0 && write) {
+		if (DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE) >
+								UINT_MAX) {
+			dirty_background_bytes = old_bytes;
+			return -ERANGE;
+		}
 		dirty_background_ratio = 0;
+	}
 	return ret;
 }
 
@@ -537,6 +555,10 @@ static int dirty_bytes_handler(struct ctl_table *table, int write,
 
 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 	if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
+		if (DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) > UINT_MAX) {
+			vm_dirty_bytes = old_bytes;
+			return -ERANGE;
+		}
 		writeback_set_ratelimit();
 		vm_dirty_ratio = 0;
 	}
-- 
2.35.3