[PATCH] zram: fix idle age_sec underflow in idle_store()

[Hao Jia <jiahao.kernel@gmail.com>]

From: Hao Jia <jiahao1@lixiang.com>

After commit 2e8ff2f51dde ("zram: use u32 for entry ac_time tracking"),
idle_store() computes the idle cutoff as follows:

	cutoff = ktime_sub((u32)ktime_get_boottime_seconds(), age_sec);

Because the left operand is cast to a 32-bit unsigned integer, when
age_sec exceeds the current uptime, the subtraction wraps modulo 2^32
and the huge result is zero-extended into the s64 cutoff.  mark_idle()
subsequently marks every entry as idle instead of matching nothing,
breaking the intended semantics of /sys/block/zramX/idle.  For instance,
running echo 86400 > idle on a machine up for only two minutes causes
all newly written pages to be marked as idle and handed over to idle
writeback and recompression.

Drop the explicit cast to perform the subtraction in signed arithmetic
again, restoring the previous behavior: an age_sec greater than uptime
yields a negative cutoff, and one equal to uptime yields 0. Since a
cutoff of 0 is now a valid computed value, it can no longer double as
the "all" sentinel. Switch to KTIME_MIN instead, which no ac_time can
be after.

Fixes: 2e8ff2f51dde ("zram: use u32 for entry ac_time tracking")
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
---
 drivers/block/zram/zram_drv.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a9b3bb1d3bef..da51f70b7121 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -438,7 +438,7 @@ static void mark_idle(struct zram *zram, ktime_t cutoff)
 		}
 
 #ifdef CONFIG_ZRAM_TRACK_ENTRY_ACTIME
-		is_idle = !cutoff ||
+		is_idle = cutoff == KTIME_MIN ||
 			ktime_after(cutoff, zram->table[index].attr.ac_time);
 #endif
 		if (is_idle)
@@ -453,7 +453,7 @@ static ssize_t idle_store(struct device *dev, struct device_attribute *attr,
 			  const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
-	ktime_t cutoff = 0;
+	ktime_t cutoff = KTIME_MIN;
 
 	if (!sysfs_streq(buf, "all")) {
 		/*
@@ -464,7 +464,7 @@ static ssize_t idle_store(struct device *dev, struct device_attribute *attr,
 
 		if (IS_ENABLED(CONFIG_ZRAM_TRACK_ENTRY_ACTIME) &&
 		    !kstrtouint(buf, 0, &age_sec))
-			cutoff = ktime_sub((u32)ktime_get_boottime_seconds(),
+			cutoff = ktime_sub(ktime_get_boottime_seconds(),
 					   age_sec);
 		else
 			return -EINVAL;
@@ -475,7 +475,7 @@ static ssize_t idle_store(struct device *dev, struct device_attribute *attr,
 		return -EINVAL;
 
 	/*
-	 * A cutoff of 0 marks everything as idle, this is the
+	 * A cutoff of KTIME_MIN marks everything as idle, this is the
 	 * "all" behavior.
 	 */
 	mark_idle(zram, cutoff);
-- 
2.34.1