[PATCH] time/namespace: Fix overflow when converting absolute deadlines

[Hungyu Lin <dennylin0707@gmail.com>]

do_timens_ktime_to_host() subtracts the namespace offset from an absolute
deadline and clamps the result to [0, KTIME_MAX]. A negative offset can
make a large deadline overflow to a negative value. Since ktime_t is
signed and KTIME_MAX is its maximum value, the subsequent comparison
against KTIME_MAX can never detect this overflow.

For example, with a -1 second boottime offset, an absolute CLOCK_BOOTTIME
timerfd deadline of KTIME_MAX can expire immediately. An absolute
clock_nanosleep() using the same clock and deadline can also return
immediately instead of waiting.

The earlier tim < offset check already handles the lower bound. A
negative subtraction result in the remaining branch therefore indicates
overflow. Detect that result and clamp it to KTIME_MAX.

Fixes: 89dd8eecfe961 ("time: Add do_timens_ktime_to_host() helper")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 kernel/time/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index 3aff27bb0a15..635f7e544bcc 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -51,7 +51,7 @@ ktime_t do_timens_ktime_to_host(clockid_t clockid, ktime_t tim,
 		tim = 0;
 	} else {
 		tim = ktime_sub(tim, offset);
-		if (unlikely(tim > KTIME_MAX))
+		if (unlikely(tim < 0))
 			tim = KTIME_MAX;
 	}
 
-- 
2.43.0