[PATCH 5.17 0670/1126] powerpc/time: Fix KVM host re-arming a timer beyond decrementer range

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 05:53:48 EST

From: Nicholas Piggin <npiggin@xxxxxxxxx>

[ Upstream commit cf74ff52e352112be78c4c4c3637a37ec36a6608 ]

If the next host timer is beyond decrementer range, timer_rearm_host_dec
will leave decrementer not programmed. This will not cause a problem for
the host it will just set the decrementer correctly when the decrementer
interrupt hits, it seems safer not to leave the next host decrementer
interrupt timing able to be influenced by a guest.

This code is only used in the P9 KVM paths so it's unlikely to be hit
practically unless large decrementer is force disabled in the host.

Fixes: 25aa145856cd ("powerpc/time: add API for KVM to re-arm the host timer/decrementer")
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220124143930.3923442-2-npiggin@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/powerpc/kernel/time.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index cd0b8b71ecdd..384f58a3f373 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -582,8 +582,9 @@ void timer_rearm_host_dec(u64 now)
local_paca->irq_happened |= PACA_IRQ_DEC;
} else {
now = *next_tb - now;
- if (now <= decrementer_max)
- set_dec_or_work(now);
+ if (now > decrementer_max)
+ now = decrementer_max;
+ set_dec_or_work(now);