From 09d2db9f46e38e2da990df8ad914d735d764251a Mon Sep 17 00:00:00 2001 From: Nicholas Piggin Date: Tue, 30 May 2023 23:12:12 +1000 Subject: [PATCH] target/ppc: Fix decrementer time underflow and infinite timer loop It is possible to store a very large value to the decrementer that it does not raise the decrementer exception so the timer is scheduled, but the next time value wraps and is treated as in the past. This can occur if (u64)-1 is stored on a zero-triggered exception, or (u64)-1 is stored twice on an underflow-triggered exception, for example. If such a value is set in DECAR, it gets stored to the decrementer by the timer function, which then immediately causes another timer, which hangs QEMU. Clamp the decrementer to the implemented width, and use that as the value for the timer calculation, effectively preventing this overflow. Reported-by: sdicaro@DDCI.com Signed-off-by: Nicholas Piggin Reviewed-by: Daniel Henrique Barboza Message-Id: <20230530131214.373524-1-npiggin@gmail.com> Signed-off-by: Daniel Henrique Barboza --- hw/ppc/ppc.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c index 4e816c68c7..d80b0adc6c 100644 --- a/hw/ppc/ppc.c +++ b/hw/ppc/ppc.c @@ -798,6 +798,8 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp, int64_t signed_decr; /* Truncate value to decr_width and sign extend for simplicity */ + value = extract64(value, 0, nr_bits); + decr = extract64(decr, 0, nr_bits); signed_value = sextract64(value, 0, nr_bits); signed_decr = sextract64(decr, 0, nr_bits);