[PATCH v1 1/2] target/ppc: Fix decrementer time underflow and infinite timer loop

[Nicholas Piggin <npiggin@gmail.com>]

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 <npiggin@gmail.com>
---
sdicaro@DDCI.com debugged and reported this, I just changed their fix
to extract variable bits so it works with large decrementer. So most
of the credit goes to them.

Thanks,
Nick

 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);
 
-- 
2.40.1