target/riscv/pmp: guard against PMP ranges with a negative size

For a TOR entry to match, the stard address must be lower than the end
address. Normally this is always the case, but correct code might still
run into the following scenario:

Initial state:

	pmpaddr3 = 0x2000	pmp3cfg = OFF
	pmpaddr4 = 0x3000	pmp4cfg = TOR

Execution:

	1. write 0x40ff to pmpaddr3
	2. write 0x32ff to pmpaddr4
	3. set pmp3cfg to NAPOT with a read-modify-write on pmpcfg0
	4. set pmp4cfg to NAPOT with a read-modify-write on pmpcfg1

When (2) is emulated, a call to pmp_update_rule() creates a negative
range for pmp4 as pmp4cfg is still set to TOR. And when (3) is emulated,
a call to tlb_flush() is performed, causing pmp_get_tlb_size() to return
a very creatively large TLB size for pmp4. This, in turn, may result in
accesses to non-existent/unitialized memory regions and a fault, so that
(4) ends up never being executed.

This is in m-mode with MPRV unset, meaning that unlocked PMP entries
should have no effect. Therefore such a behavior based on PMP content
is very unexpected.

Make sure no negative PMP range can be created, whether explicitly by
the emulated code or implicitly like the above.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <3oq0sqs1-67o0-145-5n1s-453o118804q@syhkavp.arg>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/pmp.c

@@ -167,6 +167,9 @@ void pmp_update_rule_addr(CPURISCVState *env, uint32_t pmp_index)
     case PMP_AMATCH_TOR:
         sa = prev_addr << 2; /* shift up from [xx:0] to [xx+2:2] */
         ea = (this_addr << 2) - 1u;
+        if (sa > ea) {
+            sa = ea = 0u;
+        }
         break;
 
     case PMP_AMATCH_NA4: