[Skiboot] [PATCH] flash: Handle nullptr dereference of system_flash

[Aditya Gupta]

With QEMU with NO support for MPIPL, 'p9_sbe_terminate' returns early
at:

    /* Return if MPIPL is not supported */
    if (!is_mpipl_enabled())
    	return;

But with MPIPL supported in QEMU, 'p9_sbe_terminate' continues further and
calls 'flash_unregister' which causes a Machine Check due to nullptr
dereference of 'system_flash':

    [   13.240783728,5] Reboot: OS reported error. Performing MPIPL
    [   13.241662601,5] DUMP: Crashing PIR = 0x0
    [   13.244049276,5] RESET: Fast reboot disabled: Kernel re-entered OPAL
    [    1.815018] Disabling lock debugging due to kernel taint
    [    1.815518] MCE: CPU0: machine check (Severe)  Real address Load (bad) DAR: 0000006000000098 [Not recovered]
    [    1.815544] MCE: CPU0: NIP: [0000000030040f54] 0x30040f54
    [    1.815911] MCE: CPU0: Initiator CPU
    [    1.815930] MCE: CPU0: Hardware error
    [    1.816110] opal: Hardware platform error: Unrecoverable Machine Check exception
    [    1.816338] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Tainted: G   M               6.12.0-rc4+ #1
    [    1.816531] Tainted: [M]=MACHINE_CHECK
    [    1.816546] Hardware name: IBM PowerNV (emulated by qemu) POWER10 0x801200 opal:v7.1 PowerNV
    [    1.816629] NIP:  0000000030040f54 LR: 000000003007e528 CTR: 000000003004d75c
    [    1.816646] REGS: c0000004d5e47d60 TRAP: 0200   Tainted: G   M                (6.12.0-rc4+)
    [    1.816684] MSR:  9000000002a03002 <SF,HV,VEC,VSX,FP,ME,RI>  CR: 28002284  XER: 00000000
    [    1.816863] CFAR: 000000003007e524 DAR: 0000006000000098 DSISR: 00000040 IRQMASK: 3
    [    1.816863] GPR00: 000000003007e528 0000000031c13ac0 0000000030192900 0000006000000060
    [    1.816863] GPR04: 0000000030500028 000000000000000a 0000000031c10068 0000000031c10068
    [    1.816863] GPR08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    [    1.816863] GPR12: 0000000028002284 c000000002e80000 c00000000001192c 0000000000000000
    [    1.816863] GPR16: 0000000031c10000 0000000000000000 0000000000000000 0000000000000000
    [    1.816863] GPR20: 0000000000000003 0000000000000074 0000000000000000 0000000000000000
    [    1.816863] GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    [    1.816863] GPR28: c000000002d0e8c8 00000000301257de c000000002d0e8c8 000000000000000c
    [    1.817061] NIP [0000000030040f54] 0x30040f54
    [    1.817074] LR [000000003007e528] 0x3007e528
    [    1.817165] Call Trace:
    [    1.817337] Code: 00000060 80002138 e01d0d48 00000000 01000000 00000180 a602087c 3700223d 602e29e9 100001f8 91ff21f8 180069e8 <380023e9> 0000292c 34008241 280041f8
    [   13.247702490,0] OPAL: Reboot requested due to Platform error.
    [   13.247857686,3] OPAL: failed to log an error
    [   13.248012502,2] NVRAM: Failed to load

Previously above machine check was never hit as QEMU platform didn't
had MPIPL, and hence the caller 'p9_sbe_terminate' used to return early.

Add null check to ignore the unregister request if system_flash is not set.

Signed-off-by: Aditya Gupta <adityag at linux.ibm.com>

---
Initial QEMU MPIPL support was posted to [1]. It has not been merged
yet.

[1]: https://lore.kernel.org/qemu-devel/20250217071934.86131-1-adityag@linux.ibm.com/

Changelog
==========
rfc -> v1:
  + reword warning message to say system flash not registered
---
---
 core/flash.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/core/flash.c b/core/flash.c
index a14bfc68fd1a..6d9aeb1cbfda 100644
--- a/core/flash.c
+++ b/core/flash.c
@@ -88,7 +88,16 @@ void flash_release(void)
 
 bool flash_unregister(void)
 {
-	struct blocklevel_device *bl = system_flash->bl;
+	struct blocklevel_device *bl;
+
+	if (!system_flash) {
+		prlog(PR_WARNING, "System Flash is not registered, ignoring"\
+				" unregister request\n");
+
+		return true;
+	}
+
+	bl = system_flash->bl;
 
 	if (bl->exit)
 		return bl->exit(bl);
-- 
2.48.1