[PATCH 5.19 0129/1157] powerpc/powernv: Avoid crashing if rng is NULL

[Greg Kroah-Hartman]

From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

commit 90b5d4fe0b3ba7f589c6723c6bfb559d9e83956a upstream.

On a bare-metal Power8 system that doesn't have an "ibm,power-rng", a
malicious QEMU and guest that ignore the absence of the
KVM_CAP_PPC_HWRNG flag, and calls H_RANDOM anyway, will dereference a
NULL pointer.

In practice all Power8 machines have an "ibm,power-rng", but let's not
rely on that, add a NULL check and early return in
powernv_get_random_real_mode().

Fixes: e928e9cb3601 ("KVM: PPC: Book3S HV: Add fast real-mode H_RANDOM implementation.")
Cc: stable@xxxxxxxxxxxxxxx # v4.1+
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220727143219.2684192-1-mpe@xxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/platforms/powernv/rng.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/powerpc/platforms/powernv/rng.c
+++ b/arch/powerpc/platforms/powernv/rng.c
@@ -63,6 +63,8 @@ int powernv_get_random_real_mode(unsigne
 	struct powernv_rng *rng;
 
 	rng = raw_cpu_read(powernv_rng);
+	if (!rng)
+		return 0;
 
 	*v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));