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().

Cc: sta...@vger.kernel.org # v4.1+
Fixes: e928e9cb3601 ("KVM: PPC: Book3S HV: Add fast real-mode H_RANDOM 
implementation.")
Signed-off-by: Jason A. Donenfeld <ja...@zx2c4.com>
Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
---
 arch/powerpc/platforms/powernv/rng.c | 2 ++
 1 file changed, 2 insertions(+)

v6: New patch, though change log based on Jason's original.

diff --git a/arch/powerpc/platforms/powernv/rng.c 
b/arch/powerpc/platforms/powernv/rng.c
index 3805ad13b8f3..2287c9cd0cd5 100644
--- a/arch/powerpc/platforms/powernv/rng.c
+++ b/arch/powerpc/platforms/powernv/rng.c
@@ -63,6 +63,8 @@ int powernv_get_random_real_mode(unsigned long *v)
        struct powernv_rng *rng;
 
        rng = raw_cpu_read(powernv_rng);
+       if (!rng)
+               return 0;
 
        *v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));
 
-- 
2.35.3

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