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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Michael Ellerman <mpe@ellerman.id.au>
Subject: [PATCH 5.10 062/545] powerpc/powernv: Avoid crashing if rng is NULL
Date:   Fri, 19 Aug 2022 17:37:12 +0200
Message-Id: <20220819153832.012015356@linuxfoundation.org>
In-Reply-To: <20220819153829.135562864@linuxfoundation.org>
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From: Michael Ellerman <mpe@ellerman.id.au>

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@vger.kernel.org # v4.1+
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220727143219.2684192-1-mpe@ellerman.id.au
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 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));