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Date: Tue, 27 Jul 2021 10:46:14 +1000
From: Michael Ellerman <>
Subject: Re: Linux kernel: powerpc: KVM guest to host memory corruption

Michael Ellerman <> writes:
> The Linux kernel for powerpc since v3.10 has a bug which allows a malicious KVM guest to
> corrupt host memory.
> In the handling of the H_RTAS hypercall, args.rets is made to point into the args.args
> buffer which is located on the stack:
> 	args.rets = &args.args[be32_to_cpu(args.nargs)];
> However args.nargs has not been range checked. That allows the guest to point args.rets
> anywhere up to +16GB from args.args.
> The guest does not have control of what is written to args.rets, it is always (u32)-3,
> because subsequent code does check nargs. Additionally the guest will be killed as a
> result of the nargs being out of range, so a given guest only has a single shot at
> corrupting memory.
> Only machines using Linux as the hypervisor, aka. KVM or bare metal, are affected by the
> bug.
> The bug was introduced in:
>     8e591cb72047 ("KVM: PPC: Book3S: Add infrastructure to implement kernel-side RTAS calls")
> Which was first released in v3.10.
> The upstream fix is:
>   f62f3c20647e ("KVM: PPC: Book3S: Fix H_RTAS rets buffer overflow")
> Which will be included in the v5.14 release.

This has been assigned CVE-2021-37576.


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