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Date: Tue, 19 Sep 2017 20:18:28 -0700
From: Eric Biggers <ebiggers3@...il.com>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...nel.org>,
Kevin Hao <haokexin@...il.com>, Oleg Nesterov <oleg@...hat.com>,
Wanpeng Li <wanpeng.li@...mail.com>,
Yu-cheng Yu <yu-cheng.yu@...el.com>,
Michael Halcrow <mhalcrow@...gle.com>,
Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2 3/3] x86/fpu: reinitialize FPU registers if restoring
FPU state fails
On Tue, Sep 19, 2017 at 05:44:34PM -0700, Eric Biggers wrote:
> +/*
> + * We should never get here because the fpregs_state stored in 'struct fpu'
> + * should always be readable and contain a valid FPU state. However, past bugs
> + * have allowed userspace to set reserved bits in the XSAVE area using
> + * PTRACE_SETREGSET or sys_rt_sigreturn(). These caused XRSTOR to fail when
> + * switching to the task, leaking the FPU registers of the task previously
> + * executing on the CPU. Mitigate this class of vulnerability by restoring from
> + * the initial state (essentially, zeroing out all the FPU registers) if we
> + * can't restore from the task's FPU state.
> + */
> +void __handle_bad_fpstate(union fpregs_state *fpstate, u64 mask)
> +{
> + WARN_ONCE(1, "Bad FPU state detected, reinitializing FPU registers");
> + ____copy_kernel_to_fpregs(&init_fpstate, mask);
> +}
Sorry, a small oversight here --- this needs to be exported to modules, since
kvm can be built as a module, and kvm uses __copy_kernel_to_fpregs().
Eric
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