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Date:   Fri, 24 Jul 2020 10:43:50 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     x86@kernel.org, Joerg Roedel <jroedel@suse.de>, hpa@zytor.com,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>,
        David Rientjes <rientjes@google.com>,
        Cfir Cohen <cfir@google.com>,
        Erdem Aktas <erdemaktas@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mike Stunes <mstunes@vmware.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Martin Radev <martin.b.radev@gmail.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v5 11/75] x86/boot/compressed/64: Disable red-zone usage
Message-ID: <202007241043.654ABB2@keescook>
References: <20200724160336.5435-1-joro@8bytes.org>
 <20200724160336.5435-12-joro@8bytes.org>
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On Fri, Jul 24, 2020 at 06:02:32PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> The x86-64 ABI defines a red-zone on the stack:
> 
>   The 128-byte area beyond the location pointed to by %rsp is considered
>   to be reserved and shall not be modified by signal or interrupt
>   handlers. Therefore, functions may use this area for temporary data
>   that is not needed across function calls. In particular, leaf
>   functions may use this area for their entire stack frame, rather than
>   adjusting the stack pointer in the prologue and epilogue. This area is
>   known as the red zone.
> 
> This is not compatible with exception handling, because the IRET frame
> written by the hardware at the stack pointer and the functions to handle
> the exception will overwrite the temporary variables of the interrupted
> function, causing undefined behavior. So disable red-zones for the
> pre-decompression boot code.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook