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Date:   Mon, 15 May 2023 17:03:03 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Evgeniy Baskov <baskov@ispras.ru>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexey Khoroshilov <khoroshilov@ispras.ru>,
        Peter Jones <pjones@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        Mario Limonciello <mario.limonciello@amd.com>,
        Kees Cook <keescook@chromium.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 05/20] x86: decompressor: Avoid the need for a stack
 in the 32-bit trampoline
Message-ID: <20230515140303.34lba7rl4onrltdu@box.shutemov.name>
References: <20230508070330.582131-1-ardb@kernel.org>
 <20230508070330.582131-6-ardb@kernel.org>
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On Mon, May 08, 2023 at 09:03:15AM +0200, Ard Biesheuvel wrote:
> The 32-bit trampoline no longer uses the stack for anything except
> performing a long return back to long mode. Currently, this stack is
> allocated in the same page that carries the trampoline code, which means
> this page must be mapped writable and executable, and the stack is
> therefore executable as well.
> 
> So let's do a long jump instead: that way, we can pre-calculate the
> return address and poke it into the code before we call it. In a later
> patch, we will take advantage of this by removing writable permissions
> (and adding executable ones) explicitly when booting via the EFI stub.
> 
> Not playing with the stack pointer also makes it more straight-forward
> to call the trampoline code as an ordinary 64-bit function from C code.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov