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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>
Subject: Re: [PATCH 0/8] Remove 64-bit GDT setup in efi_main + doc fixes
Date: Fri, 31 Jan 2020 14:10:46 -0500
Message-ID: <20200131191045.GA2346161@rani.riverdale.lan> (raw)
In-Reply-To: <CAKv+Gu_xw+0dJipX073ErZmJz7RVSn2ZK3SzK3HUDYhJoEOQyQ@mail.gmail.com>

On Fri, Jan 31, 2020 at 10:31:44AM +0100, Ard Biesheuvel wrote:
> Playing around with this a bit more, I think we should go with
> 
> if (IS_ENABLED(CONFIG_X86_32)) {
>   static const struct desc_struct desc[] __aligned(8) = {
>     [GDT_ENTRY_BOOT_CS] = GDT_ENTRY_INIT(0xc09b, 0, 0xfffff),
>     [GDT_ENTRY_BOOT_DS] = GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
>   };
> 
>   native_load_gdt(&(struct desc_ptr){ sizeof(desc) - 1,
>                                       (unsigned long)desc });
> }
> 

This is the way I originally did it (except I loaded the GDT with the
relocated address of desc in case efi_relocate_kernel got called). It
booted on a test, but I grew concerned because this GDT will be
somewhere in the middle of the decompression buffer and get overwritten
either during the copy to the end of the buffer or during decompression.
If anything tries to reload segment registers it will blow up.

Looking at the code though it doesn't look like anything will -- the
segments are loaded at the beginning of startup_32, and then it looks
like nothing should touch them until we get to the startup code of the
decompressed kernel, which sets up its own boot GDT first. So this might
be ok if the x86 maintainers agree.


  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-30 20:04 Arvind Sankar
2020-01-30 20:04 ` [PATCH 1/8] efi/x86: Use C wrapper instead of inline assembly Arvind Sankar
2020-01-30 20:04 ` [PATCH 2/8] efi/x86: Allocate the GDT pointer on the stack Arvind Sankar
2020-01-30 20:04 ` [PATCH 3/8] efi/x86: Factor GDT setup code into a function Arvind Sankar
2020-01-30 20:04 ` [PATCH 4/8] efi/x86: Only setup the GDT for 32-bit kernel Arvind Sankar
2020-01-30 20:04 ` [PATCH 5/8] efi/x86: Allocate only the required 32 bytes for the GDT Arvind Sankar
2020-01-30 20:04 ` [PATCH 6/8] efi/x86: Change __KERNEL_{CS,DS} to __BOOT_{CS,DS} Arvind Sankar
2020-01-30 20:04 ` [PATCH 7/8] Documentation/x86/boot: Clarify segment requirements for EFI handover Arvind Sankar
2020-01-31 19:24   ` Arvind Sankar
2020-01-30 20:04 ` [PATCH 8/8] Documentation/x86/boot: Correct segment requirements for 64-bit boot Arvind Sankar
2020-01-31  8:42 ` [PATCH 0/8] Remove 64-bit GDT setup in efi_main + doc fixes Ard Biesheuvel
2020-01-31  9:31   ` Ard Biesheuvel
2020-01-31 19:10     ` Arvind Sankar [this message]
2020-02-02 17:13 ` [PATCH v2 0/7] x86/efi,boot: GDT handling cleanup/fixes Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 1/7] x86/boot: Remove KEEP_SEGMENTS support Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 2/7] efi/x86: Don't depend on firmware GDT layout Arvind Sankar
2020-02-02 17:54     ` Ard Biesheuvel
2020-02-02 18:18       ` Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 3/7] x86/boot: Reload GDTR after copying to the end of the buffer Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 4/7] x86/boot: Clear direction and interrupt flags in startup_64 Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 5/7] efi/x86: Remove GDT setup from efi_main Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 6/7] x86/boot: GDT limit value should be size - 1 Arvind Sankar
2020-02-02 17:13   ` [PATCH v2 7/7] x86/boot: Micro-optimize GDT loading instructions Arvind Sankar
2020-02-02 18:01   ` [PATCH v2 0/7] x86/efi,boot: GDT handling cleanup/fixes Ard Biesheuvel

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