linux-kernel - Re: [PATCH v2 1/1] x86/boot/compressed: Fix reloading of GDTR post-relocation

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Message-ID: <20200227081229.GA29411@gmail.com>
Date:   Thu, 27 Feb 2020 09:12:29 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 1/1] x86/boot/compressed: Fix reloading of GDTR
 post-relocation


* Arvind Sankar <nivedita@...m.mit.edu> wrote:

> Commit ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
> introduced GDT setup into the 32-bit kernel's startup_32, and reloads
> the GDTR after relocating the kernel for paranoia's sake.
> 
> Commit 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of
> the buffer") introduced a similar GDTR reload in the 64-bit kernel.
> 
> The GDTR is adjusted by init_size - _end, however this may not be the
> correct offset to apply if the kernel was loaded at a misaligned address
> or below LOAD_PHYSICAL_ADDR, as in that case the decompression buffer
> has an additional offset from the original load address.
> 
> This should never happen for a conformant bootloader, but we're being
> paranoid anyway, so just store the new GDT address in there instead of
> adding any offsets, which is simpler as well.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Fixes: ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
> Fixes: 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of the buffer")

Have you or anyone else observed this condition practice, or have a 
suspicion that this could happen - or is this a mostly theoretical 
concern?

Thanks,

	Ingo