linux-kernel - Re: [PATCHv2 2/2] x86/boot/compressed/64: Fix moving page table out of trampoline memory

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Date:   Wed, 16 May 2018 11:53:41 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
cc:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/2] x86/boot/compressed/64: Fix moving page table out
 of trampoline memory

On Wed, 16 May 2018, Kirill A. Shutemov wrote:

> cleanup_trampoline() relocates the top-level page table out of
> trampoline memory. We use 'top_pgtable' as our new top-level page table.
> 
> But if the 'top_pgtable' would be referenced from C in a usual way,
> the address of the table will be calculated relative to RIP.
> After kernel gets relocated, the address will be in the middle of
> decompression buffer and the page table may get overwritten.
> This leads to a crash.
> 
> We calculate the address of other page tables relative to the relocation
> address. It makes them safe. We should do the same for 'top_pgtable'.
> 
> Calculate the address of 'top_pgtable' in assembly and pass down to
> cleanup_trampoline().
> 
> Move the page table to .pgtable section where the rest of page tables
> are. The section is @nobits so we save 4k in kernel image.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Fixes: e9d0e6330eb8 ("x86/boot/compressed/64: Prepare new top-level page table for trampoline")

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>