linux-kernel - Re: [PATCH] x86/mm: Fix possible index overflow when creating page table mapping

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Date:   Sat, 18 Jun 2022 03:22:20 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yuntao Wang <ytcoode@...il.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Baoquan He <bhe@...hat.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Fix possible index overflow when creating page
 table mapping

On Thu, Jun 16, 2022 at 09:55:10PM +0800, Yuntao Wang wrote:
> There are two issues in phys_p4d_init():
> 
> - The __kernel_physical_mapping_init() does not do boundary-checking for
>   paddr_end and passes it directly to phys_p4d_init(), phys_p4d_init() does
>   not do bounds checking either, so if the physical memory to be mapped is
>   large enough, 'p4d_page + p4d_index(vaddr)' will wrap around to the
>   beginning entry of the P4D table and its data will be overwritten.
> 
> - The for loop body will be executed only when 'vaddr < vaddr_end'
>   evaluates to true, but if that condition is true, 'paddr >= paddr_end'
>   will evaluate to false, thus the 'if (paddr >= paddr_end) {}' block will
>   never be executed and become dead code.
> 
> To fix these issues, use 'i < PTRS_PER_P4D' instead of 'vaddr < vaddr_end'
> as the for loop condition, this also make it more consistent with the logic
> of the phys_{pud,pmt,pte}_init() functions.

Hm. I don't see why you changed phys_p4d_init(), but not
__kernel_physical_mapping_init(). It does exactly the same thing, just
pgd_index() is hidden a bit deeper than p4d_index().

-- 
 Kirill A. Shutemov