linux-kernel - Re: [PATCH v2 1/1] x86/mm/KASLR: Store pud_page

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Date:   Wed, 14 Jun 2023 10:48:16 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Lee Jones <lee@...nel.org>
Cc:     dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] x86/mm/KASLR: Store pud_page_tramp into entry
 rather than page

On Wed, Jun 14, 2023 at 05:38:54PM +0100, Lee Jones wrote:
> set_pgd() expects to be passed whole pages to operate on, whereas
> trampoline_pgd_entry is, as the name suggests, an entry.  The
> ramifications for using set_pgd() here are that the following thread of
> execution will not only place the suggested value into the
> trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
> also attempt to replicate that value into the non-existent neighboring
> user page (located +4k away), leading to the corruption of other global
> [.bss] stored variables.
> 
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Lee Jones <lee@...nel.org>

Nice work tracking this one down!

Fixes: 0925dda5962e ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
Cc: <stable@...r.kernel.org>

> ---
>  arch/x86/mm/kaslr.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 557f0fe25dff4..37db264866b64 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(void)
>  		set_p4d(p4d_tramp,
>  			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>  
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
>  	} else {
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
>  	}
>  }
> -- 
> 2.41.0.162.gfafddb0af9-goog
>