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Date:   Wed, 14 Jun 2023 19:43:41 +0100
From:   Lee Jones <lee@kernel.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/1] x86/mm/KASLR: Store pud_page_tramp into entry
 rather than page
Message-ID: <20230614184341.GC3635807@google.com>
References: <20230614163859.924309-1-lee@kernel.org>
 <20230614163859.924309-2-lee@kernel.org>
 <ZIn9YCcETx+Hlf6Z@google.com>
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On Wed, 14 Jun 2023, Nick Desaulniers wrote:

> 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@linux.intel.com>
> > Signed-off-by: Lee Jones <lee@kernel.org>
> 
> Nice work tracking this one down!
> 
> Fixes: 0925dda5962e ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
> Cc: <stable@vger.kernel.org>

I was planning on backporting this myself once it hits Mainline.

Happy to submit a RESEND with those pieces added if required though.

Or perhaps someone would be kind enough to add them on merge?

> > ---
> >  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
> > 

-- 
Lee Jones [李琼斯]