linux-kernel - Re: [tip: x86/urgent] x86/mm: Avoid using set

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Date:   Mon, 26 Jun 2023 10:11:49 +0100
From:   Lee Jones <lee@...nel.org>
To:     Greg KH <greg@...ah.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-tip-commits@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/mm: Avoid using set_pgd() outside of real
 PGD pages

On Mon, 26 Jun 2023, Greg KH wrote:

> On Mon, Jun 26, 2023 at 09:54:50AM +0100, Lee Jones wrote:
> > Dear Stable,
> > 
> > On Fri, 16 Jun 2023, tip-bot2 for Lee Jones wrote:
> > 
> > > The following commit has been merged into the x86/urgent branch of tip:
> > > 
> > > Commit-ID:     d082d48737c75d2b3cc1f972b8c8674c25131534
> > > Gitweb:        https://git.kernel.org/tip/d082d48737c75d2b3cc1f972b8c8674c25131534
> > > Author:        Lee Jones <lee@...nel.org>
> > > AuthorDate:    Wed, 14 Jun 2023 17:38:54 +01:00
> > > Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
> > > CommitterDate: Fri, 16 Jun 2023 11:46:42 -07:00
> > > 
> > > x86/mm: Avoid using set_pgd() outside of real PGD pages
> > > 
> > > KPTI keeps around two PGDs: one for userspace and another for the
> > > kernel. Among other things, set_pgd() contains infrastructure to
> > > ensure that updates to the kernel PGD are reflected in the user PGD
> > > as well.
> > > 
> > > One side-effect of this is that set_pgd() expects to be passed whole
> > > pages.  Unfortunately, init_trampoline_kaslr() passes in a single entry:
> > > 'trampoline_pgd_entry'.
> > > 
> > > When KPTI is on, set_pgd() will update 'trampoline_pgd_entry' (an
> > > 8-Byte globally stored [.bss] variable) and will then proceed to
> > > replicate that value into the non-existent neighboring user page
> > > (located +4k away), leading to the corruption of other global [.bss]
> > > stored variables.
> > > 
> > > Fix it by directly assigning 'trampoline_pgd_entry' and avoiding
> > > set_pgd().
> > > 
> > > [ dhansen: tweak subject and changelog ]
> > > 
> > > Fixes: 0925dda5962e ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
> > > Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> > > Signed-off-by: Lee Jones <lee@...nel.org>
> > > Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> > > Cc: <stable@...r.kernel.org>
> > > Link: https://lore.kernel.org/all/20230614163859.924309-1-lee@kernel.org/g
> > > ---
> > >  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 557f0fe..37db264 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));
> > >  	}
> > >  }
> > 
> > Could we have this expedited please?  There are users waiting for it.
> > 
> > Upstream commit is:
> > 
> >   d082d48737c75 ("x86/mm: Avoid using set_pgd() outside of real PGD pages")
> 
> Please look through your emails you got this weekend, it's already
> queued up in the following stable trees:
> 	queue-5.4 queue-5.10 queue-5.15 queue-6.1 queue-6.3

Haven't reached those ones yet - only 160 to go!

Thank you though, very much appreciated.

-- 
Lee Jones [李琼斯]