Re: [tip: x86/urgent] x86/mm: Avoid using set_pgd() outside of real PGD pages

[Greg KH]

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@xxxxxxxxxx>
> > AuthorDate:    Wed, 14 Jun 2023 17:38:54 +01:00
> > Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/all/20230614163859.924309-1-lee@xxxxxxxxxx/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

greg k-h