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

[Lee Jones]

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")

Thanks muchly.

-- 
Lee Jones [李琼斯]