Re: [PATCH v2 1/1] x86/mm/KASLR: Store pud_page_tramp into entry rather than page

Lee Jones
2023-06-16T17:01:32Z

On Wed, 14 Jun 2023, 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>
> ---
> arch/x86/mm/kaslr.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)

Is there any more you need from me at this point?

Would you like me to resubmit with the Fixes: tag applied, or is
someone happy to apply it on merge?

> 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]