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

Lee Jones
2023-06-14T16:39:36Z

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(-)

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