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: 0925dda596 ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/all/20230614163859.924309-1-lee@kernel.org/g

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));
 	}
 }