Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

Jiri Kosina
2018-01-05T19:01:01Z

The previous patch was for slightly older kernel, and the logic in
_prologue() is a bit different in 4.15, but the (cofirmed) fix for
mainline is basically the same:

From: Jiri Kosina <jkosina@suse.cz>
Subject: [PATCH] PTI: unbreak EFI old_memmap

old_memmap's efi_call_phys_prolog() calls set_pgd() with swapper PGD that
has PAGE_USER set, which makes PTI set NX on it, and therefore EFI can't
execute it's code.

Fix that by forcefully clearing _PAGE_NX from the PGD (this can't be done
by the pgprot API).

_PAGE_NX will be automatically reintroduced in efi_call_phys_epilog(), as
_set_pgd() will again notice that this is _PAGE_USER, and set _PAGE_NX on
it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index d87ac96e37ed..2dd15e967c3f 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -135,7 +135,9 @@ pgd_t * __init efi_call_phys_prolog(void)
pud[j] = *pud_offset(p4d_k, vaddr);
}
}
+ pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
}
+
out:
__flush_tlb_all();

--
Jiri Kosina
SUSE Labs