efi/x86-mixed: move unmitigated RET into .rodata

Move the EFI mixed mode return trampoline RET into .rodata, so it is
normally mapped without executable permissions.  And given that this
snippet of code is really the only kernel code that we ever execute via
this 1:1 mapping, let's unmap the 1:1 mapping of the kernel .text, and
only map the page that covers the return trampoline with executable
permissions.

Note that the remainder of .rodata needs to remain mapped into the 1:1
mapping with RO/NX permissions, as literal GUIDs and strings may be
passed to the variable routines.

Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index 1f36754..b36596b 100644 (file)
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -176,7 +176,8 @@ virt_to_phys_or_null_size(void *va, unsigned long size)
 
 int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 {
-       unsigned long pfn, text, pf, rodata;
+       extern const u8 __efi64_thunk_ret_tramp[];
+       unsigned long pfn, text, pf, rodata, tramp;
        struct page *page;
        unsigned npages;
        pgd_t *pgd = efi_mm.pgd;
@@ -238,11 +239,9 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 
        npages = (_etext - _text) >> PAGE_SHIFT;
        text = __pa(_text);
-       pfn = text >> PAGE_SHIFT;
 
-       pf = _PAGE_ENC;
-       if (kernel_map_pages_in_pgd(pgd, pfn, text, npages, pf)) {
-               pr_err("Failed to map kernel text 1:1\n");
+       if (kernel_unmap_pages_in_pgd(pgd, text, npages)) {
+               pr_err("Failed to unmap kernel text 1:1 mapping\n");
                return 1;
        }
 
@@ -256,6 +255,15 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
                return 1;
        }
 
+       tramp = __pa(__efi64_thunk_ret_tramp);
+       pfn = tramp >> PAGE_SHIFT;
+
+       pf = _PAGE_ENC;
+       if (kernel_map_pages_in_pgd(pgd, pfn, tramp, 1, pf)) {
+               pr_err("Failed to map mixed mode return trampoline\n");
+               return 1;
+       }
+
        return 0;
 }
 

diff --git a/arch/x86/platform/efi/efi_thunk_64.S b/arch/x86/platform/efi/efi_thunk_64.S
index 4e5257a..c4b1144 100644 (file)
--- a/arch/x86/platform/efi/efi_thunk_64.S
+++ b/arch/x86/platform/efi/efi_thunk_64.S
@@ -23,7 +23,6 @@
 #include <linux/objtool.h>
 #include <asm/page_types.h>
 #include <asm/segment.h>
-#include <asm/nospec-branch.h>
 
        .text
        .code64
@@ -73,10 +72,18 @@ STACK_FRAME_NON_STANDARD __efi64_thunk
        pushq   %rdi                    /* EFI runtime service address */
        lretq
 
+       // This return instruction is not needed for correctness, as it will
+       // never be reached. It only exists to make objtool happy, which will
+       // otherwise complain about unreachable instructions in the callers.
+       RET
+SYM_FUNC_END(__efi64_thunk)
+
+       .section ".rodata", "a", @progbits
+       .balign 16
+SYM_DATA_START(__efi64_thunk_ret_tramp)
 1:     movq    0x20(%rsp), %rsp
        pop     %rbx
        pop     %rbp
-       ANNOTATE_UNRET_SAFE
        ret
        int3
 
@@ -84,7 +91,7 @@ STACK_FRAME_NON_STANDARD __efi64_thunk
 2:     pushl   $__KERNEL_CS
        pushl   %ebp
        lret
-SYM_FUNC_END(__efi64_thunk)
+SYM_DATA_END(__efi64_thunk_ret_tramp)
 
        .bss
        .balign 8