Thadeu reports that the retbleed mitigations have broken EFI runtime services in mixed mode, as the RET macro now expands to a relative branch that jumps to nowhere when executed from the 1:1 mapping of the kernel text that the EFI mixed mode thunk uses on its return back to the caller. So as Thadeu suggested in [1], we should switch to a bare 'ret' opcode followed by 'int3' (to limit straight line speculation). However, doing so leaves an unmitigated RET in the kernel text that is always present, even on non-EFI or non-mixed mode systems (which are quite rare these days to begin with) So let's take Thadeu's fix a bit further, and move the EFI mixed mode return trampoline that contains the 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 make the 1:1 mapping of the kernel .text non-executable as well, and only map the page that covers the return trampoline with executable permissions. Note that mapping .text and .rodata is still necessary, as otherwise, they will be covered by the default 1:1 mapping of the RAM below 4 GB, which uses read-write permissions. Also note that merging the mappings of .text and .rodata is not possible, even if they now use the same permissions, due to the fact that the hole in the middle may contain read-write data (such as the mixed mode stack) [1] https://lore.kernel.org/linux-efi/20220715194550.793957-1-cascardo@xxxxxxxxxxxxx/ Cc: tglx@xxxxxxxxxxxxx Cc: torvalds@xxxxxxxxxxxxxxxxxxxx Cc: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxx> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> --- arch/x86/platform/efi/efi_64.c | 15 ++++++++++++--- arch/x86/platform/efi/efi_thunk_64.S | 9 +++++++-- 2 files changed, 19 insertions(+), 5 deletions(-) diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c index 1f3675453a57..d8661fb31c76 100644 --- 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; @@ -240,7 +241,7 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages) text = __pa(_text); pfn = text >> PAGE_SHIFT; - pf = _PAGE_ENC; + pf = _PAGE_NX | _PAGE_ENC; if (kernel_map_pages_in_pgd(pgd, pfn, text, npages, pf)) { pr_err("Failed to map kernel text 1:1\n"); return 1; @@ -250,12 +251,20 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages) rodata = __pa(__start_rodata); pfn = rodata >> PAGE_SHIFT; - pf = _PAGE_NX | _PAGE_ENC; if (kernel_map_pages_in_pgd(pgd, pfn, rodata, npages, pf)) { pr_err("Failed to map kernel rodata 1:1\n"); 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 kernel rodata 1:1\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 9ffe2bad27d5..e436ce03741e 100644 --- a/arch/x86/platform/efi/efi_thunk_64.S +++ b/arch/x86/platform/efi/efi_thunk_64.S @@ -71,17 +71,22 @@ STACK_FRAME_NON_STANDARD __efi64_thunk pushq $__KERNEL32_CS pushq %rdi /* EFI runtime service address */ lretq +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 - RET + ret + int3 .code32 2: pushl $__KERNEL_CS pushl %ebp lret -SYM_FUNC_END(__efi64_thunk) +SYM_DATA_END(__efi64_thunk_ret_tramp) .bss .balign 8 -- 2.35.1
