Re: [RFC][PATCH 5/6] x86: Sprinkle ENDBR dust

From: Kees Cook
Date: Tue Feb 08 2022 - 18:38:10 EST

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On Tue, Nov 23, 2021 at 02:00:53PM +0000, Mark Rutland wrote:
> On Mon, Nov 22, 2021 at 06:03:06PM +0100, Peter Zijlstra wrote:
> > Kernel entry points should be having ENDBR on for IBT configs.
> >
> > The SYSCALL entry points are found through taking their respective
> > address in order to program them in the MSRs, while the exception
> > entry points are found through UNWIND_HINT_IRET_REGS.
> >
> > *Except* that latter hint is also used on exit code to denote when
> > we're down to an IRET frame. As such add an additional 'entry'
> > argument to the macro and have it default to '1' such that objtool
> > will assume it's an entry and WARN about it.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>
> On arm64 we also added BTI (analagous to ENDBR) in our SYMC_FUNC_START()
> so what we don't miss a necessary landing pad for any assembly functions
> that can be indirectly branched to.
>
> See commits:
>
> 714a8d02ca4da147 ("arm64: asm: Override SYM_FUNC_START when building the kernel with BTI")
> 2d21889f8b5c50f6 ("arm64: Don't insert a BTI instruction at inner labels")
>
> ... do you need something similar? Or do you never indirectly branch to
> an assembly function?

The crypto was doing a bunch of this, though much of it was fixed
recently. I know Sami is still dealing with needing to annotate assembly
functions for CFI, though, so I'd expect ENDBR will be needed as well.

Since objtool strips the ones it doesn't like, can't we add them
unconditionally to SYM_FUNC_START()?

-Kees

>
> Thanks,
> Mark.
>
> > ---
> > arch/x86/entry/entry_64.S | 34 ++++++++++++++++++++--------------
> > arch/x86/entry/entry_64_compat.S | 2 ++
> > arch/x86/include/asm/idtentry.h | 23 +++++++++++++++--------
> > arch/x86/include/asm/segment.h | 5 +++++
> > arch/x86/include/asm/unwind_hints.h | 18 +++++++++++++-----
> > arch/x86/kernel/head_64.S | 14 +++++++++-----
> > arch/x86/kernel/idt.c | 5 +++--
> > arch/x86/kernel/unwind_orc.c | 3 ++-
> > include/linux/objtool.h | 5 +++--
> > tools/include/linux/objtool.h | 5 +++--
> > tools/objtool/check.c | 3 ++-
> > tools/objtool/orc_dump.c | 3 ++-
> > 12 files changed, 79 insertions(+), 41 deletions(-)
> >
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -88,6 +88,7 @@
> > SYM_CODE_START(entry_SYSCALL_64)
> > UNWIND_HINT_EMPTY
> >
> > + ENDBR
> > swapgs
> > /* tss.sp2 is scratch space. */
> > movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
> > @@ -350,7 +351,8 @@ SYM_CODE_END(ret_from_fork)
> > */
> > .macro idtentry vector asmsym cfunc has_error_code:req
> > SYM_CODE_START(\asmsym)
> > - UNWIND_HINT_IRET_REGS offset=\has_error_code*8
> > + UNWIND_HINT_IRET_REGS offset=\has_error_code*8 entry=1
> > + ENDBR
> > ASM_CLAC
> >
> > .if \has_error_code == 0
> > @@ -367,7 +369,7 @@ SYM_CODE_START(\asmsym)
> > .rept 6
> > pushq 5*8(%rsp)
> > .endr
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=0
> > .Lfrom_usermode_no_gap_\@:
> > .endif
> >
> > @@ -417,7 +419,8 @@ SYM_CODE_END(\asmsym)
> > */
> > .macro idtentry_mce_db vector asmsym cfunc
> > SYM_CODE_START(\asmsym)
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > + ENDBR
> > ASM_CLAC
> >
> > pushq $-1 /* ORIG_RAX: no syscall to restart */
> > @@ -472,7 +475,8 @@ SYM_CODE_END(\asmsym)
> > */
> > .macro idtentry_vc vector asmsym cfunc
> > SYM_CODE_START(\asmsym)
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > + ENDBR
> > ASM_CLAC
> >
> > /*
> > @@ -533,7 +537,8 @@ SYM_CODE_END(\asmsym)
> > */
> > .macro idtentry_df vector asmsym cfunc
> > SYM_CODE_START(\asmsym)
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=1
> > + ENDBR
> > ASM_CLAC
> >
> > /* paranoid_entry returns GS information for paranoid_exit in EBX. */
> > @@ -626,7 +631,7 @@ SYM_INNER_LABEL(restore_regs_and_return_
> > INTERRUPT_RETURN
> >
> > SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> > /*
> > * Are we returning to a stack segment from the LDT? Note: in
> > * 64-bit mode SS:RSP on the exception stack is always valid.
> > @@ -703,7 +708,7 @@ SYM_INNER_LABEL(native_irq_return_iret,
> > popq %rdi /* Restore user RDI */
> >
> > movq %rax, %rsp
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=0
> >
> > /*
> > * At this point, we cannot write to the stack any more, but we can
> > @@ -819,13 +824,13 @@ SYM_CODE_START(xen_failsafe_callback)
> > movq 8(%rsp), %r11
> > addq $0x30, %rsp
> > pushq $0 /* RIP */
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=0
> > jmp asm_exc_general_protection
> > 1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
> > movq (%rsp), %rcx
> > movq 8(%rsp), %r11
> > addq $0x30, %rsp
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> > pushq $-1 /* orig_ax = -1 => not a system call */
> > PUSH_AND_CLEAR_REGS
> > ENCODE_FRAME_POINTER
> > @@ -1065,7 +1070,8 @@ SYM_CODE_END(error_return)
> > * when PAGE_TABLE_ISOLATION is in use. Do not clobber.
> > */
> > SYM_CODE_START(asm_exc_nmi)
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > + ENDBR
> >
> > /*
> > * We allow breakpoints in NMIs. If a breakpoint occurs, then
> > @@ -1130,13 +1136,13 @@ SYM_CODE_START(asm_exc_nmi)
> > SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
> > movq %rsp, %rdx
> > movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> > - UNWIND_HINT_IRET_REGS base=%rdx offset=8
> > + UNWIND_HINT_IRET_REGS base=%rdx offset=8 entry=0
> > pushq 5*8(%rdx) /* pt_regs->ss */
> > pushq 4*8(%rdx) /* pt_regs->rsp */
> > pushq 3*8(%rdx) /* pt_regs->flags */
> > pushq 2*8(%rdx) /* pt_regs->cs */
> > pushq 1*8(%rdx) /* pt_regs->rip */
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> > pushq $-1 /* pt_regs->orig_ax */
> > PUSH_AND_CLEAR_REGS rdx=(%rdx)
> > ENCODE_FRAME_POINTER
> > @@ -1292,7 +1298,7 @@ SYM_CODE_START(asm_exc_nmi)
> > .rept 5
> > pushq 11*8(%rsp)
> > .endr
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> >
> > /* Everything up to here is safe from nested NMIs */
> >
> > @@ -1308,7 +1314,7 @@ SYM_CODE_START(asm_exc_nmi)
> > pushq $__KERNEL_CS /* CS */
> > pushq $1f /* RIP */
> > iretq /* continues at repeat_nmi below */
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> > 1:
> > ENDBR
> > #endif
> > --- a/arch/x86/entry/entry_64_compat.S
> > +++ b/arch/x86/entry/entry_64_compat.S
> > @@ -49,6 +49,7 @@
> > SYM_CODE_START(entry_SYSENTER_compat)
> > UNWIND_HINT_EMPTY
> > /* Interrupts are off on entry. */
> > + ENDBR
> > SWAPGS
> >
> > pushq %rax
> > @@ -198,6 +199,7 @@ SYM_CODE_END(entry_SYSENTER_compat)
> > */
> > SYM_CODE_START(entry_SYSCALL_compat)
> > UNWIND_HINT_EMPTY
> > + ENDBR
> > /* Interrupts are off on entry. */
> > swapgs
> >
> > --- a/arch/x86/include/asm/idtentry.h
> > +++ b/arch/x86/include/asm/idtentry.h
> > @@ -5,6 +5,12 @@
> > /* Interrupts/Exceptions */
> > #include <asm/trapnr.h>
> >
> > +#ifdef CONFIG_X86_IBT
> > +#define IDT_ALIGN 16
> > +#else
> > +#define IDT_ALIGN 8
> > +#endif
> > +
> > #ifndef __ASSEMBLY__
> > #include <linux/entry-common.h>
> > #include <linux/hardirq.h>
> > @@ -492,33 +498,34 @@ __visible noinstr void func(struct pt_re
> > * point is to mask off the bits above bit 7 because the push is sign
> > * extending.
> > */
> > - .align 8
> > +
> > + .align IDT_ALIGN
> > SYM_CODE_START(irq_entries_start)
> > vector=FIRST_EXTERNAL_VECTOR
> > .rept NR_EXTERNAL_VECTORS
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > 0 :
> > + ENDBR
> > .byte 0x6a, vector
> > jmp asm_common_interrupt
> > - nop
> > /* Ensure that the above is 8 bytes max */
> > - . = 0b + 8
> > + .fill 0b + IDT_ALIGN - ., 1, 0x90
> > vector = vector+1
> > .endr
> > SYM_CODE_END(irq_entries_start)
> >
> > #ifdef CONFIG_X86_LOCAL_APIC
> > - .align 8
> > + .align IDT_ALIGN
> > SYM_CODE_START(spurious_entries_start)
> > vector=FIRST_SYSTEM_VECTOR
> > .rept NR_SYSTEM_VECTORS
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > 0 :
> > + ENDBR
> > .byte 0x6a, vector
> > jmp asm_spurious_interrupt
> > - nop
> > /* Ensure that the above is 8 bytes max */
> > - . = 0b + 8
> > + .fill 0b + IDT_ALIGN - ., 1, 0x90
> > vector = vector+1
> > .endr
> > SYM_CODE_END(spurious_entries_start)
> > --- a/arch/x86/include/asm/segment.h
> > +++ b/arch/x86/include/asm/segment.h
> > @@ -4,6 +4,7 @@
> >
> > #include <linux/const.h>
> > #include <asm/alternative.h>
> > +#include <asm/ibt.h>
> >
> > /*
> > * Constructor for a conventional segment GDT (or LDT) entry.
> > @@ -275,7 +276,11 @@ static inline void vdso_read_cpunode(uns
> > * vector has no error code (two bytes), a 'push $vector_number' (two
> > * bytes), and a jump to the common entry code (up to five bytes).
> > */
> > +#ifdef CONFIG_X86_IBT
> > +#define EARLY_IDT_HANDLER_SIZE 13
> > +#else
> > #define EARLY_IDT_HANDLER_SIZE 9
> > +#endif
> >
> > /*
> > * xen_early_idt_handler_array is for Xen pv guests: for each entry in
> > --- a/arch/x86/include/asm/unwind_hints.h
> > +++ b/arch/x86/include/asm/unwind_hints.h
> > @@ -11,7 +11,7 @@
> > UNWIND_HINT sp_reg=ORC_REG_UNDEFINED type=UNWIND_HINT_TYPE_CALL end=1
> > .endm
> >
> > -.macro UNWIND_HINT_REGS base=%rsp offset=0 indirect=0 extra=1 partial=0
> > +.macro UNWIND_HINT_REGS base=%rsp offset=0 indirect=0 extra=1 partial=0 entry=1
> > .if \base == %rsp
> > .if \indirect
> > .set sp_reg, ORC_REG_SP_INDIRECT
> > @@ -33,9 +33,17 @@
> > .set sp_offset, \offset
> >
> > .if \partial
> > - .set type, UNWIND_HINT_TYPE_REGS_PARTIAL
> > + .if \entry
> > + .set type, UNWIND_HINT_TYPE_REGS_ENTRY
> > + .else
> > + .set type, UNWIND_HINT_TYPE_REGS_EXIT
> > + .endif
> > .elseif \extra == 0
> > - .set type, UNWIND_HINT_TYPE_REGS_PARTIAL
> > + .if \entry
> > + .set type, UNWIND_HINT_TYPE_REGS_ENTRY
> > + .else
> > + .set type, UNWIND_HINT_TYPE_REGS_EXIT
> > + .endif
> > .set sp_offset, \offset + (16*8)
> > .else
> > .set type, UNWIND_HINT_TYPE_REGS
> > @@ -44,8 +52,8 @@
> > UNWIND_HINT sp_reg=sp_reg sp_offset=sp_offset type=type
> > .endm
> >
> > -.macro UNWIND_HINT_IRET_REGS base=%rsp offset=0
> > - UNWIND_HINT_REGS base=\base offset=\offset partial=1
> > +.macro UNWIND_HINT_IRET_REGS base=%rsp offset=0 entry=1
> > + UNWIND_HINT_REGS base=\base offset=\offset partial=1 entry=\entry
> > .endm
> >
> > .macro UNWIND_HINT_FUNC
> > --- a/arch/x86/kernel/head_64.S
> > +++ b/arch/x86/kernel/head_64.S
> > @@ -25,6 +25,7 @@
> > #include <asm/export.h>
> > #include <asm/nospec-branch.h>
> > #include <asm/fixmap.h>
> > +#include <asm/ibt.h>
> >
> > /*
> > * We are not able to switch in one step to the final KERNEL ADDRESS SPACE
> > @@ -310,7 +311,8 @@ SYM_CODE_END(start_cpu0)
> > * when .init.text is freed.
> > */
> > SYM_CODE_START_NOALIGN(vc_boot_ghcb)
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=1
> > + ENDBR
> >
> > /* Build pt_regs */
> > PUSH_AND_CLEAR_REGS
> > @@ -354,18 +356,20 @@ SYM_CODE_START(early_idt_handler_array)
> > i = 0
> > .rept NUM_EXCEPTION_VECTORS
> > .if ((EXCEPTION_ERRCODE_MASK >> i) & 1) == 0
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=1
> > + ENDBR
> > pushq $0 # Dummy error code, to make stack frame uniform
> > .else
> > - UNWIND_HINT_IRET_REGS offset=8
> > + UNWIND_HINT_IRET_REGS offset=8 entry=1
> > + ENDBR
> > .endif
> > pushq $i # 72(%rsp) Vector number
> > jmp early_idt_handler_common
> > - UNWIND_HINT_IRET_REGS
> > + UNWIND_HINT_IRET_REGS entry=0
> > i = i + 1
> > .fill early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
> > .endr
> > - UNWIND_HINT_IRET_REGS offset=16
> > + UNWIND_HINT_IRET_REGS offset=16 entry=0
> > SYM_CODE_END(early_idt_handler_array)
> >
> > SYM_CODE_START_LOCAL(early_idt_handler_common)
> > --- a/arch/x86/kernel/idt.c
> > +++ b/arch/x86/kernel/idt.c
> > @@ -10,6 +10,7 @@
> > #include <asm/proto.h>
> > #include <asm/desc.h>
> > #include <asm/hw_irq.h>
> > +#include <asm/idtentry.h>
> >
> > #define DPL0 0x0
> > #define DPL3 0x3
> > @@ -272,7 +273,7 @@ void __init idt_setup_apic_and_irq_gates
> > idt_setup_from_table(idt_table, apic_idts, ARRAY_SIZE(apic_idts), true);
> >
> > for_each_clear_bit_from(i, system_vectors, FIRST_SYSTEM_VECTOR) {
> > - entry = irq_entries_start + 8 * (i - FIRST_EXTERNAL_VECTOR);
> > + entry = irq_entries_start + IDT_ALIGN * (i - FIRST_EXTERNAL_VECTOR);
> > set_intr_gate(i, entry);
> > }
> >
> > @@ -283,7 +284,7 @@ void __init idt_setup_apic_and_irq_gates
> > * system_vectors bitmap. Otherwise they show up in
> > * /proc/interrupts.
> > */
> > - entry = spurious_entries_start + 8 * (i - FIRST_SYSTEM_VECTOR);
> > + entry = spurious_entries_start + IDT_ALIGN * (i - FIRST_SYSTEM_VECTOR);
> > set_intr_gate(i, entry);
> > }
> > #endif
> > --- a/arch/x86/kernel/unwind_orc.c
> > +++ b/arch/x86/kernel/unwind_orc.c
> > @@ -566,7 +566,8 @@ bool unwind_next_frame(struct unwind_sta
> > state->signal = true;
> > break;
> >
> > - case UNWIND_HINT_TYPE_REGS_PARTIAL:
> > + case UNWIND_HINT_TYPE_REGS_ENTRY:
> > + case UNWIND_HINT_TYPE_REGS_EXIT:
> > if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
> > orc_warn_current("can't access iret registers at %pB\n",
> > (void *)orig_ip);
> > --- a/include/linux/objtool.h
> > +++ b/include/linux/objtool.h
> > @@ -35,8 +35,9 @@ struct unwind_hint {
> > */
> > #define UNWIND_HINT_TYPE_CALL 0
> > #define UNWIND_HINT_TYPE_REGS 1
> > -#define UNWIND_HINT_TYPE_REGS_PARTIAL 2
> > -#define UNWIND_HINT_TYPE_FUNC 3
> > +#define UNWIND_HINT_TYPE_REGS_ENTRY 2
> > +#define UNWIND_HINT_TYPE_REGS_EXIT 3
> > +#define UNWIND_HINT_TYPE_FUNC 4
> >
> > #ifdef CONFIG_STACK_VALIDATION
> >
> > --- a/tools/include/linux/objtool.h
> > +++ b/tools/include/linux/objtool.h
> > @@ -35,8 +35,9 @@ struct unwind_hint {
> > */
> > #define UNWIND_HINT_TYPE_CALL 0
> > #define UNWIND_HINT_TYPE_REGS 1
> > -#define UNWIND_HINT_TYPE_REGS_PARTIAL 2
> > -#define UNWIND_HINT_TYPE_FUNC 3
> > +#define UNWIND_HINT_TYPE_REGS_ENTRY 2
> > +#define UNWIND_HINT_TYPE_REGS_EXIT 3
> > +#define UNWIND_HINT_TYPE_FUNC 4
> >
> > #ifdef CONFIG_STACK_VALIDATION
> >
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -2308,7 +2308,8 @@ static int update_cfi_state(struct instr
> > }
> >
> > if (cfi->type == UNWIND_HINT_TYPE_REGS ||
> > - cfi->type == UNWIND_HINT_TYPE_REGS_PARTIAL)
> > + cfi->type == UNWIND_HINT_TYPE_REGS_ENTRY ||
> > + cfi->type == UNWIND_HINT_TYPE_REGS_EXIT)
> > return update_cfi_state_regs(insn, cfi, op);
> >
> > switch (op->dest.type) {
> > --- a/tools/objtool/orc_dump.c
> > +++ b/tools/objtool/orc_dump.c
> > @@ -43,7 +43,8 @@ static const char *orc_type_name(unsigne
> > return "call";
> > case UNWIND_HINT_TYPE_REGS:
> > return "regs";
> > - case UNWIND_HINT_TYPE_REGS_PARTIAL:
> > + case UNWIND_HINT_TYPE_REGS_ENTRY:
> > + case UNWIND_HINT_TYPE_REGS_EXIT:
> > return "regs (partial)";
> > default:
> > return "?";
> >
> >

--
Kees Cook

Next message: Eric Biggers: "Re: [PATCH v1 6/7] random: use simpler fast key erasure flow on per-cpu keys"
Previous message: David Laight: "RE: x86: should clear_user() have alternatives?"
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