[PATCH v3 49/59] x86/retpoline: Add SKL retthunk retpolines

Peter Zijlstra
2022-09-15T11:42:13Z

From: Peter Zijlstra <peterz@infradead.org>

Ensure that retpolines do the proper call accounting so that the return
accounting works correctly.

Specifically; retpolines are used to replace both 'jmp *%reg' and
'call *%reg', however these two cases do not have the same accounting
requirements. Therefore split things up and provide two different
retpoline arrays for SKL.

The 'jmp *%reg' case needs no accounting, the
__x86_indirect_jump_thunk_array[] covers this. The retpoline is
changed to not use the return thunk; it's a simple call;ret construct.

[ strictly speaking it should do:
andq $(~0x1f), PER_CPU_VAR(__x86_call_depth)
but we can argue this can be covered by the fuzz we already have
in the accounting depth (12) vs the RSB depth (16) ]

The 'call *%reg' case does need accounting, the
__x86_indirect_call_thunk_array[] covers this. Again, this retpoline
avoids the use of the return-thunk, in this case to avoid double
accounting.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/nospec-branch.h | 12 +++++
arch/x86/kernel/alternative.c | 59 +++++++++++++++++++++++++++--
arch/x86/lib/retpoline.S | 71 +++++++++++++++++++++++++++++++----
arch/x86/net/bpf_jit_comp.c | 5 +-
4 files changed, 135 insertions(+), 12 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -288,6 +288,8 @@

typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
extern retpoline_thunk_t __x86_indirect_thunk_array[];
+extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
+extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];

extern void __x86_return_thunk(void);
extern void zen_untrain_ret(void);
@@ -317,6 +319,16 @@ static inline void x86_set_skl_return_th
#include <asm/GEN-for-each-reg.h>
#undef GEN

+#define GEN(reg) \
+ extern retpoline_thunk_t __x86_indirect_call_thunk_ ## reg;
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
+#define GEN(reg) \
+ extern retpoline_thunk_t __x86_indirect_jump_thunk_ ## reg;
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
#ifdef CONFIG_X86_64

/*
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -377,6 +377,56 @@ static int emit_indirect(int op, int reg
return i;
}

+static inline bool is_jcc32(struct insn *insn)
+{
+ /* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
+ return insn->opcode.bytes[0] == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80;
+}
+
+static int emit_call_track_retpoline(void *addr, struct insn *insn, int reg, u8 *bytes)
+{
+ u8 op = insn->opcode.bytes[0];
+ int i = 0;
+
+ /*
+ * Clang does 'weird' Jcc __x86_indirect_thunk_r11 conditional
+ * tail-calls. Deal with them.
+ */
+ if (is_jcc32(insn)) {
+ bytes[i++] = op;
+ op = insn->opcode.bytes[1];
+ goto clang_jcc;
+ }
+
+ if (insn->length == 6)
+ bytes[i++] = 0x2e; /* CS-prefix */
+
+ switch (op) {
+ case CALL_INSN_OPCODE:
+ __text_gen_insn(bytes+i, op, addr+i,
+ __x86_indirect_call_thunk_array[reg],
+ CALL_INSN_SIZE);
+ i += CALL_INSN_SIZE;
+ break;
+
+ case JMP32_INSN_OPCODE:
+clang_jcc:
+ __text_gen_insn(bytes+i, op, addr+i,
+ __x86_indirect_jump_thunk_array[reg],
+ JMP32_INSN_SIZE);
+ i += JMP32_INSN_SIZE;
+ break;
+
+ default:
+ WARN("%pS %px %*ph\n", addr, addr, 6, addr);
+ return -1;
+ }
+
+ WARN_ON_ONCE(i != insn->length);
+
+ return i;
+}
+
/*
* Rewrite the compiler generated retpoline thunk calls.
*
@@ -409,8 +459,12 @@ static int patch_retpoline(void *addr, s
BUG_ON(reg == 4);

if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
- !cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE))
+ !cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) {
+ if (cpu_feature_enabled(X86_FEATURE_CALL_DEPTH))
+ return emit_call_track_retpoline(addr, insn, reg, bytes);
+
return -1;
+ }

op = insn->opcode.bytes[0];

@@ -427,8 +481,7 @@ static int patch_retpoline(void *addr, s
* [ NOP ]
* 1:
*/
- /* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
- if (op == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80) {
+ if (is_jcc32(insn)) {
cc = insn->opcode.bytes[1] & 0xf;
cc ^= 1; /* invert condition */

--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -14,17 +14,18 @@

.section .text.__x86.indirect_thunk

-.macro RETPOLINE reg
+
+.macro POLINE reg
ANNOTATE_INTRA_FUNCTION_CALL
call .Ldo_rop_\@
-.Lspec_trap_\@:
- UNWIND_HINT_EMPTY
- pause
- lfence
- jmp .Lspec_trap_\@
+ int3
.Ldo_rop_\@:
mov %\reg, (%_ASM_SP)
UNWIND_HINT_FUNC
+.endm
+
+.macro RETPOLINE reg
+ POLINE \reg
RET
.endm

@@ -54,7 +55,6 @@ SYM_INNER_LABEL(__x86_indirect_thunk_\re
*/

#define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
-#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)

.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_thunk_array)
@@ -66,10 +66,65 @@ SYM_CODE_START(__x86_indirect_thunk_arra
.align RETPOLINE_THUNK_SIZE
SYM_CODE_END(__x86_indirect_thunk_array)

-#define GEN(reg) EXPORT_THUNK(reg)
+#define GEN(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
+#ifdef CONFIG_CALL_DEPTH_TRACKING
+.macro CALL_THUNK reg
+ .align RETPOLINE_THUNK_SIZE
+
+SYM_INNER_LABEL(__x86_indirect_call_thunk_\reg, SYM_L_GLOBAL)
+ UNWIND_HINT_EMPTY
+ ANNOTATE_NOENDBR
+
+ CALL_DEPTH_ACCOUNT
+ POLINE \reg
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+.endm
+
+ .align RETPOLINE_THUNK_SIZE
+SYM_CODE_START(__x86_indirect_call_thunk_array)
+
+#define GEN(reg) CALL_THUNK reg
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
+ .align RETPOLINE_THUNK_SIZE
+SYM_CODE_END(__x86_indirect_call_thunk_array)
+
+#define GEN(reg) __EXPORT_THUNK(__x86_indirect_call_thunk_ ## reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN

+.macro JUMP_THUNK reg
+ .align RETPOLINE_THUNK_SIZE
+
+SYM_INNER_LABEL(__x86_indirect_jump_thunk_\reg, SYM_L_GLOBAL)
+ UNWIND_HINT_EMPTY
+ ANNOTATE_NOENDBR
+ POLINE \reg
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+.endm
+
+ .align RETPOLINE_THUNK_SIZE
+SYM_CODE_START(__x86_indirect_jump_thunk_array)
+
+#define GEN(reg) JUMP_THUNK reg
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+
+ .align RETPOLINE_THUNK_SIZE
+SYM_CODE_END(__x86_indirect_jump_thunk_array)
+
+#define GEN(reg) __EXPORT_THUNK(__x86_indirect_jump_thunk_ ## reg)
+#include <asm/GEN-for-each-reg.h>
+#undef GEN
+#endif
/*
* This function name is magical and is used by -mfunction-return=thunk-extern
* for the compiler to generate JMPs to it.
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -417,7 +417,10 @@ static void emit_indirect_jump(u8 **ppro
EMIT2(0xFF, 0xE0 + reg);
} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
OPTIMIZER_HIDE_VAR(reg);
- emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
+ if (cpu_feature_enabled(X86_FEATURE_CALL_DEPTH))
+ emit_jump(&prog, &__x86_indirect_jump_thunk_array[reg], ip);
+ else
+ emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
} else {
EMIT2(0xFF, 0xE0 + reg);
}