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SubjectRe: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions
On Tue, Jun 11, 2019 at 06:21:28PM +0200, Peter Zijlstra wrote:
> although at this point I'm
> thinking we should just used the instruction decode we have instead of
> playing iffy games with packed structures.

How's something like this? It accepts jmp/32 jmp/8 call and nop5_atomic.

---
Subject: x86/alternatives: Teach text_poke_bp() to emulate instructions
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Jun 5 10:48:37 CEST 2019

In preparation for static_call support, teach text_poke_bp() to
emulate instructions, including CALL.

The current text_poke_bp() takes a @handler argument which is used as
a jump target when the temporary INT3 is hit by a different CPU.

When patching CALL instructions, this doesn't work because we'd miss
the PUSH of the return address. Instead, teach poke_int3_handler() to
emulate an instruction, typically the instruction we're patching in.

This fits almost all text_poke_bp() users, except
arch_unoptimize_kprobe() which restores random text, and for that site
we have to build an explicit emulate instruction.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/text-patching.h | 15 +++++--
arch/x86/kernel/alternative.c | 73 +++++++++++++++++++++++++----------
arch/x86/kernel/jump_label.c | 3 -
arch/x86/kernel/kprobes/opt.c | 11 +++--
4 files changed, 75 insertions(+), 27 deletions(-)

--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -37,7 +37,7 @@ extern void text_poke_early(void *addr,
extern void *text_poke(void *addr, const void *opcode, size_t len);
extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
extern int poke_int3_handler(struct pt_regs *regs);
-extern void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
+extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
extern int after_bootmem;
extern __ro_after_init struct mm_struct *poking_mm;
extern __ro_after_init unsigned long poking_addr;
@@ -48,8 +48,17 @@ static inline void int3_emulate_jmp(stru
regs->ip = ip;
}

-#define INT3_INSN_SIZE 1
-#define CALL_INSN_SIZE 5
+#define INT3_INSN_SIZE 1
+#define INT3_INSN_OPCODE 0xCC
+
+#define CALL_INSN_SIZE 5
+#define CALL_INSN_OPCODE 0xE8
+
+#define JMP_INSN_SIZE 5
+#define JMP_INSN_OPCODE 0xE9
+
+#define JMP8_INSN_SIZE 2
+#define JMP8_INSN_OPCODE 0xEB

static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
{
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -920,31 +920,45 @@ static void do_sync_core(void *info)
sync_core();
}

-static bool bp_patching_in_progress;
-static void *bp_int3_handler, *bp_int3_addr;
+static const void *bp_int3_addr;
+static const struct insn *bp_int3_insn;

int poke_int3_handler(struct pt_regs *regs)
{
+ long ip;
+
/*
* Having observed our INT3 instruction, we now must observe
- * bp_patching_in_progress.
- *
- * in_progress = TRUE INT3
- * WMB RMB
- * write INT3 if (in_progress)
+ * bp_int3_addr and bp_int3_insn:
*
- * Idem for bp_int3_handler.
+ * bp_int3_{addr,insn) = .. INT3
+ * WMB RMB
+ * write INT3 if (insn)
*/
smp_rmb();

- if (likely(!bp_patching_in_progress))
+ if (likely(!bp_int3_insn))
return 0;

if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
return 0;

- /* set up the specified breakpoint handler */
- regs->ip = (unsigned long) bp_int3_handler;
+ ip = regs->ip - INT3_INSN_SIZE + bp_int3_insn->length;
+
+ switch (bp_int3_insn->opcode.bytes[0]) {
+ case CALL_INSN_OPCODE:
+ int3_emulate_call(regs, ip + bp_int3_insn->immediate.value);
+ break;
+
+ case JMP_INSN_OPCODE:
+ case JMP8_INSN_OPCODE:
+ int3_emulate_jmp(regs, ip + bp_int3_insn->immediate.value);
+ break;
+
+ default: /* assume NOP */
+ int3_emulate_jmp(regs, ip);
+ break;
+ }

return 1;
}
@@ -955,7 +969,7 @@ NOKPROBE_SYMBOL(poke_int3_handler);
* @addr: address to patch
* @opcode: opcode of new instruction
* @len: length to copy
- * @handler: address to jump to when the temporary breakpoint is hit
+ * @emulate: opcode to emulate, when NULL use @opcode
*
* Modify multi-byte instruction by using int3 breakpoint on SMP.
* We completely avoid stop_machine() here, and achieve the
@@ -970,19 +984,40 @@ NOKPROBE_SYMBOL(poke_int3_handler);
* replacing opcode
* - sync cores
*/
-void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
+void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
{
- unsigned char int3 = 0xcc;
+ unsigned char int3 = INT3_INSN_OPCODE;
+ struct insn insn;

- bp_int3_handler = handler;
- bp_int3_addr = (u8 *)addr + sizeof(int3);
- bp_patching_in_progress = true;
+ bp_int3_addr = addr + INT3_INSN_SIZE;

lockdep_assert_held(&text_mutex);

+ if (!emulate)
+ emulate = opcode;
+
+ kernel_insn_init(&insn, emulate, MAX_INSN_SIZE);
+ insn_get_length(&insn);
+
+ BUG_ON(!insn_complete(&insn));
+ BUG_ON(insn.length != len);
+
+ switch (insn.opcode.bytes[0]) {
+ case CALL_INSN_OPCODE:
+ case JMP_INSN_OPCODE:
+ case JMP8_INSN_OPCODE:
+ break;
+
+ default:
+ BUG_ON(len != 5);
+ BUG_ON(memcmp(emulate, ideal_nops[NOP_ATOMIC5], 5));
+ }
+
+ bp_int3_insn = &insn;
+
/*
* Corresponding read barrier in int3 notifier for making sure the
- * in_progress and handler are correctly ordered wrt. patching.
+ * in_progress and opcode are correctly ordered wrt. patching.
*/
smp_wmb();

@@ -1011,6 +1046,6 @@ void text_poke_bp(void *addr, const void
* sync_core() implies an smp_mb() and orders this store against
* the writing of the new instruction.
*/
- bp_patching_in_progress = false;
+ bp_int3_insn = NULL;
}

--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -87,8 +87,7 @@ static void __ref __jump_label_transform
return;
}

- text_poke_bp((void *)jump_entry_code(entry), code, JUMP_LABEL_NOP_SIZE,
- (void *)jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
+ text_poke_bp((void *)jump_entry_code(entry), code, JUMP_LABEL_NOP_SIZE, NULL);
}

void arch_jump_label_transform(struct jump_entry *entry,
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -437,8 +437,7 @@ void arch_optimize_kprobes(struct list_h
insn_buff[0] = RELATIVEJUMP_OPCODE;
*(s32 *)(&insn_buff[1]) = rel;

- text_poke_bp(op->kp.addr, insn_buff, RELATIVEJUMP_SIZE,
- op->optinsn.insn);
+ text_poke_bp(op->kp.addr, insn_buff, RELATIVEJUMP_SIZE, NULL);

list_del_init(&op->list);
}
@@ -448,12 +447,18 @@ void arch_optimize_kprobes(struct list_h
void arch_unoptimize_kprobe(struct optimized_kprobe *op)
{
u8 insn_buff[RELATIVEJUMP_SIZE];
+ u8 emulate_buff[RELATIVEJUMP_SIZE];

/* Set int3 to first byte for kprobes */
insn_buff[0] = BREAKPOINT_INSTRUCTION;
memcpy(insn_buff + 1, op->optinsn.copied_insn, RELATIVE_ADDR_SIZE);
+
+ emulate_buff[0] = RELATIVEJUMP_OPCODE;
+ *(s32 *)(&emulate_buff[1]) = (s32)((long)op->optinsn.insn -
+ ((long)op->kp.addr + RELATIVEJUMP_SIZE));
+
text_poke_bp(op->kp.addr, insn_buff, RELATIVEJUMP_SIZE,
- op->optinsn.insn);
+ emulate_buff);
}

/*
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