Re: [PATCH -v5 12/17] x86/kprobes: Fix ordering

Paul E. McKenney
2019-11-13T14:31:46Z

On Mon, Nov 11, 2019 at 02:13:04PM +0100, Peter Zijlstra wrote:
> Kprobes does something like:
>
> register:
> arch_arm_kprobe()
> text_poke(INT3)
> /* guarantees nothing, INT3 will become visible at some point, maybe */
>
> kprobe_optimizer()
> /* guarantees the bytes after INT3 are unused */
> syncrhonize_rcu_tasks();
> text_poke_bp(JMP32);
> /* implies IPI-sync, kprobe really is enabled */
>
>
> unregister:
> __disarm_kprobe()
> unoptimize_kprobe()
> text_poke_bp(INT3 + tail);
> /* implies IPI-sync, so tail is guaranteed visible */
> arch_disarm_kprobe()
> text_poke(old);
> /* guarantees nothing, old will maybe become visible */
>
> synchronize_rcu()
>
> free-stuff
>
> Now the problem is that on register, the synchronize_rcu_tasks() does
> not imply sufficient to guarantee all CPUs have already observed INT3
> (although in practise this is exceedingly unlikely not to have
> happened) (similar to how MEMBARRIER_CMD_PRIVATE_EXPEDITED does not
> imply MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE).
>
> Worse, even if it did, we'd have to do 2 synchronize calls to provide
> the guarantee we're looking for, the first to ensure INT3 is visible,
> the second to guarantee nobody is then still using the instruction
> bytes after INT3.
>
> Similar on unregister; the synchronize_rcu() between
> __unregister_kprobe_top() and __unregister_kprobe_bottom() does not
> guarantee all CPUs are free of the INT3 (and observe the old text).
>
> Therefore, sprinkle some IPI-sync love around. This guarantees that
> all CPUs agree on the text and RCU once again provides the required
> guaranteed.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: hpa@zytor.com
> Cc: paulmck@kernel.org

With a phrase like "IPI-sync love" in the commit log... ;-)

Acked-by: Paul E. McKenney <paulmck@kernel.org>

> Cc: mathieu.desnoyers@efficios.com
> ---
> arch/x86/include/asm/text-patching.h | 1 +
> arch/x86/kernel/alternative.c | 11 ++++++++---
> arch/x86/kernel/kprobes/core.c | 2 ++
> arch/x86/kernel/kprobes/opt.c | 12 ++++--------
> 4 files changed, 15 insertions(+), 11 deletions(-)
>
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -42,6 +42,7 @@ extern void text_poke_early(void *addr,
> * an inconsistent instruction while you patch.
> */
> extern void *text_poke(void *addr, const void *opcode, size_t len);
> +extern void text_poke_sync(void);
> 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, const void *emulate);
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -936,6 +936,11 @@ static void do_sync_core(void *info)
> sync_core();
> }
>
> +void text_poke_sync(void)
> +{
> + on_each_cpu(do_sync_core, NULL, 1);
> +}
> +
> struct text_poke_loc {
> s32 rel_addr; /* addr := _stext + rel_addr */
> s32 rel32;
> @@ -1085,7 +1090,7 @@ static void text_poke_bp_batch(struct te
> for (i = 0; i < nr_entries; i++)
> text_poke(text_poke_addr(&tp[i]), &int3, sizeof(int3));
>
> - on_each_cpu(do_sync_core, NULL, 1);
> + text_poke_sync();
>
> /*
> * Second step: update all but the first byte of the patched range.
> @@ -1107,7 +1112,7 @@ static void text_poke_bp_batch(struct te
> * not necessary and we'd be safe even without it. But
> * better safe than sorry (plus there's not only Intel).
> */
> - on_each_cpu(do_sync_core, NULL, 1);
> + text_poke_sync();
> }
>
> /*
> @@ -1123,7 +1128,7 @@ static void text_poke_bp_batch(struct te
> }
>
> if (do_sync)
> - on_each_cpu(do_sync_core, NULL, 1);
> + text_poke_sync();
>
> /*
> * sync_core() implies an smp_mb() and orders this store against
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -502,11 +502,13 @@ int arch_prepare_kprobe(struct kprobe *p
> void arch_arm_kprobe(struct kprobe *p)
> {
> text_poke(p->addr, ((unsigned char []){INT3_INSN_OPCODE}), 1);
> + text_poke_sync();
> }
>
> void arch_disarm_kprobe(struct kprobe *p)
> {
> text_poke(p->addr, &p->opcode, 1);
> + text_poke_sync();
> }
>
> void arch_remove_kprobe(struct kprobe *p)
> --- a/arch/x86/kernel/kprobes/opt.c
> +++ b/arch/x86/kernel/kprobes/opt.c
> @@ -444,14 +444,10 @@ void arch_optimize_kprobes(struct list_h
> /* Replace a relative jump with a breakpoint (int3). */
> void arch_unoptimize_kprobe(struct optimized_kprobe *op)
> {
> - u8 insn_buff[JMP32_INSN_SIZE];
> -
> - /* Set int3 to first byte for kprobes */
> - insn_buff[0] = INT3_INSN_OPCODE;
> - memcpy(insn_buff + 1, op->optinsn.copied_insn, DISP32_SIZE);
> -
> - text_poke_bp(op->kp.addr, insn_buff, JMP32_INSN_SIZE,
> - text_gen_insn(JMP32_INSN_OPCODE, op->kp.addr, op->optinsn.insn));
> + arch_arm_kprobe(&op->kp);
> + text_poke(op->kp.addr + INT3_INSN_SIZE,
> + op->optinsn.copied_insn, DISP32_SIZE);
> + text_poke_sync();
> }
>
> /*
>
>