Re: [PATCH v3] kprobe/ftrace: bail out if ftrace was killed

Guo Ren
2024-05-01T17:35:29Z

On Thu, May 2, 2024 at 12:30 AM Stephen Brennan
<stephen.s.brennan@oracle.com> wrote:
>
> If an error happens in ftrace, ftrace_kill() will prevent disarming
> kprobes. Eventually, the ftrace_ops associated with the kprobes will be
> freed, yet the kprobes will still be active, and when triggered, they
> will use the freed memory, likely resulting in a page fault and panic.
>
> This behavior can be reproduced quite easily, by creating a kprobe and
> then triggering a ftrace_kill(). For simplicity, we can simulate an
> ftrace error with a kernel module like [1]:
>
> [1]: https://github.com/brenns10/kernel_stuff/tree/master/ftrace_killer
>
> sudo perf probe --add commit_creds
> sudo perf trace -e probe:commit_creds
> # In another terminal
> make
> sudo insmod ftrace_killer.ko # calls ftrace_kill(), simulating bug
> # Back to perf terminal
> # ctrl-c
> sudo perf probe --del commit_creds
>
> After a short period, a page fault and panic would occur as the kprobe
> continues to execute and uses the freed ftrace_ops. While ftrace_kill()
> is supposed to be used only in extreme circumstances, it is invoked in
> FTRACE_WARN_ON() and so there are many places where an unexpected bug
> could be triggered, yet the system may continue operating, possibly
> without the administrator noticing. If ftrace_kill() does not panic the
> system, then we should do everything we can to continue operating,
> rather than leave a ticking time bomb.
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
> ---
> Changes in v3:
> Don't expose ftrace_is_dead(). Create a "kprobe_ftrace_disabled"
> variable and check it directly in the kprobe handlers.
> Link to v1/v2 discussion:
> https://lore.kernel.org/all/20240426225834.993353-1-stephen.s.brennan@oracle.com/
>
> arch/csky/kernel/probes/ftrace.c | 3 +++
> arch/loongarch/kernel/ftrace_dyn.c | 3 +++
> arch/parisc/kernel/ftrace.c | 3 +++
> arch/powerpc/kernel/kprobes-ftrace.c | 3 +++
> arch/riscv/kernel/probes/ftrace.c | 3 +++
> arch/s390/kernel/ftrace.c | 3 +++
> arch/x86/kernel/kprobes/ftrace.c | 3 +++
> include/linux/kprobes.h | 7 +++++++
> kernel/kprobes.c | 6 ++++++
> kernel/trace/ftrace.c | 1 +
> 10 files changed, 35 insertions(+)
>
> diff --git a/arch/csky/kernel/probes/ftrace.c b/arch/csky/kernel/probes/ftrace.c
> index 834cffcfbce3..7ba4b98076de 100644
> --- a/arch/csky/kernel/probes/ftrace.c
> +++ b/arch/csky/kernel/probes/ftrace.c
> @@ -12,6 +12,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe_ctlblk *kcb;
> struct pt_regs *regs;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
For csky part.
Acked-by: Guo Ren <guoren@kernel.org>

> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/arch/loongarch/kernel/ftrace_dyn.c b/arch/loongarch/kernel/ftrace_dyn.c
> index 73858c9029cc..bff058317062 100644
> --- a/arch/loongarch/kernel/ftrace_dyn.c
> +++ b/arch/loongarch/kernel/ftrace_dyn.c
> @@ -287,6 +287,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe *p;
> struct kprobe_ctlblk *kcb;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> index 621a4b386ae4..c91f9c2e61ed 100644
> --- a/arch/parisc/kernel/ftrace.c
> +++ b/arch/parisc/kernel/ftrace.c
> @@ -206,6 +206,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe *p;
> int bit;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> index 072ebe7f290b..f8208c027148 100644
> --- a/arch/powerpc/kernel/kprobes-ftrace.c
> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
> @@ -21,6 +21,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
> struct pt_regs *regs;
> int bit;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(nip, parent_nip);
> if (bit < 0)
> return;
> diff --git a/arch/riscv/kernel/probes/ftrace.c b/arch/riscv/kernel/probes/ftrace.c
> index 7142ec42e889..a69dfa610aa8 100644
> --- a/arch/riscv/kernel/probes/ftrace.c
> +++ b/arch/riscv/kernel/probes/ftrace.c
> @@ -11,6 +11,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe_ctlblk *kcb;
> int bit;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
> index c46381ea04ec..7f6f8c438c26 100644
> --- a/arch/s390/kernel/ftrace.c
> +++ b/arch/s390/kernel/ftrace.c
> @@ -296,6 +296,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe *p;
> int bit;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
> index dd2ec14adb77..15af7e98e161 100644
> --- a/arch/x86/kernel/kprobes/ftrace.c
> +++ b/arch/x86/kernel/kprobes/ftrace.c
> @@ -21,6 +21,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct kprobe_ctlblk *kcb;
> int bit;
>
> + if (unlikely(kprobe_ftrace_disabled))
> + return;
> +
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (bit < 0)
> return;
> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> index 0ff44d6633e3..5fcbc254d186 100644
> --- a/include/linux/kprobes.h
> +++ b/include/linux/kprobes.h
> @@ -378,11 +378,15 @@ static inline void wait_for_kprobe_optimizer(void) { }
> extern void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> struct ftrace_ops *ops, struct ftrace_regs *fregs);
> extern int arch_prepare_kprobe_ftrace(struct kprobe *p);
> +/* Set when ftrace has been killed: kprobes on ftrace must be disabled for safety */
> +extern bool kprobe_ftrace_disabled __read_mostly;
> +extern void kprobe_ftrace_kill(void);
> #else
> static inline int arch_prepare_kprobe_ftrace(struct kprobe *p)
> {
> return -EINVAL;
> }
> +static inline void kprobe_ftrace_kill(void) {}
> #endif /* CONFIG_KPROBES_ON_FTRACE */
>
> /* Get the kprobe at this addr (if any) - called with preemption disabled */
> @@ -495,6 +499,9 @@ static inline void kprobe_flush_task(struct task_struct *tk)
> static inline void kprobe_free_init_mem(void)
> {
> }
> +static inline void kprobe_ftrace_kill(void)
> +{
> +}
> static inline int disable_kprobe(struct kprobe *kp)
> {
> return -EOPNOTSUPP;
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 65adc815fc6e..166ebf81dc45 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1068,6 +1068,7 @@ static struct ftrace_ops kprobe_ipmodify_ops __read_mostly = {
>
> static int kprobe_ipmodify_enabled;
> static int kprobe_ftrace_enabled;
> +bool kprobe_ftrace_disabled;
>
> static int __arm_kprobe_ftrace(struct kprobe *p, struct ftrace_ops *ops,
> int *cnt)
> @@ -1136,6 +1137,11 @@ static int disarm_kprobe_ftrace(struct kprobe *p)
> ipmodify ? &kprobe_ipmodify_ops : &kprobe_ftrace_ops,
> ipmodify ? &kprobe_ipmodify_enabled : &kprobe_ftrace_enabled);
> }
> +
> +void kprobe_ftrace_kill()
> +{
> + kprobe_ftrace_disabled = true;
> +}
> #else /* !CONFIG_KPROBES_ON_FTRACE */
> static inline int arm_kprobe_ftrace(struct kprobe *p)
> {
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index da1710499698..96db99c347b3 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -7895,6 +7895,7 @@ void ftrace_kill(void)
> ftrace_disabled = 1;
> ftrace_enabled = 0;
> ftrace_trace_function = ftrace_stub;
> + kprobe_ftrace_kill();
> }
>
> /**
> --
> 2.39.3
>

--
Best Regards
Guo Ren