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

Masami Hiramatsu
2024-05-02T02:03:58Z

On Thu, 2 May 2024 01:35:16 +0800
Guo Ren <guoren@kernel.org> wrote:

> 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>

Thanks Stephen, Guo and Steve!

Let me pick this to probes/for-next!

Thank you,

>
> > 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

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>