Re: [PATCH] riscv: kprobe: Fixup kernel panic when probing an illegal position

[Guo Ren]

On Sat, Jan 28, 2023 at 10:55 AM liaochang (A) <liaochang1@xxxxxxxxxx> wrote:
>
>
>
> 在 2023/1/26 21:05, guoren@xxxxxxxxxx 写道:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > The kernel would panic when probed for an illegal position. eg:
> >
> > (CONFIG_RISCV_ISA_C=n)
> >
> > echo 'p:hello kernel_clone+0x16 a0=%a0' >> kprobe_events
> > echo 1 > events/kprobes/hello/enable
> > cat trace
> >
> > Kernel panic - not syncing: stack-protector: Kernel stack
> > is corrupted in: __do_sys_newfstatat+0xb8/0xb8
> > CPU: 0 PID: 111 Comm: sh Not tainted
> > 6.2.0-rc1-00027-g2d398fe49a4d #490
> > Hardware name: riscv-virtio,qemu (DT)
> > Call Trace:
> > [<ffffffff80007268>] dump_backtrace+0x38/0x48
> > [<ffffffff80c5e83c>] show_stack+0x50/0x68
> > [<ffffffff80c6da28>] dump_stack_lvl+0x60/0x84
> > [<ffffffff80c6da6c>] dump_stack+0x20/0x30
> > [<ffffffff80c5ecf4>] panic+0x160/0x374
> > [<ffffffff80c6db94>] generic_handle_arch_irq+0x0/0xa8
> > [<ffffffff802deeb0>] sys_newstat+0x0/0x30
> > [<ffffffff800158c0>] sys_clone+0x20/0x30
> > [<ffffffff800039e8>] ret_from_syscall+0x0/0x4
> > ---[ end Kernel panic - not syncing: stack-protector:
> > Kernel stack is corrupted in: __do_sys_newfstatat+0xb8/0xb8 ]---
> >
> > That is because the kprobe's ebreak instruction broke the kernel's
> > original code. The user should guarantee the correction of the probe
> > position, but it couldn't make the kernel panic.
> >
> > This patch adds arch_check_kprobe in arch_prepare_kprobe to prevent an
> > illegal position (Such as the middle of an instruction).
> >
> > Fixes: c22b0bcb1dd0 ("riscv: Add kprobes supported")
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > ---
> >  arch/riscv/kernel/probes/kprobes.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> > index f21592d20306..475989f06d6d 100644
> > --- a/arch/riscv/kernel/probes/kprobes.c
> > +++ b/arch/riscv/kernel/probes/kprobes.c
> > @@ -48,6 +48,21 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
> >       post_kprobe_handler(p, kcb, regs);
> >  }
> >
> > +static bool __kprobes arch_check_kprobe(struct kprobe *p)
> > +{
> > +     unsigned long tmp  = (unsigned long)p->addr - p->offset;
> > +     unsigned long addr = (unsigned long)p->addr;
> > +
> > +     while (tmp <= addr) {
> > +             if (tmp == addr)
> > +                     return true;
> > +
> > +             tmp += GET_INSN_LENGTH(*(kprobe_opcode_t *)tmp);
> > +     }
> > +
> > +     return false;
> > +}
>
> LGTM.
>
> I have submit a patch to fix the same problem, found at:
>
> https://lore.kernel.org/lkml/20230127130541.1250865-11-chenguokai17@xxxxxxxxxxxxxxxx/
I have a question on your OPTKPROBE patch:
https://lore.kernel.org/lkml/20230127130541.1250865-10-chenguokai17@xxxxxxxxxxxxxxxx/

@@ -490,7 +573,14 @@ int arch_prepare_optimized_kprobe(struct
optimized_kprobe *op,
  * to detour buffer, ra is used to form JR jumping back from detour
  * buffer.
  */
- find_free_registers(orig, op, &rd, &ra);
+ if (used_reg == ALL_REG_OCCUPIED) {
+ find_free_registers(orig, op, &rd, &ra);
+ } else {
+ /* Choose one unused caller-saved register. */
+ rd = ffz(used_reg);
+ ra = rd;
+ }
+
  if (rd == 0 || ra == 0) {
  ^^^^^^^^^^^^^^^^^^
After no opt_used_dst_reg & no free caller-saved register (Of cause,
it's very rare for no available tmp regs):

Why not try:

     0: REG_S  ra, -SZREG(sp)
     4: auipc  ra, ?
     8: jalr   ?(ra)
    12: REG_L  ra, -SZREG(sp)

Besides taking up more instruction slots, does it have other problems?

>
> So this boundary check is necessary no matter CONFIG_RISCV_ISA_C is enable or not, right?
>
>
> > +
> >  int __kprobes arch_prepare_kprobe(struct kprobe *p)
> >  {
> >       unsigned long probe_addr = (unsigned long)p->addr;
> > @@ -55,6 +70,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
> >       if (probe_addr & 0x1)
> >               return -EILSEQ;
> >
> > +     if (!arch_check_kprobe(p))
> > +             return -EILSEQ;
> > +
> >       /* copy instruction */
> >       p->opcode = *p->addr;
> >
>
> --
> BR,
> Liao, Chang



-- 
Best Regards
 Guo Ren