[PATCH 3/5] powerpc/kprobes: Fix warnings from __this_cpu_read() on preempt kernels

[Masami Hiramatsu]

On Thu, 14 Sep 2017 12:17:20 +0530
"Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:

> On 2017/09/13 05:36PM, Masami Hiramatsu wrote:
> > On Thu, 14 Sep 2017 02:50:34 +0530
> > "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:
> > 
> > > Kamalesh pointed out that we are getting the below call traces with
> > > livepatched functions when we enable CONFIG_PREEMPT:
> > > 
> > > [  495.470721] BUG: using __this_cpu_read() in preemptible [00000000] code: cat/8394
> > > [  495.471167] caller is is_current_kprobe_addr+0x30/0x90
> > > [  495.471171] CPU: 4 PID: 8394 Comm: cat Tainted: G              K 4.13.0-rc7-nnr+ #95
> > > [  495.471173] Call Trace:
> > > [  495.471178] [c00000008fd9b960] [c0000000009f039c] dump_stack+0xec/0x160 (unreliable)
> > > [  495.471184] [c00000008fd9b9a0] [c00000000059169c] check_preemption_disabled+0x15c/0x170
> > > [  495.471187] [c00000008fd9ba30] [c000000000046460] is_current_kprobe_addr+0x30/0x90
> > > [  495.471191] [c00000008fd9ba60] [c00000000004e9a0] ftrace_call+0x1c/0xb8
> > > [  495.471195] [c00000008fd9bc30] [c000000000376fd8] seq_read+0x238/0x5c0
> > > [  495.471199] [c00000008fd9bcd0] [c0000000003cfd78] proc_reg_read+0x88/0xd0
> > > [  495.471203] [c00000008fd9bd00] [c00000000033e5d4] __vfs_read+0x44/0x1b0
> > > [  495.471206] [c00000008fd9bd90] [c0000000003402ec] vfs_read+0xbc/0x1b0
> > > [  495.471210] [c00000008fd9bde0] [c000000000342138] SyS_read+0x68/0x110
> > > [  495.471214] [c00000008fd9be30] [c00000000000bc6c] system_call+0x58/0x6c
> > > 
> > > Commit c05b8c4474c030 ("powerpc/kprobes: Skip livepatch_handler() for
> > > jprobes") introduced a helper is_current_kprobe_addr() to help determine
> > > if the current function has been livepatched or if it has a jprobe
> > > installed, both of which modify the NIP.
> > > 
> > > In the case of a jprobe, kprobe_ftrace_handler() disables pre-emption
> > > before calling into setjmp_pre_handler() which returns without disabling
> > > pre-emption. This is done to ensure that the jprobe han dler won't
> > > disappear beneath us if the jprobe is unregistered between the
> > > setjmp_pre_handler() and the subsequent longjmp_break_handler() called
> > > from the jprobe handler. Due to this, we can use __this_cpu_read() in
> > > is_current_kprobe_addr() with the pre-emption check as we know that
> > > pre-emption will be disabled.
> > > 
> > > However, if this function has been livepatched, we are still doing this
> > > check and when we do so, pre-emption won't necessarily be disabled. This
> > > results in the call trace shown above.
> > > 
> > > Fix this by only invoking is_current_kprobe_addr() when pre-emption is
> > > disabled. And since we now guard this within a pre-emption check, we can
> > > instead use raw_cpu_read() to get the current_kprobe value skipping the
> > > check done by __this_cpu_read().
> > 
> > Hmm, can you disable preempt temporary at this function and read it?
> 
> Yes, but I felt this approach is more optimal specifically for live 
> patching. We don't normally expect preemption to be disabled while 
> handling a livepatched function, so the simple 'if (!preemptible())'
> check helps us bypass looking at current_kprobe.

Ah, I see. this is for checking "jprobes", since kprobes/kretprobes
usually already done there.

> 
> The check also ensures we can use raw_cpu_read() safely in other 
> scenarios. Do you see any other concerns with this approach?

Yes, there are 2 reasons.

At first, if user's custom kprobe pre-handler changes NIP for their
custom handwriting livepatching, such handler will enable preemption
before return. We don't prohibit it. I think this function can not
detect it.

And also, the function "name" is very confusing :)
Especially, since this symbol is exported, if you are checking jprobes
context, it should be renamed, at least it starts with "__" so that
show it as local use.

Thank you,


> 
> Thanks,
> Naveen
> 
> > 
> > Thank you,
> > 
> > > 
> > > Fixes: c05b8c4474c030 ("powerpc/kprobes: Skip livepatch_handler() for jprobes")
> > > Reported-by: Kamalesh Babulal <kamalesh at linux.vnet.ibm.com>
> > > Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
> > > ---
> > >  arch/powerpc/kernel/kprobes.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> > > index e848fe2c93fb..db40b13fd3d1 100644
> > > --- a/arch/powerpc/kernel/kprobes.c
> > > +++ b/arch/powerpc/kernel/kprobes.c
> > > @@ -45,8 +45,12 @@ struct kretprobe_blackpoint kretprobe_blacklist[] = {{NULL, NULL}};
> > >  
> > >  int is_current_kprobe_addr(unsigned long addr)
> > >  {
> > > -	struct kprobe *p = kprobe_running();
> > > -	return (p && (unsigned long)p->addr == addr) ? 1 : 0;
> > > +	if (!preemptible()) {
> > > +		struct kprobe *p = raw_cpu_read(current_kprobe);
> > > +		return (p && (unsigned long)p->addr == addr) ? 1 : 0;
> > > +	}
> > > +
> > > +	return 0;
> > >  }
> > >  
> > >  bool arch_within_kprobe_blacklist(unsigned long addr)
> > > -- 
> > > 2.14.1
> > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu <mhiramat at kernel.org>
> > 
> 


-- 
Masami Hiramatsu <mhiramat at kernel.org>