[PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

syzbot complains smp_processor_id() from harden_branch_predictor()
 from page fault path [1]. Explicitly disable preemption and use
raw_smp_processor_id().

Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
Reported-by: syzbot <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel space")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
This patch is completely untested.

 arch/arm/include/asm/system_misc.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
index 98b37340376b..a92446769acd 100644
--- a/arch/arm/include/asm/system_misc.h
+++ b/arch/arm/include/asm/system_misc.h
@@ -20,8 +20,11 @@ typedef void (*harden_branch_predictor_fn_t)(void);
 DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
 static inline void harden_branch_predictor(void)
 {
-	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
-						  smp_processor_id());
+	harden_branch_predictor_fn_t fn;
+
+	preempt_disable_notrace();
+	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
+	preempt_enable_no_resched_notrace();
 	if (fn)
 		fn();
 }
-- 
2.18.4

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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Russell King (Oracle)]

On Wed, Jun 22, 2022 at 03:49:21PM +0900, Tetsuo Handa wrote:
> syzbot complains smp_processor_id() from harden_branch_predictor()
>  from page fault path [1]. Explicitly disable preemption and use
> raw_smp_processor_id().
> 
> Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
> Reported-by: syzbot <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
> Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel space")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

This may "fix" the warning, but...

> ---
> This patch is completely untested.
> 
>  arch/arm/include/asm/system_misc.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
> index 98b37340376b..a92446769acd 100644
> --- a/arch/arm/include/asm/system_misc.h
> +++ b/arch/arm/include/asm/system_misc.h
> @@ -20,8 +20,11 @@ typedef void (*harden_branch_predictor_fn_t)(void);
>  DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
>  static inline void harden_branch_predictor(void)
>  {
> -	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
> -						  smp_processor_id());
> +	harden_branch_predictor_fn_t fn;
> +
> +	preempt_disable_notrace();
> +	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
> +	preempt_enable_no_resched_notrace();
>  	if (fn)
>  		fn();

The idea is to get the function for the specific CPU, and then to run it
_on_ that CPU, and in theory the CPU that took the fault. However, I
seem to remember there are issues trying to achieve that, and I don't
have a solution for it.

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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

On 2022/06/22 20:21, Russell King (Oracle) wrote:
>>  static inline void harden_branch_predictor(void)
>>  {
>> -	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
>> -						  smp_processor_id());
>> +	harden_branch_predictor_fn_t fn;
>> +
>> +	preempt_disable_notrace();
>> +	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
>> +	preempt_enable_no_resched_notrace();
>>  	if (fn)
>>  		fn();
> 
> The idea is to get the function for the specific CPU, and then to run it
> _on_ that CPU, and in theory the CPU that took the fault. However, I
> seem to remember there are issues trying to achieve that, and I don't
> have a solution for it.

Hmm, since some ARM processors support Asymmetric Multiprocessing, whether
invalidation is needed depends on which processor was assigned, and that's
the reason smp_processor_id() is used?

I'm not familiar with hardware, but if a CPU assigned to current thread
changes, wouldn't sufficient invalidation take place? In other words, do
we need to worry about fn() call if the CPU that took the fault and the CPU
which runs the code after harden_branch_predictor() returned differs?

If some instructions for CPU-A is not supported by CPU-B, I guess that
reading instructions for CPU-A at per_cpu(harden_branch_predictor_fn) and
executing instructions on CPU-B at fn() causes problems. Is it guaranteed
that all instructions used by fn() are supported by all processors?
If it is not guaranteed, we would need to make sure that fn() runs on a CPU
which per_cpu(harden_branch_predictor_fn) was done for?


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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Marc Zyngier]

On 2022-06-22 07:49, Tetsuo Handa wrote:
> syzbot complains smp_processor_id() from harden_branch_predictor()
>  from page fault path [1]. Explicitly disable preemption and use
> raw_smp_processor_id().
> 
> Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
> Reported-by: syzbot 
> <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
> Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel 
> space")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> This patch is completely untested.
> 
>  arch/arm/include/asm/system_misc.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/system_misc.h
> b/arch/arm/include/asm/system_misc.h
> index 98b37340376b..a92446769acd 100644
> --- a/arch/arm/include/asm/system_misc.h
> +++ b/arch/arm/include/asm/system_misc.h
> @@ -20,8 +20,11 @@ typedef void (*harden_branch_predictor_fn_t)(void);
>  DECLARE_PER_CPU(harden_branch_predictor_fn_t, 
> harden_branch_predictor_fn);
>  static inline void harden_branch_predictor(void)
>  {
> -	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
> -						  smp_processor_id());
> +	harden_branch_predictor_fn_t fn;
> +
> +	preempt_disable_notrace();
> +	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
> +	preempt_enable_no_resched_notrace();
>  	if (fn)
>  		fn();
>  }

I don't think that's required.

harden_branch_predictor() is always called on the fault path,
from __do_user_fault(), and that's always non-preemptible.

My hunch is that we're missing some tracking that indicates
to the kernel that we're already non-preemptible by virtue
of being in an exception handler.

Russell, what do you think?

         M.
-- 
Jazz is not dead. It just smells funny...

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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Russell King (Oracle)]

On Wed, Jun 22, 2022 at 02:50:15PM +0100, Marc Zyngier wrote:
> On 2022-06-22 07:49, Tetsuo Handa wrote:
> > syzbot complains smp_processor_id() from harden_branch_predictor()
> >  from page fault path [1]. Explicitly disable preemption and use
> > raw_smp_processor_id().
> > 
> > Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
> > Reported-by: syzbot
> > <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
> > Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel
> > space")
> > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > ---
> > This patch is completely untested.
> > 
> >  arch/arm/include/asm/system_misc.h | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/system_misc.h
> > b/arch/arm/include/asm/system_misc.h
> > index 98b37340376b..a92446769acd 100644
> > --- a/arch/arm/include/asm/system_misc.h
> > +++ b/arch/arm/include/asm/system_misc.h
> > @@ -20,8 +20,11 @@ typedef void (*harden_branch_predictor_fn_t)(void);
> >  DECLARE_PER_CPU(harden_branch_predictor_fn_t,
> > harden_branch_predictor_fn);
> >  static inline void harden_branch_predictor(void)
> >  {
> > -	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
> > -						  smp_processor_id());
> > +	harden_branch_predictor_fn_t fn;
> > +
> > +	preempt_disable_notrace();
> > +	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
> > +	preempt_enable_no_resched_notrace();
> >  	if (fn)
> >  		fn();
> >  }
> 
> I don't think that's required.
> 
> harden_branch_predictor() is always called on the fault path,
> from __do_user_fault(), and that's always non-preemptible.
> 
> My hunch is that we're missing some tracking that indicates
> to the kernel that we're already non-preemptible by virtue
> of being in an exception handler.
> 
> Russell, what do you think?

There is one path that we can hit this while we're preemptible - a
page fault (handled via do_page_fault) at an address in kernel space
triggered by user space (e.g. userspace trying to access vmalloc or
module space). 

I suppose, since we know that's never going to be fixed up, we could
detect that the address >= TASK_SIZE and we're entering from usespace
before enabling interrupts, and do the hardening early in that path.
We'd need to move the other cases where we call the hardening as well.

The down side is more tests in the page fault fast-path... so there
will be a performance penalty to be paid just to shut up this warning -
a warning that is "userspace is trying to do real bad stuff" that
basically means userspace is trying to run an exploit...

Which makes me think... having the loud complaint from the kernel there
is actually a good thing, it makes people sit up and notice that
something is wrong.

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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

On 2022/06/22 23:04, Russell King (Oracle) wrote:
> Which makes me think... having the loud complaint from the kernel there
> is actually a good thing, it makes people sit up and notice that
> something is wrong.

OK. Then, would you change the code not to emit "BUG:" message, for
syzkaller stops testing upon encountering "BUG:" string?


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Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

On 2022/06/23 0:02, Tetsuo Handa wrote:
> On 2022/06/22 23:04, Russell King (Oracle) wrote:
>> Which makes me think... having the loud complaint from the kernel there
>> is actually a good thing, it makes people sit up and notice that
>> something is wrong.
> 
> OK. Then, would you change the code not to emit "BUG:" message, for
> syzkaller stops testing upon encountering "BUG:" string?
> 

Something like this?

 arch/arm/include/asm/system_misc.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
index 98b37340376b..09d5c2262165 100644
--- a/arch/arm/include/asm/system_misc.h
+++ b/arch/arm/include/asm/system_misc.h
@@ -20,10 +20,22 @@ typedef void (*harden_branch_predictor_fn_t)(void);
 DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
 static inline void harden_branch_predictor(void)
 {
-	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
-						  smp_processor_id());
+	static DEFINE_RATELIMIT_STATE(predictor_rs, 10 * HZ, 1);
+	const bool is_preemptible = preemptible();
+	harden_branch_predictor_fn_t fn;
+
+	if (unlikely(is_preemptible)) {
+		ratelimit_set_flags(&predictor_rs, RATELIMIT_MSG_ON_RELEASE);
+		if (__ratelimit(&predictor_rs))
+			pr_err("%s[%d] page fault with preemption enabled (exploit attempt?)\n",
+			       current->comm, task_pid_nr(current));
+		preempt_disable_notrace();
+	}
+	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
 	if (fn)
 		fn();
+	if (unlikely(is_preemptible))
+		preempt_enable_no_resched_notrace();
 }
 #else
 #define harden_branch_predictor() do { } while (0)
-- 
2.18.4

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[PATCH v2] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

syzbot is reporting that CONFIG_HARDEN_BRANCH_PREDICTOR=y +
CONFIG_DEBUG_PREEMPT=y on ARM32 causes "BUG: using smp_processor_id() in
preemptible code" message [1], for this check was not designed to handle
attempts to access kernel memory like

  ----------
  int main() { return *(char *) -1; }
  ----------

. Although Russell King commented that this BUG: message might help finding
possible exploit attempts [2], this is not a kernel's problem that worth
giving up fuzz testing.

This patch explicitly disables preemption and uses raw_smp_processor_id().

Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
Link: https://lkml.kernel.org/r/YrMhVAev9wMAA8tl@shell.armlinux.org.uk [2]
Reported-by: syzbot <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel space")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 arch/arm/include/asm/system_misc.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
index 98b37340376b..670e8d116770 100644
--- a/arch/arm/include/asm/system_misc.h
+++ b/arch/arm/include/asm/system_misc.h
@@ -20,10 +20,13 @@ typedef void (*harden_branch_predictor_fn_t)(void);
 DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
 static inline void harden_branch_predictor(void)
 {
-	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
-						  smp_processor_id());
+	harden_branch_predictor_fn_t fn;
+
+	preempt_disable_notrace();
+	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
 	if (fn)
 		fn();
+	preempt_enable_no_resched_notrace();
 }
 #else
 #define harden_branch_predictor() do { } while (0)
-- 
2.34.1


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Re: [PATCH v2] ARM: spectre-v2: fix smp_processor_id() warning

[Russell King (Oracle)]

On Fri, Jul 15, 2022 at 10:09:01PM +0900, Tetsuo Handa wrote:
> syzbot is reporting that CONFIG_HARDEN_BRANCH_PREDICTOR=y +
> CONFIG_DEBUG_PREEMPT=y on ARM32 causes "BUG: using smp_processor_id() in
> preemptible code" message [1], for this check was not designed to handle
> attempts to access kernel memory like
> 
>   ----------
>   int main() { return *(char *) -1; }
>   ----------
> 
> . Although Russell King commented that this BUG: message might help finding
> possible exploit attempts [2], this is not a kernel's problem that worth
> giving up fuzz testing.

But shutting up a valid warning when the real problem is still there is
also not acceptable.

As I've said many times, the workaround needs to be run on the _same_
CPU that faulted. The warning is telling us that we're preemptible at
this point, which means that can't be guaranteed.

So bodging it by disabling preemption around here DOES NOT FIX THE
PROBLEM. It _SHUTS UP THE VALID WARNING_. And shutting up a valid
warning is A VERY BAD PRACTICE.

NAK. MAK. NAK. NAK. NAK.

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Re: [PATCH v2] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

On 2022/07/15 22:36, Russell King (Oracle) wrote:
> On Fri, Jul 15, 2022 at 10:09:01PM +0900, Tetsuo Handa wrote:
>> syzbot is reporting that CONFIG_HARDEN_BRANCH_PREDICTOR=y +
>> CONFIG_DEBUG_PREEMPT=y on ARM32 causes "BUG: using smp_processor_id() in
>> preemptible code" message [1], for this check was not designed to handle
>> attempts to access kernel memory like
>>
>>   ----------
>>   int main() { return *(char *) -1; }
>>   ----------
>>
>> . Although Russell King commented that this BUG: message might help finding
>> possible exploit attempts [2], this is not a kernel's problem that worth
>> giving up fuzz testing.
> 
> But shutting up a valid warning when the real problem is still there is
> also not acceptable.

Then, at least for now can we stop emitting the BUG: string? I showed an idea at
https://lkml.kernel.org/r/fa786d1c-db06-f7f1-9ac9-6a468c1e8d81@I-love.SAKURA.ne.jp
but I got no response.

Since syzkaller stops fuzz testing upon encountering BUG: or WARNING: string,
ARM32 might be failing to find other bugs for 491 days due to this problem.

If you don't want to stop emitting this BUG: string, we might want to teach
syzkaller to build ARM32 kernels with CONFIG_HARDEN_BRANCH_PREDICTOR=n.


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