Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Waiman Long]

On 02/20/2018 08:26 PM, Kees Cook wrote:
> On Tue, Feb 20, 2018 at 3:17 PM, Andrew Morton
>  wrote:
>> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>>
>>> Even with clamped sysctl parameters, it is still not that straight
>>> forward to figure out the exact range of those parameters. One may
>>> try to write extreme parameter values to see if they get clamped.
>>> To make it easier, a warning with the expected range will now be
>>> printed in the kernel ring buffer when a clamped sysctl parameter
>>> receives an out of range value.
>> This assumes that do_proc_dointvec_minmax_conv() and
>> do_proc_douintvec_minmax_conv() are only ever called by privileged
>> userspace.  Because we mustn't give unprivileged applications a way to
>> spam the kernel logs.
>>
>> That's presumably true in the case of the caller you just added, but I
>> don't see what we can do to guarantee this in the future, so perhaps we
>> should add some permission check to the pr_warn()?
> How about pr_warn_ratelimited() instead?
>
> -Kees
>
My current thinking is to issue at most one warning per sysctl parameter
as additional warning of the same kind does not provide additional
information.

-Longman

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Waiman Long]

On 02/20/2018 08:26 PM, Kees Cook wrote:
> On Tue, Feb 20, 2018 at 3:17 PM, Andrew Morton
>  wrote:
>> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>>
>>> Even with clamped sysctl parameters, it is still not that straight
>>> forward to figure out the exact range of those parameters. One may
>>> try to write extreme parameter values to see if they get clamped.
>>> To make it easier, a warning with the expected range will now be
>>> printed in the kernel ring buffer when a clamped sysctl parameter
>>> receives an out of range value.
>> This assumes that do_proc_dointvec_minmax_conv() and
>> do_proc_douintvec_minmax_conv() are only ever called by privileged
>> userspace.  Because we mustn't give unprivileged applications a way to
>> spam the kernel logs.
>>
>> That's presumably true in the case of the caller you just added, but I
>> don't see what we can do to guarantee this in the future, so perhaps we
>> should add some permission check to the pr_warn()?
> How about pr_warn_ratelimited() instead?
>
> -Kees
>
My current thinking is to issue at most one warning per sysctl parameter
as additional warning of the same kind does not provide additional
information.

-Longman

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Waiman Long]

On 02/20/2018 06:17 PM, Andrew Morton wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>
>> Even with clamped sysctl parameters, it is still not that straight
>> forward to figure out the exact range of those parameters. One may
>> try to write extreme parameter values to see if they get clamped.
>> To make it easier, a warning with the expected range will now be
>> printed in the kernel ring buffer when a clamped sysctl parameter
>> receives an out of range value.
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
>
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?
>
Good point. Beside adding security check, another alternative is to use
some kind of warn_once() for each sysctl parameter. That will limit the
amount of spamming that is possible. It will require adding a flag to
the ctl_table to mark an  entry as warned. I think that will be less
tricky than adding permission check.

I can also use the new flag to designate that a sysctl parameter need to
be clamped to the range instead of failing when out of range. With that,
I don't need to introduce new clamping APIs. I can use the existing
ones. I will work on v2 patch with that change.

Thanks,
Longman

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Waiman Long]

On 02/20/2018 06:17 PM, Andrew Morton wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>
>> Even with clamped sysctl parameters, it is still not that straight
>> forward to figure out the exact range of those parameters. One may
>> try to write extreme parameter values to see if they get clamped.
>> To make it easier, a warning with the expected range will now be
>> printed in the kernel ring buffer when a clamped sysctl parameter
>> receives an out of range value.
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
>
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?
>
Good point. Beside adding security check, another alternative is to use
some kind of warn_once() for each sysctl parameter. That will limit the
amount of spamming that is possible. It will require adding a flag to
the ctl_table to mark an  entry as warned. I think that will be less
tricky than adding permission check.

I can also use the new flag to designate that a sysctl parameter need to
be clamped to the range instead of failing when out of range. With that,
I don't need to introduce new clamping APIs. I can use the existing
ones. I will work on v2 patch with that change.

Thanks,
Longman

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Kees Cook]

On Tue, Feb 20, 2018 at 3:17 PM, Andrew Morton
 wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>
>> Even with clamped sysctl parameters, it is still not that straight
>> forward to figure out the exact range of those parameters. One may
>> try to write extreme parameter values to see if they get clamped.
>> To make it easier, a warning with the expected range will now be
>> printed in the kernel ring buffer when a clamped sysctl parameter
>> receives an out of range value.
>
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
>
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?

How about pr_warn_ratelimited() instead?

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Kees Cook]

On Tue, Feb 20, 2018 at 3:17 PM, Andrew Morton
 wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
>
>> Even with clamped sysctl parameters, it is still not that straight
>> forward to figure out the exact range of those parameters. One may
>> try to write extreme parameter values to see if they get clamped.
>> To make it easier, a warning with the expected range will now be
>> printed in the kernel ring buffer when a clamped sysctl parameter
>> receives an out of range value.
>
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
>
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?

How about pr_warn_ratelimited() instead?

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Luis R. Rodriguez]

On Tue, Feb 20, 2018 at 03:17:05PM -0800, Andrew Morton wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
> 
> > Even with clamped sysctl parameters, it is still not that straight
> > forward to figure out the exact range of those parameters. One may
> > try to write extreme parameter values to see if they get clamped.
> > To make it easier, a warning with the expected range will now be
> > printed in the kernel ring buffer when a clamped sysctl parameter
> > receives an out of range value.
> 
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
> 
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?

Beyond what we do with sysctl_perm() on proc_sys_call_handler()?

  Luis

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Luis R. Rodriguez]

On Tue, Feb 20, 2018 at 03:17:05PM -0800, Andrew Morton wrote:
> On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:
> 
> > Even with clamped sysctl parameters, it is still not that straight
> > forward to figure out the exact range of those parameters. One may
> > try to write extreme parameter values to see if they get clamped.
> > To make it easier, a warning with the expected range will now be
> > printed in the kernel ring buffer when a clamped sysctl parameter
> > receives an out of range value.
> 
> This assumes that do_proc_dointvec_minmax_conv() and
> do_proc_douintvec_minmax_conv() are only ever called by privileged
> userspace.  Because we mustn't give unprivileged applications a way to
> spam the kernel logs.
> 
> That's presumably true in the case of the caller you just added, but I
> don't see what we can do to guarantee this in the future, so perhaps we
> should add some permission check to the pr_warn()?

Beyond what we do with sysctl_perm() on proc_sys_call_handler()?

  Luis

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Andrew Morton]

On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:

> Even with clamped sysctl parameters, it is still not that straight
> forward to figure out the exact range of those parameters. One may
> try to write extreme parameter values to see if they get clamped.
> To make it easier, a warning with the expected range will now be
> printed in the kernel ring buffer when a clamped sysctl parameter
> receives an out of range value.

This assumes that do_proc_dointvec_minmax_conv() and
do_proc_douintvec_minmax_conv() are only ever called by privileged
userspace.  Because we mustn't give unprivileged applications a way to
spam the kernel logs.

That's presumably true in the case of the caller you just added, but I
don't see what we can do to guarantee this in the future, so perhaps we
should add some permission check to the pr_warn()?

Re: [PATCH 2/3] sysctl: Warn when a clamped sysctl parameter is set out of range

[Andrew Morton]

On Mon, 19 Feb 2018 11:53:50 -0500 Waiman Long  wrote:

> Even with clamped sysctl parameters, it is still not that straight
> forward to figure out the exact range of those parameters. One may
> try to write extreme parameter values to see if they get clamped.
> To make it easier, a warning with the expected range will now be
> printed in the kernel ring buffer when a clamped sysctl parameter
> receives an out of range value.

This assumes that do_proc_dointvec_minmax_conv() and
do_proc_douintvec_minmax_conv() are only ever called by privileged
userspace.  Because we mustn't give unprivileged applications a way to
spam the kernel logs.

That's presumably true in the case of the caller you just added, but I
don't see what we can do to guarantee this in the future, so perhaps we
should add some permission check to the pr_warn()?