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Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user

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  Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user Mark Rutland <mark.rutland@arm.com> - 2016-10-17 15:10 +0200
    Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user Kees Cook <keescook@chromium.org> - 2016-10-18 22:50 +0200

#1501959 — Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user

On Fri, Aug 26, 2016 at 02:57:58PM -0400, Kees Cook wrote:
> On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > The strncpy_from_user() accessor is effectively a copy_from_user()
> > specialised to copy strings, terminating early at a NUL byte if
> > possible. In other respects it is identical, and can be used to copy an
> > arbitrarily large buffer from userspace into the kernel. Conceptually,
> > it exposes a similar attack surface.
> >
> > As with copy_from_user(), we check the destination range when the kernel
> > is built with KASAN, but unlike copy_from_user() we do not check the
> > destination buffer when using HARDENED_USERCOPY. As strncpy_from_user()
> > calls get_user() in a loop, we must call check_object_size() explicitly.
> >
> > This patch adds this instrumentation to strncpy_from_user(), per the
> > same rationale as with the regular copy_from_user(). In the absence of
> > hardened usercopy this will have no impact as the instrumentation
> > expands to an empty static inline function.

[...]

> Ah, yes, good catch! (And to repeat what you mentioned to me in
> passing in the hall: there appear to be other users of get_user() in a
> loop in other places in the kernel that will likely need some
> attention too.)

I was reminded of this as it just hit mainline; is it worth dropping a
TODO on the KSPP wiki? I suspect I won't have the time to delve much
further into this in the near term, and it might be a good intro task
for someone.

Thanks,
Mark.

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#1503337

On Mon, Oct 17, 2016 at 6:04 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Fri, Aug 26, 2016 at 02:57:58PM -0400, Kees Cook wrote:
>> On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <mark.rutland@arm.com> wrote:
>> > The strncpy_from_user() accessor is effectively a copy_from_user()
>> > specialised to copy strings, terminating early at a NUL byte if
>> > possible. In other respects it is identical, and can be used to copy an
>> > arbitrarily large buffer from userspace into the kernel. Conceptually,
>> > it exposes a similar attack surface.
>> >
>> > As with copy_from_user(), we check the destination range when the kernel
>> > is built with KASAN, but unlike copy_from_user() we do not check the
>> > destination buffer when using HARDENED_USERCOPY. As strncpy_from_user()
>> > calls get_user() in a loop, we must call check_object_size() explicitly.
>> >
>> > This patch adds this instrumentation to strncpy_from_user(), per the
>> > same rationale as with the regular copy_from_user(). In the absence of
>> > hardened usercopy this will have no impact as the instrumentation
>> > expands to an empty static inline function.
>
> [...]
>
>> Ah, yes, good catch! (And to repeat what you mentioned to me in
>> passing in the hall: there appear to be other users of get_user() in a
>> loop in other places in the kernel that will likely need some
>> attention too.)
>
> I was reminded of this as it just hit mainline; is it worth dropping a
> TODO on the KSPP wiki? I suspect I won't have the time to delve much
> further into this in the near term, and it might be a good intro task
> for someone.

Ah, right. I've updated the kernserc TODO list with this (recently the
csum_* routines were pointed out), and added a bunch more TODOs that
were in my notes.

-Kees

-- 
Kees Cook
Nexus Security

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