[kernel-hardening] Re: [PATCHv2 1/2] arch: Move CONFIG_DEBUG_RODATA and CONFIG_SET_MODULE_RONX to be common

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Feb 03, 2017 at 01:08:40PM -0800, Kees Cook wrote:
> On Fri, Feb 3, 2017 at 12:29 PM, Russell King - ARM Linux
> <linux@armlinux.org.uk> wrote:
> > On Fri, Feb 03, 2017 at 11:45:56AM -0800, Kees Cook wrote:
> >> On Fri, Feb 3, 2017 at 9:52 AM, Laura Abbott <labbott@redhat.com> wrote:
> >> > diff --git a/arch/Kconfig b/arch/Kconfig
> >> > index 99839c2..22ee01e 100644
> >> > --- a/arch/Kconfig
> >> > +++ b/arch/Kconfig
> >> > @@ -781,4 +781,32 @@ config VMAP_STACK
> >> >           the stack to map directly to the KASAN shadow map using a formula
> >> >           that is incorrect if the stack is in vmalloc space.
> >> >
> >> > +config ARCH_NO_STRICT_RWX_DEFAULTS
> >> > +       def_bool n
> >> > +
> >> > +config ARCH_HAS_STRICT_KERNEL_RWX
> >> > +       def_bool n
> >> > +
> >> > +config DEBUG_RODATA
> >> > +       def_bool y if !ARCH_NO_STRICT_RWX_DEFAULTS
> >> > +       prompt "Make kernel text and rodata read-only" if ARCH_NO_STRICT_RWX_DEFAULTS
> >>
> >> Ah! Yes, perfect. I totally forgot about using conditional "prompt"
> >> lines. Nice!
> >
> > It's no different from the more usual:
> >
> >         bool "Make kernel text and rodata read-only" if ARCH_NO_STRICT_RWX_DEFAULTS
> >         default y if !ARCH_NO_STRICT_RWX_DEFAULTS
> >         depends on ARCH_HAS_STRICT_KERNEL_RWX
> >
> > But... I really don't like this - way too many negations and negatives
> > which make it difficult to figure out what's going on here.
> >
> > The situation we have today is:
> >
> > -config DEBUG_RODATA
> > -       bool "Make kernel text and rodata read-only"
> > -       depends on MMU && !XIP_KERNEL
> > -       default y if CPU_V7
> >
> > which is "allow the user to select DEBUG_RODATA if building a MMU non-XIP
> > kernel", suggesting that the user turns it on for ARMv7 CPUs.
> >
> > That changes with this and the above:
> >
> > +       select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
> > +       select ARCH_HAS_STRICT_MODULE_RWX if MMU
> > +       select ARCH_NO_STRICT_RWX_DEFAULTS if !CPU_V7
> >
> > This means that ARCH_HAS_STRICT_KERNEL_RWX is set for a MMU non-XIP
> > kernel, which carries the same pre-condition for DEBUG_RODATA - no
> > problem there.
> >
> > However, ARCH_NO_STRICT_RWX_DEFAULTS is set for non-ARMv7 CPUs, which
> > means the "Make kernel text and rodata read-only" prompt _is_ provided
> > for those.  However, for all ARMv7 systems, we go from "suggesting that
> > the user enables the option" to "you don't have a choice, you get this
> > whether you want it or not."
> >
> > I'd prefer to keep it off for my development systems, where I don't
> > care about kernel security.  If we don't wish to do that as a general
> > rule, can we make it dependent on EMBEDDED?
> >
> > Given that on ARM it can add up to 4MB to the kernel image - there
> > _will_ be about 1MB before the .text section, the padding on between
> > __modver and __ex_table which for me is around 626k, the padding
> > between .notes and the init sections start with .vectors (the space
> > between __ex_table and end of .notes is only 4124, which gets padded
> > up to 1MB) and lastly the padding between the .init section and the
> > data section (for me around 593k).  This all adds up to an increase
> > in kernel image size of 3.2MB on 14.2MB - an increase of 22%.
> >
> > So no, I'm really not happy with that.
> 
> Ah yeah, good point. We have three cases: unsupported, mandatory,
> optional, but we have the case of setting the default for the optional
> case. Maybe something like this?
> 
> config STRICT_KERNEL_RWX
>   bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
>   depends on ARCH_HAS_STRICT_KERNEL_RWX
>   default ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> unsupported:
> !ARCH_HAS_STRICT_KERNEL_RWX
> 
> mandatory:
> ARCH_HAS_STRICT_KERNEL_RWX
> !ARCH_OPTIONAL_KERNEL_RWX
> 
> optional:
> ARCH_HAS_STRICT_KERNEL_RWX
> ARCH_OPTIONAL_KERNEL_RWX
> with default controlled by ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> 
> Then arm is:
>   select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>   select ARCH_HAS_STRICT_MODULE_RWX if MMU
>   select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
> 
> x86 and arm64 are:
>   select ARCH_HAS_STRICT_KERNEL_RWX
>   select ARCH_HAS_STRICT_MODULE_RWX

Looks to me like it will do the job.

In passing, I noticed that, on ARM:

  3 .rodata       002212b4  c0703000  c0703000  00703000  2**6
                  CONTENTS, ALLOC, LOAD, DATA

a lack of READONLY there - which suggests something in this lot isn't
actually read-only data:

        .rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {           \
                VMLINUX_SYMBOL(__start_rodata) = .;                     \
                *(.rodata) *(.rodata.*)                                 \
                RO_AFTER_INIT_DATA      /* Read only after init */      \
                KEEP(*(__vermagic))     /* Kernel version magic */      \
                . = ALIGN(8);                                           \
                VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .;         \
                KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
                VMLINUX_SYMBOL(__stop___tracepoints_ptrs) = .;          \
                *(__tracepoints_strings)/* Tracepoints: strings */      \
        }                                                               \

I suspect it's the RO_AFTER_INIT_DATA section - maybe this should be
placed into its own separate output section?  I don't think it's doing
any harm, it's just odd that the .rodata section is marked as a normal
data section.

-- 
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