Re: [PATCH v5 4/7] introduce post-init read-only memory

[Andy Lutomirski]

On Mon, Mar 7, 2016 at 4:16 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Mon, Mar 7, 2016 at 5:00 AM, Christian Borntraeger
> <borntraeger@xxxxxxxxxx> wrote:
>> On 02/17/2016 11:41 PM, Kees Cook wrote:
>>> One of the easiest ways to protect the kernel from attack is to reduce
>>> the internal attack surface exposed when a "write" flaw is available. By
>>> making as much of the kernel read-only as possible, we reduce the
>>> attack surface.
>>>
>>> Many things are written to only during __init, and never changed
>>> again. These cannot be made "const" since the compiler will do the wrong
>>> thing (we do actually need to write to them). Instead, move these items
>>> into a memory region that will be made read-only during mark_rodata_ro()
>>> which happens after all kernel __init code has finished.
>>>
>>> This introduces __ro_after_init as a way to mark such memory, and adds
>>> some documentation about the existing __read_mostly marking.
>>>
>>> Based on work by PaX Team and Brad Spengler.
>>
>> This seems to crash in the lkdtm module on s390, if the module
>> is compiled in.
>>
>> [    0.360571] Unable to handle kernel pointer dereference in virtual kernel address space
>> [    0.360574] Failing address: 0000000000996000 TEID: 0000000000996407
>> [    0.360575] Fault in home space mode while using kernel ASCE.
>> [    0.360577] AS:0000000000cac007 R3:000000003ffd1007 S:0000000000901600
>> [    0.360665] Oops: 0004 ilc:3 [#1] SMP
>> [    0.360668] Modules linked in:
>> [    0.360674] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc6-next-20160304+ #165
>> [    0.360676] task: 000000003de40000 ti: 000000003de48000 task.ti: 000000003de48000
>> [    0.360678] Krnl PSW : 0704d00180000000 0000000000c480e8 (lkdtm_module_init+0x28/0x278)
>> [    0.360680]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3
>> Krnl GPRS: ffffffffffffffee 0000000055aa55aa 00000000016503a0 00000000000004ef
>> [    0.360682]            00000000001001ca 00000000000004f0 0000000000c93370 0000000000000000
>> [    0.360683]            0000000000c57008 000000000000006a 0000000000000002 0000000000000000
>> [    0.360685]            0000000000c480c0 0000000001650380 000000003de4bd88 000000003de4bd28
>> [    0.360698] Krnl Code: 0000000000c480d8: a51b00aa            oill    %r1,170
>>            0000000000c480dc: e3f0ffa0ff71       lay     %r15,-96(%r15)
>>           #0000000000c480e2: e3e0f0980024       stg     %r14,152(%r15)
>>           >0000000000c480e8: c41bffea76c8       stgrl   %r1,996e78
>>            0000000000c480ee: c020ffe95e5c       larl    %r2,973da6
>>            0000000000c480f4: a7390000           lghi    %r3,0
>>            0000000000c480f8: c0d0ffdee974       larl    %r13,8253e0
>>            0000000000c480fe: a7c90009           lghi    %r12,9
>> [    0.360710] Call Trace:
>> [    0.360713] ([<000000003de4be28>] 0x3de4be28)
>> [    0.360718] ([<00000000001001da>] do_one_initcall+0xa2/0x1b0)
>> [    0.360724] ([<0000000000c0dcf4>] kernel_init_freeable+0x1e4/0x298)
>> [    0.360729] ([<00000000007b3cda>] kernel_init+0x2a/0x128)
>> [    0.360731] ([<00000000007bcc8e>] kernel_thread_starter+0x6/0xc)
>> [    0.360733] ([<00000000007bcc88>] kernel_thread_starter+0x0/0xc)
>> [    0.360733] Last Breaking-Event-Address:
>> [    0.360735]  [<00000000001001d8>] do_one_initcall+0xa0/0x1b0
>> [    0.360735]
>> [    0.360738] Kernel panic - not syncing: Fatal exception: panic_on_oops
>>
>>
>> The code in question is
>>         /* Make sure we can write to __ro_after_init values during __init */
>>         ro_after_init |= 0xAA;
>>
>>
>> The problem is that s390 does not call mark_rodata_ro, instead sections are
>> marked read-only earlier. Maybe we should change that. Heiko, Martin?
>
> Since you've got rodata starting life as rodata, perhaps s390 could
> use a different section for __ro_after_init and only mark that section
> as RO during mark_rodata_ro()? It'd be nice to have that level of
> granularity, actually.
>
> I'll send an utterly untested patch... :P

virtme-defconfig --arch=s390x
make [arch that virtme-defconfig tells you]
virtme-run --kdir=. --arch=s390x

:)

--Andy

>
> -Kees
>
>>
>>>
>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> ---
>>>  arch/parisc/include/asm/cache.h   |  3 +++
>>>  include/asm-generic/vmlinux.lds.h |  1 +
>>>  include/linux/cache.h             | 14 ++++++++++++++
>>>  3 files changed, 18 insertions(+)
>>>
>>> diff --git a/arch/parisc/include/asm/cache.h b/arch/parisc/include/asm/cache.h
>>> index 3d0e17bcc8e9..df0f52bd18b4 100644
>>> --- a/arch/parisc/include/asm/cache.h
>>> +++ b/arch/parisc/include/asm/cache.h
>>> @@ -22,6 +22,9 @@
>>>
>>>  #define __read_mostly __attribute__((__section__(".data..read_mostly")))
>>>
>>> +/* Read-only memory is marked before mark_rodata_ro() is called. */
>>> +#define __ro_after_init      __read_mostly
>>> +
>>>  void parisc_cache_init(void);        /* initializes cache-flushing */
>>>  void disable_sr_hashing_asm(int); /* low level support for above */
>>>  void disable_sr_hashing(void);   /* turns off space register hashing */
>>> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>>> index e9a81a6a109f..8f5a12ab2f2b 100644
>>> --- a/include/asm-generic/vmlinux.lds.h
>>> +++ b/include/asm-generic/vmlinux.lds.h
>>> @@ -255,6 +255,7 @@
>>>       .rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {           \
>>>               VMLINUX_SYMBOL(__start_rodata) = .;                     \
>>>               *(.rodata) *(.rodata.*)                                 \
>>> +             *(.data..ro_after_init) /* Read only after init */      \
>>>               *(__vermagic)           /* Kernel version magic */      \
>>>               . = ALIGN(8);                                           \
>>>               VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .;         \
>>> diff --git a/include/linux/cache.h b/include/linux/cache.h
>>> index 17e7e82d2aa7..1be04f8c563a 100644
>>> --- a/include/linux/cache.h
>>> +++ b/include/linux/cache.h
>>> @@ -12,10 +12,24 @@
>>>  #define SMP_CACHE_BYTES L1_CACHE_BYTES
>>>  #endif
>>>
>>> +/*
>>> + * __read_mostly is used to keep rarely changing variables out of frequently
>>> + * updated cachelines. If an architecture doesn't support it, ignore the
>>> + * hint.
>>> + */
>>>  #ifndef __read_mostly
>>>  #define __read_mostly
>>>  #endif
>>>
>>> +/*
>>> + * __ro_after_init is used to mark things that are read-only after init (i.e.
>>> + * after mark_rodata_ro() has been called). These are effectively read-only,
>>> + * but may get written to during init, so can't live in .rodata (via "const").
>>> + */
>>> +#ifndef __ro_after_init
>>> +#define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
>>> +#endif
>>> +
>>>  #ifndef ____cacheline_aligned
>>>  #define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
>>>  #endif
>>>
>>
>
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security



-- 
Andy Lutomirski
AMA Capital Management, LLC