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

[Christian Borntraeger]

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?

> 
> 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
>