Re: [PATCH 2/2] ARM: mm: keep rodata non-executable

[Laura Abbott]

On 2/17/2014 4:34 AM, Dave Martin wrote:
> On Fri, Feb 14, 2014 at 11:11:07AM -0800, Kees Cook wrote:
>> On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin <Dave.Martin@xxxxxxx> wrote:
>>> On Thu, Feb 13, 2014 at 05:04:10PM -0800, Kees Cook wrote:
>>>> Introduce "CONFIG_DEBUG_RODATA" to mostly match the x86 config, though
>>>> the behavior is different: it depends on STRICT_KERNMEM_PERMS, which
>>>> sets rodata read-only (but executable), where as this option additionally
>>>> splits rodata from the kernel text (resulting in potentially more memory
>>>> lost to padding) and sets it non-executable as well. The end result is
>>>> that on builds with CONFIG_DEBUG_RODATA=y (like x86) the rodata with be
>>>> marked purely read-only.
>>>
>>> This triggers an Oops in kexec, because we have a block of code in .text
>>> which is a template for generating baremetal code to relocate the new
>>> kernel, and some literal words are written into it before copying.
>>
>> You're writing into the text area? I would imagine that
>> CONFIG_ARM_KERNMEM_PERMS would break that. However, that's not the
>> right place to be building code -- shouldn't the module area be used
>> for that?
>>
>>> Possibly this should be in .rodata, not .text.
>>
>> Well, rodata should be neither writable nor executable.
> 
> We're not writing into code exactly.
> 
> This code is never executed in-place in vmlinux.  It gets copied, and
> only copies are ever executed.
> 
> Some pointers and offsets get poked into the code to configure it.
> 
> I think it would be better simply to put the code in .rodata, and
> poke paramaters into the copy, not the original -- but that's a bit
> more awkward to code up, since the values can't be poked simply by
> writing global variables.
> 
>>
>>> There may be a few other instances of this kind of thing.
>>
>> This config will certainly find them! :) But, that's why it's behind a config.
> 
> I haven't tested exhaustively, but it think this is sufficient for a
> Tested-by.  The patch does seem to be doing what it is intended to
> do, and doesn't seem to be triggering false positives all over the
> place.
> 
>>
>>> Are you aware of similar situations on other arches?
>>
>> I think there were some problems a long time ago on x86 for rodata too.
> 
> It would be good to get this kexec case fixed -- I'll try to hack up
> a separate patch.
>

FWIW, we've hit issues not just with kexec but kprobes as well. The same
problems exist with this series:

/ # echo p:nl 0xc01d5c00 >> /sys/kernel/debug/tracing/kprobe_events
/ # echo 1 > /sys/kernel/debug/tracing/events/kprobes/nl/enable
[ 1639.739629] Unable to handle kernel paging request at virtual address c01d5c00
[ 1639.739655] pgd = edbc4000
[ 1639.745730] [c01d5c00] *pgd=0001141e(bad)
[ 1639.752413] Internal error: Oops: 80d [#1] PREEMPT SMP ARM
[ 1639.752503] Modules linked in:
[ 1639.760920] CPU: 0 PID: 58 Comm: sh Not tainted 3.14.0-rc7-next-20140318-00004-ga0191b7-dirty #170
[ 1639.761015] task: edb90d80 ti: ed018000 task.ti: ed018000
[ 1639.769870] PC is at patch_text+0x4/0x10
[ 1639.775333] LR is at arm_kprobe+0x28/0x38
[ 1639.779327] pc : [<c058acec>]    lr : [<c058bcc4>]    psr: 20000013
[ 1639.779327] sp : ed019f10  ip : a0000013  fp : 01e7fb34
[ 1639.783241] r10: 00000000  r9 : 01e80ab8  r8 : 00000002
[ 1639.794517] r7 : ed019f80  r6 : ed900bc4  r5 : edb19fa0  r4 : edb19f08
[ 1639.799727] r3 : c01d5c00  r2 : ed019f08  r1 : e7f001f8  r0 : c01d5c00
[ 1639.806326] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[ 1639.812837] Control: 10c5787d  Table: 2dbc406a  DAC: 00000015
[ 1639.820040] Process sh (pid: 58, stack limit = 0xed018240)
[ 1639.825768] Stack: (0xed019f10 to 0xed01a000)
[ 1639.831150] 9f00:                                     00000000 c058bd5c ed900ba0 c018b724
[ 1639.835584] 9f20: ed6fab00 00000000 edb2d240 00000002 ed019f80 c018bdc4 ffffffff 00000001
[ 1639.843743] 9f40: edb2d240 01e80ab8 ed019f80 00000002 00000002 c01e1b1c 01e7fb34 c011c3c0
[ 1639.851902] 9f60: 00000003 00000000 00000000 edb2d240 edb2d240 00000002 01e80ab8 c01e2108
[ 1639.860063] 9f80: 00000000 00000000 00200200 00157ecc 00000001 01e80ab8 00000004 c0106e64
[ 1639.868222] 9fa0: ed018000 c0106ce0 00157ecc 00000001 00000001 01e80ab8 00000002 00000000
[ 1639.876381] 9fc0: 00157ecc 00000001 01e80ab8 00000004 00000020 01e7fb48 01e7fb14 01e7fb34
[ 1639.884542] 9fe0: 00000000 bef4562c 0001ee5d 0000a8cc 60000010 00000001 00000000 00000000
[ 1639.892709] [<c058acec>] (patch_text) from [<c058bcc4>] (arm_kprobe+0x28/0x38)
[ 1639.900862] [<c058bcc4>] (arm_kprobe) from [<c058bd5c>] (enable_kprobe+0x88/0x94)
[ 1639.907983] [<c058bd5c>] (enable_kprobe) from [<c018b724>] (__ftrace_event_enable_disable+0x13c/0x200)
[ 1639.915537] [<c018b724>] (__ftrace_event_enable_disable) from [<c018bdc4>] (event_enable_write+0x78/0xd4)
[ 1639.924741] [<c018bdc4>] (event_enable_write) from [<c01e1b1c>] (vfs_write+0xac/0x188)
[ 1639.934372] [<c01e1b1c>] (vfs_write) from [<c01e2108>] (SyS_write+0x40/0x94)
[ 1639.942187] [<c01e2108>] (SyS_write) from [<c0106ce0>] (ret_fast_syscall+0x0/0x30)
[ 1639.949386] Code: e4831004 e1a01003 eaee298d e1a03000 (e4831004)
[ 1639.956766] ---[ end trace b548269e2c7a3190 ]---


We had some functions that allowed the text to be temporarily made writable but something
uniform for kexec would be useful as well (our kexec solution has been 'turn it off')


> Cheers
> ---Dave
> 

Thanks,
Laura
 
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