On Sun, Jan 4, 2015 at 8:28 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
> Oded Gabbay <oded.gabbay@xxxxxxx> writes:
>> On 12/24/2014 01:01 AM, Rusty Russell wrote:
>>> Oded Gabbay <oded.gabbay@xxxxxxx> writes:
>>>> I didn't say it doesn't always work.
>>>> The actual thing that doesn't work is the define symbol_get and only in a
>>>> specific case of 32bit kernel AND CONFIG_MODULES is unset AND
>>>> CONFIG_RANDOMIZE_BASE is set.
>>>> The define in that case is:
>>>> #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
>>>>
>>>> Why it doesn't work (doesn't return NULL when symbol doesn't exists) ?
>>>
>>> Hmm, I'd guess CONFIG_RANDOMIZE_BASE is relocating NULL symbols...
>>>
>>> No, I can't reproduce this. Please send your .config privately.
>>>
>>> Here's my test case:
>>>
>>> diff --git a/init/main.c b/init/main.c
>>> index 61b993767db5..a3ee1ec97ec3 100644
>>> --- a/init/main.c
>>> +++ b/init/main.c
>>> @@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
>>>
>>> ftrace_init();
>>>
>>> + {
>>> + extern void nonexistent_fn(void);
>>> + printk("symbol_get(nonexistent_fn) = %p\n",
>>> + symbol_get(nonexistent_fn));
>>> + }
>>> +
>>> /* Do the rest non-__init'ed, we're now alive */
>>> rest_init();
>>> }
>>>
>>> Thanks,
>>> Rusty.
>>>
>> Hi Rusty,
>>
>> Attached is the bad config file. (config-bad)
>> I have narrowed the changes you need to do to the config file in order to
>> reproduce this bug.
>> The base assumption is a 32-bit kernel and without modules support. Rest of the
>> config file is pretty standard, IMO.
>> Then, its not enough to enable CONFIG_RANDOMIZE_BASE like I wrote in my original
>> post. You need also to unset CONFIG_HIBERNATION.
>
> Indeed, thanks; your config breaks as reported. With CONFIG_HIBERNATION
> the kernel offset is 0, so we don't see this.
>
> Kees, as far as I can tell you need another 0-terminated vmlinux.relocs
> section for weak symbols. These should not be relocated if already 0.
>
> Put this somewhere to test. It fails for x86_64, too:
>
> diff --git a/init/main.c b/init/main.c
> index 61b993767db5..c9e0195c792a 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -683,6 +683,12 @@ asmlinkage __visible void __init start_kernel(void)
>
> ftrace_init();
>
> + {
> + extern void __attribute__((weak)) nonexistent_fn(void);
> + printk("nonexistent_fn = %p\n", nonexistent_fn);
> + BUG_ON(nonexistent_fn != NULL);
> + }
> +
> /* Do the rest non-__init'ed, we're now alive */
> rest_init();
> }
Hm, I can't reproduce this on v3.19-rc4. My nonexistent_fn comes back
NULL regardless of CONFIG and kaslr on/off states I've tried. Could
this be a (yet another) linker bug? What was the toolchain used? I
built with gcc 4.8.2 and binutils 2.24.
-Kees
--
Kees Cook
Chrome OS Security
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