Re: RFC: Audit external function called indirectly via GOT

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Mar 19, 2018 at 1:21 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 03/17/2018 02:31 PM, H.J. Lu wrote:
>>
>> Auditing of external function calls and their return values relies on
>> lazy binding with PLT.  When external functions are called indirectly
>> via GOT without using PLT, auditing stops working.
>>
>> Here is a proposal to support auditing of external function called
>> indirectly via GOT:
>>
>> 1. Add optional dynamic tags:
>>
>>   #define DT_GNU_PLT     0x6ffffef4  /* Address of PLT section  */
>>   #define DT_GNU_PLTSZ   0x6ffffdf1  /* Size of PLT section  */
>>   #define DT_GNU_PLTENT  0x6ffffdf2  /* Size of one PLT entry  */
>>   #define DT_GNU_PLT0SZ  0x6ffffdf3  /* Size of the first PLT entry  */
>>   #define DT_GNU_PLTGOTSZ 0x6ffffdf4 /* Size of PLTGOT section  */
>>
>> and update DT_FLAGS_1 with:
>>
>>   #define DF_1_JMPRELIGN 0x10000000  /* DT_JMPREL can be ignored  */
>> 2. Linker creates PLT entries for auditing external function calls via
>> GOT and sets DT_GNU_PLT, DT_GNU_PLTSZ, DT_GNU_PLTENT, DT_GNU_PLT0SZ and
>> DT_GNU_PLTGOTSZ.  If PLT isn't required for lazy binding, set the
>> DF_1_JMPRELIGN bit in DT_FLAGS_1.
>
>
> Could we ship a template for the PLT entries in ld.so instead?  And if
> needed, map it from the file together with an address array, like this?

This won't work since linker needs to know exactly PLT layout to generate
JUMP_SLOT relocations for LD_AUDIT.

>   Data page with pointer
>   PLT template from ld.so (loading pointers from the previous page)
>
> This process can get be repeated, to obtain as many PLT stubs as needed.
> It's not a real JIT, so SELinux will still be happy.
>
> The data page would probably contain two pointers per PLT entry, not just
> one, so that the reserved PLT entries aren't necessary.
>
>> 3. When auditing is enabled at run-time, dynamic linker resolves GLOB_DAT
>> relocation to its corresponding PLT entry by finding JUMP_SLOT relocation
>> against the same function and use its PLT slot as the function address.
>
>
> This step would stay the same.
>
> I wonder if this would make it possible to restore audit support for
> existing binaries which lack PLT entries today.
>

I don't think so.


-- 
H.J.