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[PATCH v2 1/3] elf: Add GNU_PROPERTY_MEMORY_SEAL gnu property

Adhemerval Zanella Netto adhemerval.zanella@linaro.org
Wed Oct 9 13:47:44 GMT 2024 
On 03/10/24 11:00, Adhemerval Zanella Netto wrote:
> 
> 
> On 03/10/24 09:43, Adhemerval Zanella Netto wrote:
>>
>>
>> On 01/10/24 03:45, Jan Beulich wrote:
>>> On 30.09.2024 22:08, Adhemerval Zanella wrote:
>>>> Along with -Wl,memory-seal/-Wl,nomemory-seal options to ld.bfd.
>>>>
>>>> The new attribute indicates that an ET_EXEC or ET_DYN ELF object should
>>>> be memory-sealed if the loader supports it. Memory sealing is useful as
>>>> a hardening mechanism to avoid either remapping the memory segments or
>>>> changing the memory protection segments layout by the dynamic loader
>>>> (for instance, the RELRO hardening). The Linux 6.10
>>>> (8be7258aad44b5e25977a98db136f677fa6f4370) added the mseal syscall
>>>> accomplishes it.
>>>>
>>>> A GNU property is used instead of a new dynamic section tag (like the
>>>> one proposed for DT_GNU_FLAGS_1) because the memory sealing should be
>>>> selectable for ET_EXEC and not only for ET_DYN. It also fits new opt-in
>>>> security features like x86 CET or AArch64 BTI.
>>>
>>> What's wrong with DT_* in ET_EXEC type files? The first random executables
>>> I looked at (as-new and ld-new) have a fair set of DT_*, so I don't see
>>> why a note would need using instead. As per H.J.'s question, a note would
>>> also pose a possible issue (as to how to deal with it) when found in an
>>> object file.
>>
>> I had the impression that this was not really required for ET_EXEC, but it
>> seems I am wrong.  I will change to use a DT_FLAGS_1, similar to DF_GNU_1_UNIQUE.
>>
>>
> 
> In fact the main problem with DT_FLAGS_1 is it requires a dynamic section and
> it might be tricky to get this with for non-pie static binaries (since a lot
> of fields are mandatory).  We can just make it supported for PT_DYNAMIC, although
> with GNU attribute it is easier to avoid this constraint.


So I am more inclined to keep using the current strategy of a gnu property
to avoid the PT_DYNAMIC constraint (specially because some glibc ports 
still do not have static-pie support).  Do you see any problem with this
approach?
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