[llvm-dev] [lld][ELF] Add option to make .dynamic read only

[Petr Hosek via llvm-dev]

On Wed, May 17, 2017 at 1:44 PM Peter Collingbourne <peter at pcc.me.uk> wrote:

> On Wed, May 17, 2017 at 1:32 PM, Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Wed, May 17, 2017 at 1:11 PM, Petr Hosek <phosek at chromium.org> wrote:
>>
>>> The motivation is not only memory savings but also security:
>>> can-never-be-written is strictly better than RELRO in all cases. The
>>> biggest win is when .dynamic is the sole reason for having a writable
>>> segment at all. The distinction is fairly small for exploitability, but not
>>> negligible.
>>>
>>
>> I'm not even sure if it is strictly better to make .dynamic read-only by
>> default is better than relro. It seems almost the same to me. What are
>> attack scenario you can think of in which you can exploit only when
>> .dynamic sections are read-only from beginning?
>>
> Maybe Fuchsia OS does something special for executable that contain
>> read-only sections only?
>>
>
There's one use case we have which is the vDSO. Our kernel has to be able
to load this vDSO into every process, but we don't want to include full ELF
loader in our kernel, so we use the read-only layout for the vDSO. Combined
with page-aligned segments which LLD uses by default, the logic for loading
vDSO is trivial. We're considering other use cases as well.


>  I'd be interested in this as well, CFI relies on relro for its vtable
> protection to work. Is the issue just the window of opportunity between
> mapping the pages and protecting them?
>

Correct, it eliminates the short window between mapping the page and
protecting it. There's also the memory saving; it's only a single page per
dynamic library which isn't a lot, by if you multiply that by the number of
processes times the number of shared libraries used by these it can become
more significant.


> LLD already has several command-line options that are not supported by or
>>> are different from ld or gold, so this wouldn't be the first one (and
>>> probably not the last). If LLD supported per-OS configuration, we would
>>> make this default for Fuchsia and wouldn't need the new option, but since
>>> these don't exists, it's something we would handle through the Clang driver.
>>>
>>
>> Yes, we have bunch of options that are not supported by GNU linkers,
>> particularly for LTO. So adding new options is fine as long as doing it
>> makes sense. But the bar should be higher than implementing compatible
>> options.
>>
>
One of the design principles we're trying to follow is to make everything
read-only, unless it has be writable. The only reason for .dynamic to be
writable is DT_DEBUG which is something we never intend to support. FWIW in
Fuchsia all we need is a read-only .dynamic without emitting DT_DEBUG
altogether, but we wanted to make sure that this flag is also usable
elsewhere hence implementing DT_DEBUG_INDIRECT which is already supported
by musl as Jake pointed out.


> Having no host OS-specific default options in the linker is more like a
>> feature than a lack of a feature. We rely on compilers to give appropriate
>> options to us. That makes the linker's behavior more predictable, and it
>> also makes it very easy to support cross-linking.
>>
>
> Note that you could avoid needing to add a new flag by defining a new
> ELFOSABI for Fuchsia and making this behaviour conditional on it.
>

I'd rather have a flag and have our driver set it. ELFOSABI requires driver
support which means it becomes difficult to produce Fuchsia binaries with
compilers that don't have the right driver (e.g. we have some developers
using a combination of GCC with LLD and this would be an issue for them).
I'm also not sure how well is ELFOSABI supported with LTO?
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