Re: [PATCH] fs: Preventing READ_IMPLIES_EXEC Propagation

[dongbo (E)]

On 2017/4/24 23:58, Catalin Marinas wrote:
> On Mon, Apr 24, 2017 at 04:40:23PM +0100, Will Deacon wrote:
>> On Wed, Apr 19, 2017 at 11:33:14AM +0100, Catalin Marinas wrote:
>>> On Tue, Apr 18, 2017 at 09:01:52PM +0100, Peter Maydell wrote:
>>>> On 18 April 2017 at 18:01, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>>>>> On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote:
>>>>>> From: Dong Bo <dongbo4@xxxxxxxxxx>
>>>>>>
>>>>>> In load_elf_binary(), once the READ_IMPLIES_EXEC flag is set,
>>>>>> the flag is propagated to its child processes, even the elf
>>>>>> files are marked as not requiring executable stack. It may
>>>>>> cause superfluous operations on some arch, e.g.
>>>>>> __sync_icache_dcache on aarch64 due to a PROT_READ mmap is
>>>>>> also marked as PROT_EXEC.
>>>>
>>>>> That's affecting most architectures with a risk of ABI breakage. We
>>>>> could do it on arm64 only, though I'm not yet clear on the ABI
>>>>> implications (at a first look, there shouldn't be any).
>>>>
>>>> Is there a reason why it isn't just straightforwardly a bug
>>>> (which we could fix) to make READ_IMPLIES_EXEC propagate to
>>>> child processes?
>>>
>>> While I agree that it looks like a bug, if there are user programs
>>> relying on such bug we call it "ABI". On arm64, I don't think there is
>>> anything relying on inheriting READ_IMPLIES_EXEC but I wouldn't change
>>> the compat task handling without the corresponding change in arch/arm.
>>>
>>>> AFAICT this should be per-process: just because
>>>> init happens not to have been (re)compiled to permit non-executable
>>>> stacks doesn't mean every process on the system needs to have
>>>> an executable stack.
>>>
>>> I think this also affects the heap if brk(2) is used (via
>>> VM_DATA_DEFAULT_FLAGS though I guess malloc mostly uses mmap these
>>> days).
>>
>> I think it also affects mprotect, which is more worrying imo, particularly
>> for things like JIT code that is ported from 32-bit (although a quick look
>> at v8, ionmonkey and art suggests they all pass PROT_EXEC when needed).
> 
> As Peter said, the default behaviour is READ_IMPLIES_EXEC off, so JIT
> code must already pass PROT_EXEC if it wants executable permission. The
> question is whether any user code relies on READ_IMPLIES_EXEC being
> passed down to child processes. I don't think so but I would be
> reluctant to make an such cross-arch change (happy to do it for arm64
> though).
> 

OK, I have re-built a patch for arm64 as you suggested. Thanks.

> Since linux-arch was cc'ed in the middle of this thread, I doubt people
> would reply. I suggest that the original patch is re-posted to
> linux-arch directly.
> 
Re-posted.

Bo Dong
.