[PATCH] fs: Preventing READ_IMPLIES_EXEC Propagation

[Will Deacon]

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 at arm.com> wrote:
> > > On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote:
> > >> From: Dong Bo <dongbo4 at huawei.com>
> > >>
> > >> 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).

Will