[PATCH] fs: Preventing READ_IMPLIES_EXEC Propagation

[will.deacon@arm.com (Will Deacon)]

On Mon, Apr 24, 2017 at 04:58:41PM +0100, 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@arm.com> wrote:
> > > > > On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote:
> > > > >> From: Dong Bo <dongbo4@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).
> 
> 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).

I was thinking of the case where init has an executable stack, but that's
not the common case and such code would be broken under a different init
anyway, so I agree that we should make this change for arm64 (but not compat
without arch/arm/).

Will