[PATCH] fs: Preventing READ_IMPLIES_EXEC Propagation

[Catalin Marinas]

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 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).

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).

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.

-- 
Catalin