Re: [PATCH v3 5/7] arm32/64, elf: Split READ_IMPLIES_EXEC from executable GNU_STACK

Catalin Marinas
2020-02-12T09:28:03Z

On Mon, Feb 10, 2020 at 11:30:47AM -0800, Kees Cook wrote:
> The READ_IMPLIES_EXEC work-around was designed for old toolchains that
> lacked the ELF PT_GNU_STACK marking under the assumption that toolchains
> that couldn't specify executable permission flags for the stack may not
> know how to do it correctly for any memory region.
>
> This logic is sensible for having ancient binaries coexist in a system
> with possibly NX memory, but was implemented in a way that equated having
> a PT_GNU_STACK marked executable as being as "broken" as lacking the
> PT_GNU_STACK marking entirely. Things like unmarked assembly and stack
> trampolines may cause PT_GNU_STACK to need an executable bit, but they
> do not imply all mappings must be executable.
>
> This confusion has led to situations where modern programs with explicitly
> marked executable stack are forced into the READ_IMPLIES_EXEC state when
> no such thing is needed. (And leads to unexpected failures when mmap()ing
> regions of device driver memory that wish to disallow VM_EXEC[1].)
>
> In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann
> Horn noted that glibc thread stacks have always been marked RWX (until
> 2003 when they started tracking the PT_GNU_STACK flag instead[2]). And
> musl doesn't support executable stacks at all[3]. As such, no breakage
> for multithreaded applications is expected from this change.
>
> This changes arm32 and arm64 compat together, to keep behavior the same.
>
> [1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com
> [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882
> [3] https://lkml.kernel.org/r/20190423192534.GN23599@brightrain.aerifal.cx
>
> Suggested-by: Hector Marco-Gisbert <hecmargi@upv.es>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>