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@xxxxxxxxxxxx > [2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=54ee14b3882 > [3] https://lkml.kernel.org/r/20190423192534.GN23599@xxxxxxxxxxxxxxxxxxxxx > > Suggested-by: Hector Marco-Gisbert <hecmargi@xxxxxx> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
