Re: [PATCH] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs

[Jann Horn]

+linux-api, +musl

On Tue, Apr 23, 2019 at 8:12 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> The READ_IMPLIES_EXEC work-around was designed for old CPUs lacking NX
> (to have the visible permission flags on memory regions reflect reality:
> they are all executable), and for old toolchains that lacked the ELF
> PT_GNU_STACK marking (under the assumption than toolchains that couldn't
> even specify memory protection flags may have it wrong for all memory
> regions).
>
> This logic is sensible, 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. This is not a reasonable assumption
> for CPUs that have had NX support from the start (or very close to
> the start). This confusion has led to situations where modern 64-bit
> 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].)
>
> To fix this, elf_read_implies_exec() is adjusted on arm64 (where NX has
> always existed and all toolchains include PT_GNU_STACK), and x86 is
> adjusted to handle this combination of possible outcomes:
>
>               CPU: | lacks NX  | has NX, ia32     | has NX, x86_64   |
>  ELF:              |           |                  |                  |
>  ------------------------------|------------------|------------------|
>  missing GNU_STACK | needs RIE | needs RIE        | no RIE           |
>  GNU_STACK == RWX  | needs RIE | no RIE: stack X  | no RIE: stack X  |
>  GNU_STACK == RW   | needs RIE | no RIE: stack NX | no RIE: stack NX |
>
> This has the effect of making binfmt_elf's EXSTACK_DEFAULT actually take
> on the correct architecture default of being non-executable on arm64 and
> x86_64, and being executable on ia32.

It's probably worth going a bit more into detail in this description
on how libraries typically allocate thread stacks.

It looks like glibc will be fine; before commit 54ee14b3882
(https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=nptl/allocatestack.c;h=dc501650b8629eda4502f2016016f09106cfb526;hp=6ada1fe1381de104153c0627e27f09fe5ad02caa;hb=54ee14b3882;hpb=16a76cd23ce9d3924fa192395e730423e3dc8b36),
thread stacks were always RWX, and since then, from what I can tell,
thread stacks were executable depending on the executable's ELF
headers (as parsed by glibc).

But e.g. musl's __pthread_create() seems to hardcode
PROT_READ|PROT_WRITE, which I think would mean that if someone built a
multithreaded program with nested functions and linked with musl, that
program would stop working? Or maybe I'm just reading the code wrong.

Then again, I'm not sure whether anyone actually uses nested functions...