Re: [PATCH] MIPS: Modernize READ_IMPLIES_EXEC

[Kees Cook <keescook@chromium.org>]

Friendly ping. :)

On Wed, Sep 01, 2021 at 12:42:08PM -0700, Kees Cook wrote:
> I'm doing some thread necromancy of
> https://lore.kernel.org/lkml/202007081624.82FA0CC1EA@keescook/
> 
> x86, arm64, and arm32 adjusted their READ_IMPLIES_EXEC logic to better
> align with the safer defaults and the interactions with other mappings,
> which I illustrated with this comment on x86:
> 
> /*
>  * An executable for which elf_read_implies_exec() returns TRUE will
>  * have the READ_IMPLIES_EXEC personality flag set automatically.
>  *
>  * The decision process for determining the results are:
>  *
>  *                 CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 |
>  * ELF:                 |            |                  |                |
>  * ---------------------|------------|------------------|----------------|
>  * missing PT_GNU_STACK | exec-all   | exec-all         | exec-none      |
>  * PT_GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     |
>  * PT_GNU_STACK == RW   | exec-none  | exec-none        | exec-none      |
>  *
>  *  exec-all  : all PROT_READ user mappings are executable, except when
>  *              backed by files on a noexec-filesystem.
>  *  exec-none : only PROT_EXEC user mappings are executable.
>  *  exec-stack: only the stack and PROT_EXEC user mappings are
>  *  executable.
>  *
>  *  *this column has no architectural effect: NX markings are ignored by
>  *   hardware, but may have behavioral effects when "wants X" collides with
>  *   "cannot be X" constraints in memory permission flags, as in
>  *   https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com
>  *
>  */
> 
> For MIPS, the "lacks NX" above is the "!cpu_has_rixi" check. On x86,
> we decided that the READ_IMPLIES_EXEC flag needed to reflect the
> expectations, not the architectural behavior due to bad interactions
> as noted above, as always returning "1" on non-NX hardware breaks
> some mappings.
> 
> The other part of the issue is "what does the MIPS toolchain do for
> PT_GNU_STACK?" The answer seems to be "by default, include PT_GNU_STACK,
> but mark it executable" (likely due to concerns over non-NX hardware):
> 
> $ mipsel-linux-gnu-gcc -o hello_world hello_world.c
> $ llvm-readelf -lW hellow_world | grep GNU_STACK
>   GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10
> 
> Given that older hardware doesn't support non-executable memory, it
> seems safe to make the "PT_GNU_STACK is absent" logic mean "assume
> non-executable", but this might break very old software running on
> modern MIPS. This situation matches the ia32-on-x86_64 logic x86
> uses (which assumes needing READ_IMPLIES_EXEC in that situation). But
> modern toolchains on modern MIPS hardware should follow a safer default
> (assume NX stack).
> 
> A follow-up to this change would be to switch the MIPS toolchain to emit
> a non-executable PT_GNU_STACK, as this seems to be unneeded.
> 
> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
> Cc: Xuefeng Li <lixuefeng@loongson.cn>
> Cc: Juxin Gao <gaojuxin@loongson.cn>
> Cc: linux-mips@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/mips/kernel/elf.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
> index 7b045d2a0b51..5582a4ca1e9e 100644
> --- a/arch/mips/kernel/elf.c
> +++ b/arch/mips/kernel/elf.c
> @@ -328,16 +328,10 @@ void mips_set_personality_nan(struct arch_elf_state *state)
>  
>  int mips_elf_read_implies_exec(void *elf_ex, int exstack)
>  {
> -	if (exstack != EXSTACK_DISABLE_X) {
> -		/* The binary doesn't request a non-executable stack */
> -		return 1;
> -	}
> -
> -	if (!cpu_has_rixi) {
> -		/* The CPU doesn't support non-executable memory */
> -		return 1;
> -	}
> -
> -	return 0;
> +	/*
> +	 * Set READ_IMPLIES_EXEC only on non-NX systems that
> +	 * do not request a specific state via PT_GNU_STACK.
> +	 */
> +	return (!cpu_has_rixi && exstack == EXSTACK_DEFAULT);
>  }
>  EXPORT_SYMBOL(mips_elf_read_implies_exec);
> -- 
> 2.30.2
> 

-- 
Kees Cook