linux-kernel - Re: [PATCH] fs: binfmt_elf_efpic: fix personality for fdpic ELF

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Message-ID: <202307110901.7E9A0D0AE5@keescook>
Date:   Tue, 11 Jul 2023 09:11:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Ungerer <gerg@...nel.org>
Cc:     linux-arm@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
        ebiederm@...ssion.com, brauner@...nel.org
Subject: Re: [PATCH] fs: binfmt_elf_efpic: fix personality for fdpic ELF

On Tue, Jul 11, 2023 at 11:39:55PM +1000, Greg Ungerer wrote:
> The elf-fdpic loader hard sets the process personality to either
> PER_LINUX_FDPIC for true elf-fdpic binaries or to PER_LINUX for
> normal ELF binaries (in this case they would be constant displacement
> compiled with -pie for example). The problem with that is that it
> will lose any other bits that may be in the ELF header personality
> (such as the "bug emulation" bits).
> 
> On the ARM architecture the ADDR_LIMIT_32BIT flag is used to signify
> a normal 32bit binary - as opposed to a legacy 26bit address binary.
> This matters since start_thread() will set the ARM CPSR register as
> required based on this flag. If the elf-fdpic loader loses this bit
> the process will be mis-configured and crash out pretty quickly.
> 
> Modify elf-fdpic loaders personality setting for ELF binaries so that
> it preserves the upper three bytes by using the SET_PERSONALITY macro
> to set it. This macro in the generic case sets PER_LINUX but and
> preserves the upper bytes. Architectures can override this for their
> specific use case, and ARM does exactly this.

Thanks for tracking this down!

There are some twisty macros in use across all the architectures here!

I notice the bare set_personality() call remains, though. Is that right?

For example, ARM (and sh and xtensa) also sets:

#define elf_check_fdpic(x) ((x)->e_ident[EI_OSABI] == ELFOSABI_ARM_FDPIC)

so it's possible the first half of the "if" below could get executed,
and ARM (and possibly other architectures) would again lose the other
flags, if I'm reading correctly.

(And the fact that PER_LINUX is actually 0x0 is oddly handled, leaving
it implicit in most architectures.)

What seems perhaps more correct is to remove the "if" entirely and make
sure that SET_PERSONALITY() checks the header flags on all architectures?

But I'm less familiar with this area, so please let me know what I'm
missing. :)

> 
> Signed-off-by: Greg Ungerer <gerg@...nel.org>
> ---
> 
> Is anyone out there using elf-fdpic on ARM?

It would seem you're the first? :) (_Should_ it be usable on ARM?)

-Kees

> This seems to break it rather badly due to the loss of that ADDR_LIMIT_32BIT
> bit from the process personality.
> 
>  fs/binfmt_elf_fdpic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index a05eafcacfb2..f29ae1d96fd7 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -348,7 +348,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
>  	if (elf_check_fdpic(&exec_params.hdr))
>  		set_personality(PER_LINUX_FDPIC);
>  	else
> -		set_personality(PER_LINUX);
> +		SET_PERSONALITY(exec_params.hdr);
>  	if (elf_read_implies_exec(&exec_params.hdr, executable_stack))
>  		current->personality |= READ_IMPLIES_EXEC;
>  
> -- 
> 2.25.1
> 

-- 
Kees Cook