linux-hardening - Re: [PATCH v2 1/1] binfmt

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Message-ID: <20241204190429.0a18dd22@mordecai.tesarici.cz>
Date: Wed, 4 Dec 2024 19:04:29 +0100
From: Petr Tesařík <petr@...arici.cz>
To: jeffxu@...omium.org
Cc: akpm@...ux-foundation.org, keescook@...omium.org, jannh@...gle.com,
 sroettger@...gle.com, adhemerval.zanella@...aro.org, ojeda@...nel.org,
 adobriyan@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 jorgelo@...omium.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 1/1] binfmt_elf: mseal address zero

On Tue,  6 Aug 2024 21:49:27 +0000
jeffxu@...omium.org wrote:

> From: Jeff Xu <jeffxu@...omium.org>
> 
> Some legacy SVr4 apps might depend on page on address zero
> to be readable, however I can't find a reason that the page
> ever becomes writeable, so seal it.
> 
> If there is a compain, we can make this configurable.
> 
> Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> ---
>  fs/binfmt_elf.c    |  5 +++++
>  include/linux/mm.h | 10 ++++++++++
>  mm/mseal.c         |  2 +-
>  3 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 19fa49cd9907..f839fa228509 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1314,6 +1314,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		   emulate the SVr4 behavior. Sigh. */
>  		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
>  				MAP_FIXED | MAP_PRIVATE, 0);
> +
> +		retval = do_mseal(0, PAGE_SIZE, 0);
> +		if (retval)
> +			pr_warn("pid=%d, couldn't seal address 0, ret=%d.\n",
> +					task_pid_nr(current), retval);
>  	}
>  
>  	regs = current_pt_regs();
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c4b238a20b76..a178c15812eb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4201,4 +4201,14 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
>  
>  int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
>  
> +#ifdef CONFIG_64BIT

Strictly speaking, this should be

#if defined(CONFIG_64BIT) && defined(CONFIG_MMU)

But since we do not support any 64-bit architecture without MMU, I'm
just making this marginal note, so it can be found in ML archives if
needed.

Petr T