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Date:   Tue, 7 Apr 2020 08:58:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: [PATCH 1/3] binfmt: Move install_exec_creds after setup_new_exec
 to match binfmt_elf
Message-ID: <202004070858.4644D9F529@keescook>
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On Mon, Apr 06, 2020 at 08:31:25PM -0500, Eric W. Biederman wrote:
> 
> In 2016 Linus moved install_exec_creds immediately after
> setup_new_exec, in binfmt_elf as a cleanup and as part of closing a
> potential information leak.
> 
> Perform the same cleanup for the other binary formats.
> 
> Different binary formats doing the same things the same way makes exec
> easier to reason about and easier to maintain.
> 
> Putting install_exec_creds immediate after setup_new_exec makes many
> simplifications possible in the code.
> 
> Ref: 9f834ec18def ("binfmt_elf: switch to new creds when switching to new mm")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  arch/x86/ia32/ia32_aout.c | 3 +--
>  fs/binfmt_aout.c          | 2 +-
>  fs/binfmt_elf_fdpic.c     | 2 +-
>  fs/binfmt_flat.c          | 3 +--
>  4 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/ia32/ia32_aout.c b/arch/x86/ia32/ia32_aout.c
> index 9bb71abd66bd..37b36a8ce5fa 100644
> --- a/arch/x86/ia32/ia32_aout.c
> +++ b/arch/x86/ia32/ia32_aout.c
> @@ -140,6 +140,7 @@ static int load_aout_binary(struct linux_binprm *bprm)
>  	set_personality_ia32(false);
>  
>  	setup_new_exec(bprm);
> +	install_exec_creds(bprm);
>  
>  	regs->cs = __USER32_CS;
>  	regs->r8 = regs->r9 = regs->r10 = regs->r11 = regs->r12 =
> @@ -156,8 +157,6 @@ static int load_aout_binary(struct linux_binprm *bprm)
>  	if (retval < 0)
>  		return retval;
>  
> -	install_exec_creds(bprm);
> -
>  	if (N_MAGIC(ex) == OMAGIC) {
>  		unsigned long text_addr, map_size;
>  
> diff --git a/fs/binfmt_aout.c b/fs/binfmt_aout.c
> index 8e8346a81723..ace587b66904 100644
> --- a/fs/binfmt_aout.c
> +++ b/fs/binfmt_aout.c
> @@ -162,6 +162,7 @@ static int load_aout_binary(struct linux_binprm * bprm)
>  	set_personality(PER_LINUX);
>  #endif
>  	setup_new_exec(bprm);
> +	install_exec_creds(bprm);
>  
>  	current->mm->end_code = ex.a_text +
>  		(current->mm->start_code = N_TXTADDR(ex));
> @@ -174,7 +175,6 @@ static int load_aout_binary(struct linux_binprm * bprm)
>  	if (retval < 0)
>  		return retval;
>  
> -	install_exec_creds(bprm);
>  
>  	if (N_MAGIC(ex) == OMAGIC) {
>  		unsigned long text_addr, map_size;
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index 240f66663543..6c94c6d53d97 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -353,6 +353,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
>  		current->personality |= READ_IMPLIES_EXEC;
>  
>  	setup_new_exec(bprm);
> +	install_exec_creds(bprm);
>  
>  	set_binfmt(&elf_fdpic_format);
>  
> @@ -434,7 +435,6 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
>  	current->mm->start_stack = current->mm->start_brk + stack_size;
>  #endif
>  
> -	install_exec_creds(bprm);
>  	if (create_elf_fdpic_tables(bprm, current->mm,
>  				    &exec_params, &interp_params) < 0)
>  		goto error;
> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
> index 831a2b25ba79..1a1d1fcb893f 100644
> --- a/fs/binfmt_flat.c
> +++ b/fs/binfmt_flat.c
> @@ -541,6 +541,7 @@ static int load_flat_file(struct linux_binprm *bprm,
>  		/* OK, This is the point of no return */
>  		set_personality(PER_LINUX_32BIT);
>  		setup_new_exec(bprm);
> +		install_exec_creds(bprm);
>  	}
>  
>  	/*
> @@ -963,8 +964,6 @@ static int load_flat_binary(struct linux_binprm *bprm)
>  		}
>  	}
>  
> -	install_exec_creds(bprm);
> -
>  	set_binfmt(&flat_format);
>  
>  #ifdef CONFIG_MMU
> -- 
> 2.25.0
> 

-- 
Kees Cook