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Date: Thu, 21 Mar 2024 10:05:02 -0700
From: Kees Cook <keescook@chromium.org>
To: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org,
	Eric Biederman <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Rich Felker <dalias@libc.org>, stable@vger.kernel.org
Subject: Re: [PATCH] exec: fix linux_binprm::exec in transfer_args_to_stack()
Message-ID: <202403211004.19F5EE27F@keescook>
References: <20240320182607.1472887-1-jcmvbkbc@gmail.com>
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In-Reply-To: <20240320182607.1472887-1-jcmvbkbc@gmail.com>

On Wed, Mar 20, 2024 at 11:26:07AM -0700, Max Filippov wrote:
> In NUMMU kernel the value of linux_binprm::p is the offset inside the
> temporary program arguments array maintained in separate pages in the
> linux_binprm::page. linux_binprm::exec being a copy of linux_binprm::p
> thus must be adjusted when that array is copied to the user stack.
> Without that adjustment the value passed by the NOMMU kernel to the ELF
> program in the AT_EXECFN entry of the aux array doesn't make any sense
> and it may break programs that try to access memory pointed to by that
> entry.
> 
> Adjust linux_binprm::exec before the successful return from the
> transfer_args_to_stack().

What's the best way to test this? (Is there a qemu setup I can use to
see the before/after of AT_EXECFN?)

How did you encounter the problem?

> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
> ---
>  fs/exec.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index af4fbb61cd53..5ee2545c3e18 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -895,6 +895,7 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
>  			goto out;
>  	}
>  
> +	bprm->exec += *sp_location - MAX_ARG_PAGES * PAGE_SIZE;
>  	*sp_location = sp;
>  
>  out:
> -- 
> 2.39.2
> 

-- 
Kees Cook