linux-kernel - Re: [PATCH] fs: correctly check for errors from replace_fd() in receive_fd

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Message-ID: <20250804-fechten-glukose-1cb2e2b0413a@brauner>
Date: Mon, 4 Aug 2025 10:38:35 +0200
From: Christian Brauner <brauner@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	Sargun Dhillon <sargun@...gun.me>, Kees Cook <kees@...nel.org>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] fs: correctly check for errors from replace_fd() in
 receive_fd_replace()

On Fri, Aug 01, 2025 at 09:38:38AM +0200, Thomas Weißschuh wrote:
> replace_fd() returns either a negative error number or the number of the
> new file descriptor. The current code misinterprets any positive file
> descriptor number as an error.
> 
> Only check for negative error numbers, so that __receive_sock() is called
> correctly for valid file descriptors.
> 
> Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
> Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
> Cc: stable@...r.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> ---
> Untested, it stuck out while reading the code.
> ---
>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 6d2275c3be9c6967d16c75d1b6521f9b58980926..56c3a045121d8f43a54cf05e6ce1962f896339ac 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1387,7 +1387,7 @@ int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
>  	if (error)
>  		return error;
>  	error = replace_fd(new_fd, file, o_flags);
> -	if (error)
> +	if (error < 0)
>  		return error;

What in the holy fsck? Why did the seccomp selftests not fail
horrendously explode because of that.