linux-kernel - Re: [PATCH] exec: do not leave bprm->interp on stack

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Date:	Sat, 24 Nov 2012 08:12:36 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	ppandit@...hat.com
Cc:	keescook@...omium.org, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	josh@...htriplett.org, serge.hallyn@...onical.com,
	linux-fsdevel@...r.kernel.org, me@...fdog.net
Subject: Re: [PATCH] exec: do not leave bprm->interp on stack

P J P wrote:
> 
>    Hello Kees, all,
> 
> Please have a look at a *NEW* patch at the end of this mail. It seems to fix 
> both the issues, stack disclosure + undue recursions.
> 
> It uses modprobe "--first-time" option which returns an error code when trying 
> to load a module which is already present or unload one which is not present.

It might fix both "stack disclosure" + "undue recursions" issues, but it
introduces a regression "only one of concurrent requesters succeeds" which
ruins the value of automatic module loading feature.

> @@ -1423,7 +1423,14 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
>  				break; /* -ENOEXEC */
>  			if (try)
>  				break; /* -ENOEXEC */
> -			request_module("binfmt-%04x", *(unsigned short *)(&bprm->buf[2]));

What happens if more than one processes are requesting execve() of a program
which needs to call request_module("binfmt-%04x") to succeed, and more than two
of them concurrently reached at this line? Only one process will succeed. This
means that execve() of a program which needs to call request_module() will fail
if concurrently reached here.

> +            if (request_module("binfmt-%04x",
> +                                *(unsigned short *)(&bprm->buf[2])))
> +            {
> +                printk(KERN_WARNING
> +                        "request_module: failed to load: binfmt-%04x",
> +                         *(unsigned short *)(&bprm->buf[2]));
> +                break;
> +            }
>  		}
>  #else
>  		break;
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