linux-kernel - Re: [RFC][PATCH] exec: Fix use after free of tracepoint trace_sched_process

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Message-ID: <20140205135203.GA16229@redhat.com>
Date:	Wed, 5 Feb 2014 14:52:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...isplace.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Smith <dsmith@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Igor Zhbanov <i.zhbanov@...sung.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC][PATCH] exec: Fix use after free of tracepoint
	trace_sched_process_exec

On 02/04, Linus Torvalds wrote:
>
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -239,7 +239,7 @@ static int ____call_usermodehelper(void *data)
>
>  	commit_creds(new);
>
> -	retval = do_execve(sub_info->path,
> +	retval = do_execve(getname_kernel(sub_info->path),
>  			   (const char __user *const __user *)sub_info->argv,
>  			   (const char __user *const __user *)sub_info->envp);

Great, this naturally duplicates filename unconditionally, and we can
kill bprm->tcomm[].

But,

> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -37,7 +37,7 @@ struct linux_binprm {
>  	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */
>  	unsigned int per_clear;	/* bits to clear in current->personality */
>  	int argc, envc;
> -	const char * filename;	/* Name of binary as seen by procps */
> +	struct filename *filename;	/* Name of binary as seen by procps */

Do we really need this change? If not (afaics), the patch can be
much simpler, see below...



> -void free_bprm(struct linux_binprm *bprm)
> +static void free_bprm(struct linux_binprm *bprm)
>  {
>  	free_arg_pages(bprm);
>  	if (bprm->cred) {
> @@ -1174,15 +1179,17 @@ void free_bprm(struct linux_binprm *bprm)
>  		fput(bprm->file);
>  	}
>  	/* If a binfmt changed the interp, free it. */
> -	if (bprm->interp != bprm->filename)
> +	if (bprm->interp != bprm->filename->name)
>  		kfree(bprm->interp);
> +	if (bprm->filename)
> +		putname(bprm->filename);

Even if we actually need to turn bprm->filename into "struct filename"
this free_bprm()->putname() only complicates the code, unless I missed
something. The caller, do_execve(), can do putname() unconditionally and
avoid if/NULL games.

IOW, doesn't the change below (on top of your patch) obviously makes
sense or I am totally confused?

Oleg.

--- x/fs/exec.c
+++ x/fs/exec.c
@@ -1183,8 +1183,6 @@ static void free_bprm(struct linux_binpr
 	/* If a binfmt changed the interp, free it. */
 	if (bprm->interp != bprm->filename->name)
 		kfree(bprm->interp);
-	if (bprm->filename)
-		putname(bprm->filename);
 	kfree(bprm);
 }
 
@@ -1478,9 +1476,6 @@ static int do_execve_common(struct filen
 	if (!bprm)
 		goto out_files;
 
-	bprm->filename = filename;
-	bprm->interp = filename->name;
-
 	retval = prepare_bprm_creds(bprm);
 	if (retval)
 		goto out_free;
@@ -1496,6 +1491,8 @@ static int do_execve_common(struct filen
 	sched_exec();
 
 	bprm->file = file;
+	bprm->filename = filename;
+	bprm->interp = filename->name;
 
 	retval = bprm_mm_init(bprm);
 	if (retval)
@@ -1538,7 +1535,7 @@ static int do_execve_common(struct filen
 	free_bprm(bprm);
 	if (displaced)
 		put_files_struct(displaced);
-	return retval;
+	goto out_ret;
 
 out:
 	if (bprm->mm) {
@@ -1552,14 +1549,12 @@ out_unmark:
 
 out_free:
 	free_bprm(bprm);
-	filename = NULL;
 
 out_files:
 	if (displaced)
 		reset_files_struct(displaced);
 out_ret:
-	if (filename)
-		putname(filename);
+	putname(filename);
 	return retval;
 }
 

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