linux-kernel - Re: [PATCH] Smack: fix use-after-free in smk_write_relabel

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Message-ID: <20200721003830.GC7464@sol.localdomain>
Date:   Mon, 20 Jul 2020 17:38:30 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-security-module@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>
Cc:     syzkaller-bugs@...glegroups.com, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        syzbot+e6416dabb497a650da40@...kaller.appspotmail.com
Subject: Re: [PATCH] Smack: fix use-after-free in smk_write_relabel_self()

On Wed, Jul 08, 2020 at 01:15:20PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> smk_write_relabel_self() frees memory from the task's credentials with
> no locking, which can easily cause a use-after-free because multiple
> tasks can share the same credentials structure.
> 
> Fix this by using prepare_creds() and commit_creds() to correctly modify
> the task's credentials.
> 
> Reproducer for "BUG: KASAN: use-after-free in smk_write_relabel_self":
> 
> 	#include <fcntl.h>
> 	#include <pthread.h>
> 	#include <unistd.h>
> 
> 	static void *thrproc(void *arg)
> 	{
> 		int fd = open("/sys/fs/smackfs/relabel-self", O_WRONLY);
> 		for (;;) write(fd, "foo", 3);
> 	}
> 
> 	int main()
> 	{
> 		pthread_t t;
> 		pthread_create(&t, NULL, thrproc, NULL);
> 		thrproc(NULL);
> 	}
> 
> Reported-by: syzbot+e6416dabb497a650da40@...kaller.appspotmail.com
> Fixes: 38416e53936e ("Smack: limited capability for changing process label")
> Cc: <stable@...r.kernel.org> # v4.4+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

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