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Subject[patch 1/1] selinux: fix sb_lock/sb_security_lock nesting (Was: Re: 2.6.17-rc5-mm1)
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Date
On Tue, 2006-05-30 at 21:04 +0200, Arjan van de Ven wrote:
> On Tue, 2006-05-30 at 20:39 +0200, Michal Piotrowski wrote:
> > Hi,
> >
> > On 30/05/06, Andrew Morton <akpm@osdl.org> wrote:
> > >
> > > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.17-rc5/2.6.17-rc5-mm1/
> > >
> >
> > I get this on 2.6.17-rc5-mm1 + hot fixes + Arjan's net/ipv4/igmp.c patch.
> >
> > May 30 20:25:56 ltg01-fedora kernel:
> > May 30 20:25:56 ltg01-fedora kernel:
> > =====================================================
> > May 30 20:25:56 ltg01-fedora kernel: [ BUG: possible circular locking
> > deadlock detected! ]
> > May 30 20:25:56 ltg01-fedora kernel:
> > -----------------------------------------------------
> > May 30 20:25:56 ltg01-fedora kernel: umount/2322 is trying to acquire lock:
> > May 30 20:25:56 ltg01-fedora kernel: (sb_security_lock){--..}, at:
> > [<c01d6400>] selinux_sb_free_security+0x17/0x4e
>
>
> ok so selinux_complete_init() does
> spin_lock(&sb_security_lock);
> next_sb:
> if (!list_empty(&superblock_security_head)) {
> struct superblock_security_struct *sbsec =
> list_entry(superblock_security_head.next,
> struct superblock_security_struct,
> list);
> struct super_block *sb = sbsec->sb;
> spin_lock(&sb_lock);
> sb->s_count++;
> spin_unlock(&sb_lock);
> spin_unlock(&sb_security_lock);
>
> nesting sb_lock inside sb_security_lock
>
> while
>
> put_super() takes the sb_lock, then calls __put_super() which calls
> selinux_sb_free_security which calls superblock_free_security() which takes sb_security_lock
> which means the nesting is opposite.
>
>
> textbook AB-BA deadlock

Yes, looks that way, although oddly I don't see this warning myself upon
performing a umount (w/ 2.6.17-rc5-mm1-lockdep). Patch below should
fix.

---

Fix unsafe nesting of sb_lock inside sb_security_lock in selinux_complete_init.
Detected by the kernel locking validator.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

---

security/selinux/hooks.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)

--- linux-2.6.17-rc5-mm1/security/selinux/hooks.c 2006-05-30 14:26:11.000000000 -0400
+++ linux-2.6.17-rc5-mm1-x/security/selinux/hooks.c 2006-05-31 07:29:23.000000000 -0400
@@ -4448,6 +4448,7 @@ void selinux_complete_init(void)

/* Set up any superblocks initialized prior to the policy load. */
printk(KERN_INFO "SELinux: Setting up existing superblocks.\n");
+ spin_lock(&sb_lock);
spin_lock(&sb_security_lock);
next_sb:
if (!list_empty(&superblock_security_head)) {
@@ -4456,19 +4457,20 @@ next_sb:
struct superblock_security_struct,
list);
struct super_block *sb = sbsec->sb;
- spin_lock(&sb_lock);
sb->s_count++;
- spin_unlock(&sb_lock);
spin_unlock(&sb_security_lock);
+ spin_unlock(&sb_lock);
down_read(&sb->s_umount);
if (sb->s_root)
superblock_doinit(sb, NULL);
drop_super(sb);
+ spin_lock(&sb_lock);
spin_lock(&sb_security_lock);
list_del_init(&sbsec->list);
goto next_sb;
}
spin_unlock(&sb_security_lock);
+ spin_unlock(&sb_lock);
}

/* SELinux requires early initialization in order to label
--
Stephen Smalley
National Security Agency

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