null-ptr-deref in integrity_inode_free()

[Mimi Zohar]

Hi Christian,

On Wed, 2021-04-14 at 13:50 +0200, Christian Brauner wrote:
> Hey,
> 
> [Resending since the previous mail somehow hasn't made it onto the list.]
> 
> While working on a patch to port ecryptfs to use clone_private_mount() I
> hit the splat in [1] with v5.12-rc6 (Without any of my changes.). To
> reproduce this you can use the config in [5]. Then run the scripts [2]
> and [3] in two terminals. The kernel command line was [4]:
> 
> while true; do ./test.sh; done
> while true; do ./test2.sh; done
> 
> and wait for [1] to appear.
> 
> The two test scripts aren't specifically designed to trigger this issue.
> They were stress tests for my ecryptfs clone_private_mount() port. They
> just allow to trigger this issue.
> 
> From a very uninformed position it seemed that:
> 
> void integrity_inode_free(struct inode *inode)
> {
>         struct integrity_iint_cache *iint;
> 
>         if (!IS_IMA(inode))
>                 return;

Thank you for all the details.

A builtin IMA policy wasn't specified on the boot command line.  Unless
a custom IMA policy is loaded, it shouldn't get beyond this
point.   Before looking any farther, I would appreciate your confirming
that you've loaded a custom IMA policy.  To see if a policy has been
loaded, cat /sys/kernel/security/ima/policy.

thanks,

Mimi

> 
>         write_lock(&integrity_iint_lock);
>         iint = __integrity_iint_find(inode);
>         ^^^^^
> >>>>>>> is somehow NULL?
> 
>         rb_erase(&iint->rb_node, &integrity_iint_tree);
>         write_unlock(&integrity_iint_lock);
> 
>         iint_free(iint);
> }
> 
> So I've placed a BUG_ON() in there:
> 
> diff --git a/security/integrity/iint.c b/security/integrity/iint.c
> index 0ba01847e836..d5152b5ef517 100644
> --- a/security/integrity/iint.c
> +++ b/security/integrity/iint.c
> @@ -152,6 +152,7 @@ void integrity_inode_free(struct inode *inode)
> 
>         write_lock(&integrity_iint_lock);
>         iint = __integrity_iint_find(inode);
> +       BUG_ON(!iint);
>         rb_erase(&iint->rb_node, &integrity_iint_tree);
>         write_unlock(&integrity_iint_lock);
> 
> and indeed, I get:
> 
> [  266.308227] ------------[ cut here ]------------
> [  266.308592] kernel BUG at security/integrity/iint.c:155!
> [  266.312120] invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> [  266.313898] CPU: 0 PID: 5867 Comm: rm Tainted: G            E     5.12.0-rc6-clone-private-mount-8a97af3f33b5 #380
> [  266.317651] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS 0.0.0 02/06/2015