Re: [PATCH v2] ceph: Fix use-after-free in __ceph_remove_cap

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2019-10-25 at 14:05 +0100, Luis Henriques wrote:
> KASAN reports a use-after-free when running xfstest generic/531, with the
> following trace:
> 
> [  293.903362]  kasan_report+0xe/0x20
> [  293.903365]  rb_erase+0x1f/0x790
> [  293.903370]  __ceph_remove_cap+0x201/0x370
> [  293.903375]  __ceph_remove_caps+0x4b/0x70
> [  293.903380]  ceph_evict_inode+0x4e/0x360
> [  293.903386]  evict+0x169/0x290
> [  293.903390]  __dentry_kill+0x16f/0x250
> [  293.903394]  dput+0x1c6/0x440
> [  293.903398]  __fput+0x184/0x330
> [  293.903404]  task_work_run+0xb9/0xe0
> [  293.903410]  exit_to_usermode_loop+0xd3/0xe0
> [  293.903413]  do_syscall_64+0x1a0/0x1c0
> [  293.903417]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> This happens because __ceph_remove_cap() may queue a cap release
> (__ceph_queue_cap_release) which can be scheduled before that cap is
> removed from the inode list with
> 
> 	rb_erase(&cap->ci_node, &ci->i_caps);
> 
> And, when this finally happens, the use-after-free will occur.
> 
> This can be fixed by removing the cap from the inode list before being
> removed from the session list, and thus eliminating the risk of an UAF.
> 
> Signed-off-by: Luis Henriques <lhenriques@suse.com>
> ---
> Hi!
> 
> So, after spending some time trying to find possible races throught code
> review and testing, I modified the fix according to Jeff's suggestion.
> 
> Cheers,
> Luis
> 
> fs/ceph/caps.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> index d3b9c9d5c1bd..a9ce858c37d0 100644
> --- a/fs/ceph/caps.c
> +++ b/fs/ceph/caps.c
> @@ -1058,6 +1058,11 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
>  
>  	dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
>  
> +	/* remove from inode list */
> +	rb_erase(&cap->ci_node, &ci->i_caps);
> +	if (ci->i_auth_cap == cap)
> +		ci->i_auth_cap = NULL;
> +
>  	/* remove from session list */
>  	spin_lock(&session->s_cap_lock);
>  	if (session->s_cap_iterator == cap) {
> @@ -1091,11 +1096,6 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
>  
>  	spin_unlock(&session->s_cap_lock);
>  
> -	/* remove from inode list */
> -	rb_erase(&cap->ci_node, &ci->i_caps);
> -	if (ci->i_auth_cap == cap)
> -		ci->i_auth_cap = NULL;
> -
>  	if (removed)
>  		ceph_put_cap(mdsc, cap);
>  

Looks good. Merged with a slight modification to the comment:

+       /* remove from inode's cap rbtree, and clear auth cap */

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>