Re: [PATCH v2] ceph: fix potential use-after-free bug when trimming caps

["Luís Henriques" <lhenriques@suse.de>]

xiubli@redhat.com writes:

> From: Xiubo Li <xiubli@redhat.com>
>
> When trimming the caps and just after the 'session->s_cap_lock' is
> released in ceph_iterate_session_caps() the cap maybe removed by
> another thread, and when using the stale cap memory in the callbacks
> it will trigger use-after-free crash.
>
> We need to check the existence of the cap just after the 'ci->i_ceph_lock'
> being acquired. And do nothing if it's already removed.
>
> Cc: stable@vger.kernel.org
> URL: https://bugzilla.redhat.com/show_bug.cgi?id=2186264

I didn't had time to look closer at what this patch is fixing but the
above URL requires a account to access it.  So I guess it should be
dropped or replaced by another one from the tracker...?

Also, just skimming through the patch, there are at least 2 obvious issues
with it.  See below.

> Signed-off-by: Xiubo Li <xiubli@redhat.com>
> ---
>
> V2:
> - Fix this in ceph_iterate_session_caps instead.
>
>
>  fs/ceph/debugfs.c    |  7 +++++-
>  fs/ceph/mds_client.c | 56 ++++++++++++++++++++++++++++++--------------
>  fs/ceph/mds_client.h |  2 +-
>  3 files changed, 46 insertions(+), 19 deletions(-)
>
> diff --git a/fs/ceph/debugfs.c b/fs/ceph/debugfs.c
> index bec3c4549c07..5c0f07df5b02 100644
> --- a/fs/ceph/debugfs.c
> +++ b/fs/ceph/debugfs.c
> @@ -248,14 +248,19 @@ static int metrics_caps_show(struct seq_file *s, void *p)
>  	return 0;
>  }
>  
> -static int caps_show_cb(struct inode *inode, struct ceph_cap *cap, void *p)
> +static int caps_show_cb(struct inode *inode, struct rb_node *ci_node, void *p)
>  {
> +	struct ceph_inode_info *ci = ceph_inode(inode);
>  	struct seq_file *s = p;
> +	struct ceph_cap *cap;
>  
> +	spin_lock(&ci->i_ceph_lock);
> +	cap = rb_entry(ci_node, struct ceph_cap, ci_node);
>  	seq_printf(s, "0x%-17llx%-3d%-17s%-17s\n", ceph_ino(inode),
>  		   cap->session->s_mds,
>  		   ceph_cap_string(cap->issued),
>  		   ceph_cap_string(cap->implemented));
> +	spin_unlock(&ci->i_ceph_lock);
>  	return 0;
>  }
>  
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index 294af79c25c9..7fcfbddd534d 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -1786,7 +1786,7 @@ static void cleanup_session_requests(struct ceph_mds_client *mdsc,
>   * Caller must hold session s_mutex.
>   */
>  int ceph_iterate_session_caps(struct ceph_mds_session *session,
> -			      int (*cb)(struct inode *, struct ceph_cap *,
> +			      int (*cb)(struct inode *, struct rb_node *ci_node,
>  					void *), void *arg)
>  {
>  	struct list_head *p;
> @@ -1799,6 +1799,8 @@ int ceph_iterate_session_caps(struct ceph_mds_session *session,
>  	spin_lock(&session->s_cap_lock);
>  	p = session->s_caps.next;
>  	while (p != &session->s_caps) {
> +		struct rb_node *ci_node;
> +
>  		cap = list_entry(p, struct ceph_cap, session_caps);
>  		inode = igrab(&cap->ci->netfs.inode);
>  		if (!inode) {
> @@ -1806,6 +1808,7 @@ int ceph_iterate_session_caps(struct ceph_mds_session *session,
>  			continue;
>  		}
>  		session->s_cap_iterator = cap;
> +		ci_node = &cap->ci_node;
>  		spin_unlock(&session->s_cap_lock);
>  
>  		if (last_inode) {
> @@ -1817,7 +1820,7 @@ int ceph_iterate_session_caps(struct ceph_mds_session *session,
>  			old_cap = NULL;
>  		}
>  
> -		ret = cb(inode, cap, arg);
> +		ret = cb(inode, ci_node, arg);
>  		last_inode = inode;
>  
>  		spin_lock(&session->s_cap_lock);
> @@ -1850,17 +1853,22 @@ int ceph_iterate_session_caps(struct ceph_mds_session *session,
>  	return ret;
>  }
>  
> -static int remove_session_caps_cb(struct inode *inode, struct ceph_cap *cap,
> +static int remove_session_caps_cb(struct inode *inode, struct rb_node *ci_node,
>  				  void *arg)
>  {
>  	struct ceph_inode_info *ci = ceph_inode(inode);
>  	bool invalidate = false;
> +	struct ceph_cap *cap;
>  	int iputs;
>  
> -	dout("removing cap %p, ci is %p, inode is %p\n",
> -	     cap, ci, &ci->netfs.inode);
>  	spin_lock(&ci->i_ceph_lock);
> -	iputs = ceph_purge_inode_cap(inode, cap, &invalidate);

This will leave iputs uninitialized if the statement below returns NULL.
Which will cause issues later in the function.

> +	cap = rb_entry(ci_node, struct ceph_cap, ci_node);
> +	if (cap) {
> +		dout(" removing cap %p, ci is %p, inode is %p\n",
> +		     cap, ci, &ci->netfs.inode);
> +
> +		iputs = ceph_purge_inode_cap(inode, cap, &invalidate);
> +	}
>  	spin_unlock(&ci->i_ceph_lock);
>  
>  	wake_up_all(&ci->i_cap_wq);
> @@ -1934,11 +1942,11 @@ enum {
>   *
>   * caller must hold s_mutex.
>   */
> -static int wake_up_session_cb(struct inode *inode, struct ceph_cap *cap,
> -			      void *arg)
> +static int wake_up_session_cb(struct inode *inode, struct rb_node *ci_node, void *arg)
>  {
>  	struct ceph_inode_info *ci = ceph_inode(inode);
>  	unsigned long ev = (unsigned long)arg;
> +	struct ceph_cap *cap;
>  
>  	if (ev == RECONNECT) {
>  		spin_lock(&ci->i_ceph_lock);
> @@ -1949,7 +1957,9 @@ static int wake_up_session_cb(struct inode *inode, struct ceph_cap *cap,
>  		if (cap->cap_gen < atomic_read(&cap->session->s_cap_gen)) {

Since we're replacing the 'cap' argument by the 'ci_node', the
above statement will have garbage in 'cap'.

Cheers,
-- 
Luís

>  			/* mds did not re-issue stale cap */
>  			spin_lock(&ci->i_ceph_lock);
> -			cap->issued = cap->implemented = CEPH_CAP_PIN;
> +			cap = rb_entry(ci_node, struct ceph_cap, ci_node);
> +			if (cap)
> +				cap->issued = cap->implemented = CEPH_CAP_PIN;
>  			spin_unlock(&ci->i_ceph_lock);
>  		}
>  	} else if (ev == FORCE_RO) {
> @@ -2113,16 +2123,22 @@ static bool drop_negative_children(struct dentry *dentry)
>   * Yes, this is a bit sloppy.  Our only real goal here is to respond to
>   * memory pressure from the MDS, though, so it needn't be perfect.
>   */
> -static int trim_caps_cb(struct inode *inode, struct ceph_cap *cap, void *arg)
> +static int trim_caps_cb(struct inode *inode, struct rb_node *ci_node, void *arg)
>  {
>  	int *remaining = arg;
>  	struct ceph_inode_info *ci = ceph_inode(inode);
>  	int used, wanted, oissued, mine;
> +	struct ceph_cap *cap;
>  
>  	if (*remaining <= 0)
>  		return -1;
>  
>  	spin_lock(&ci->i_ceph_lock);
> +	cap = rb_entry(ci_node, struct ceph_cap, ci_node);
> +	if (!cap) {
> +		spin_unlock(&ci->i_ceph_lock);
> +		return 0;
> +	}
>  	mine = cap->issued | cap->implemented;
>  	used = __ceph_caps_used(ci);
>  	wanted = __ceph_caps_file_wanted(ci);
> @@ -4265,26 +4281,23 @@ static struct dentry* d_find_primary(struct inode *inode)
>  /*
>   * Encode information about a cap for a reconnect with the MDS.
>   */
> -static int reconnect_caps_cb(struct inode *inode, struct ceph_cap *cap,
> +static int reconnect_caps_cb(struct inode *inode, struct rb_node *ci_node,
>  			  void *arg)
>  {
>  	union {
>  		struct ceph_mds_cap_reconnect v2;
>  		struct ceph_mds_cap_reconnect_v1 v1;
>  	} rec;
> -	struct ceph_inode_info *ci = cap->ci;
> +	struct ceph_inode_info *ci = ceph_inode(inode);
>  	struct ceph_reconnect_state *recon_state = arg;
>  	struct ceph_pagelist *pagelist = recon_state->pagelist;
>  	struct dentry *dentry;
> +	struct ceph_cap *cap;
>  	char *path;
> -	int pathlen = 0, err;
> +	int pathlen = 0, err = 0;
>  	u64 pathbase;
>  	u64 snap_follows;
>  
> -	dout(" adding %p ino %llx.%llx cap %p %lld %s\n",
> -	     inode, ceph_vinop(inode), cap, cap->cap_id,
> -	     ceph_cap_string(cap->issued));
> -
>  	dentry = d_find_primary(inode);
>  	if (dentry) {
>  		/* set pathbase to parent dir when msg_version >= 2 */
> @@ -4301,6 +4314,15 @@ static int reconnect_caps_cb(struct inode *inode, struct ceph_cap *cap,
>  	}
>  
>  	spin_lock(&ci->i_ceph_lock);
> +	cap = rb_entry(ci_node, struct ceph_cap, ci_node);
> +	if (!cap) {
> +		spin_lock(&ci->i_ceph_lock);
> +		goto out_err;
> +	}
> +	dout(" adding %p ino %llx.%llx cap %p %lld %s\n",
> +	     inode, ceph_vinop(inode), cap, cap->cap_id,
> +	     ceph_cap_string(cap->issued));
> +
>  	cap->seq = 0;        /* reset cap seq */
>  	cap->issue_seq = 0;  /* and issue_seq */
>  	cap->mseq = 0;       /* and migrate_seq */
> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
> index 0f70ca3cdb21..001b69d04307 100644
> --- a/fs/ceph/mds_client.h
> +++ b/fs/ceph/mds_client.h
> @@ -569,7 +569,7 @@ extern void ceph_queue_cap_reclaim_work(struct ceph_mds_client *mdsc);
>  extern void ceph_reclaim_caps_nr(struct ceph_mds_client *mdsc, int nr);
>  extern int ceph_iterate_session_caps(struct ceph_mds_session *session,
>  				     int (*cb)(struct inode *,
> -					       struct ceph_cap *, void *),
> +					       struct rb_node *ci_node, void *),
>  				     void *arg);
>  extern void ceph_mdsc_pre_umount(struct ceph_mds_client *mdsc);
>  
> -- 
>
> 2.39.2
>