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

[Xiubo Li <xiubli@redhat.com>]

On 4/17/23 23:55, Luís Henriques wrote:
> 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...?

There already one old tracker to follow this 
https://tracker.ceph.com/issues/43272.

- Xiubo


> 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,