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SubjectRe: [V9fs-developer] [PATCH 2/2] 9p: Add refcount to p9_req_t
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Hi Tomas & Dominique,

Could you help paste the reason of the crash bug to help others understand
more clearly? And I have another question below.

On 2018/8/11 22:42, Tomas Bortoli wrote:
> To avoid use-after-free(s), use a refcount to keep track of the
> usable references to any instantiated struct p9_req_t.
>
> This commit adds p9_req_put(), p9_req_get() and p9_req_try_get() as
> wrappers to kref_put(), kref_get() and kref_get_unless_zero().
> These are used by the client and the transports to keep track of
> valid requests' references.
>
> p9_free_req() is added back and used as callback by kref_put().
>
> Add SLAB_TYPESAFE_BY_RCU as it ensures that the memory freed by
> kmem_cache_free() will not be reused for another type until the rcu
> synchronisation period is over, so an address gotten under rcu read
> lock is safe to inc_ref() without corrupting random memory while
> the lock is held.
>
> Co-developed-by: Dominique Martinet <dominique.martinet@cea.fr>
> Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
> Reported-by: syzbot+467050c1ce275af2a5b8@syzkaller.appspotmail.com
> Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
> ---
> include/net/9p/client.h | 14 +++++++++++++
> net/9p/client.c | 54 +++++++++++++++++++++++++++++++++++++++++++------
> net/9p/trans_fd.c | 11 +++++++++-
> net/9p/trans_rdma.c | 1 +
> 4 files changed, 73 insertions(+), 7 deletions(-)
>
> diff --git a/include/net/9p/client.h b/include/net/9p/client.h
> index 735f3979d559..947a570307a6 100644
> --- a/include/net/9p/client.h
> +++ b/include/net/9p/client.h
> @@ -94,6 +94,7 @@ enum p9_req_status_t {
> struct p9_req_t {
> int status;
> int t_err;
> + struct kref refcount;
> wait_queue_head_t wq;
> struct p9_fcall tc;
> struct p9_fcall rc;
> @@ -233,6 +234,19 @@ int p9_client_lock_dotl(struct p9_fid *fid, struct p9_flock *flock, u8 *status);
> int p9_client_getlock_dotl(struct p9_fid *fid, struct p9_getlock *fl);
> void p9_fcall_fini(struct p9_fcall *fc);
> struct p9_req_t *p9_tag_lookup(struct p9_client *, u16);
> +
> +static inline void p9_req_get(struct p9_req_t *r)
> +{
> + kref_get(&r->refcount);
> +}
> +
> +static inline int p9_req_try_get(struct p9_req_t *r)
> +{
> + return kref_get_unless_zero(&r->refcount);
> +}
> +
> +int p9_req_put(struct p9_req_t *r);
> +
> void p9_client_cb(struct p9_client *c, struct p9_req_t *req, int status);
>
> int p9_parse_header(struct p9_fcall *, int32_t *, int8_t *, int16_t *, int);
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 7942c0bfcc5b..83f2f0aadc14 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -310,6 +310,18 @@ p9_tag_alloc(struct p9_client *c, int8_t type, unsigned int max_size)
> if (tag < 0)
> goto free;
>
> + /* Init ref to two because in the general case there is one ref
> + * that is put asynchronously by a writer thread, one ref
> + * temporarily given by p9_tag_lookup and put by p9_client_cb
> + * in the recv thread, and one ref put by p9_remove_tag in the
> + * main thread. The only exception is virtio that does not use
> + * p9_tag_lookup but does not have a writer thread either
> + * (the write happens synchronously in the request/zc_request
> + * callback), so p9_client_cb eats the second ref there
> + * as the pointer is duplicated directly by virtqueue_add_sgs()
> + */
> + refcount_set(&req->refcount.refcount, 2);
> +
> return req;
>
> free:
> @@ -333,10 +345,21 @@ struct p9_req_t *p9_tag_lookup(struct p9_client *c, u16 tag)
> struct p9_req_t *req;
>
> rcu_read_lock();
> +again:
> req = idr_find(&c->reqs, tag);
> - /* There's no refcount on the req; a malicious server could cause
> - * us to dereference a NULL pointer
> - */
> + if (req) {
> + /* We have to be careful with the req found under rcu_read_lock
> + * Thanks to SLAB_TYPESAFE_BY_RCU we can safely try to get the
> + * ref again without corrupting other data, then check again
> + * that the tag matches once we have the ref
> + */
> + if (!p9_req_try_get(req))
> + goto again;
> + if (req->tc.tag != tag) {
> + p9_req_put(req);
> + goto again;
> + }
> + }
> rcu_read_unlock();
>
> return req;
> @@ -350,7 +373,7 @@ EXPORT_SYMBOL(p9_tag_lookup);
> *
> * Context: Any context.
> */
> -static void p9_tag_remove(struct p9_client *c, struct p9_req_t *r)
> +static int p9_tag_remove(struct p9_client *c, struct p9_req_t *r)
> {
> unsigned long flags;
> u16 tag = r->tc.tag;
> @@ -359,11 +382,23 @@ static void p9_tag_remove(struct p9_client *c, struct p9_req_t *r)
> spin_lock_irqsave(&c->lock, flags);
> idr_remove(&c->reqs, tag);
> spin_unlock_irqrestore(&c->lock, flags);
> + return p9_req_put(r);
> +}
> +
> +static void p9_req_free(struct kref *ref)
> +{
> + struct p9_req_t *r = container_of(ref, struct p9_req_t, refcount);
> p9_fcall_fini(&r->tc);
> p9_fcall_fini(&r->rc);
> kmem_cache_free(p9_req_cache, r);
> }
>
> +int p9_req_put(struct p9_req_t *r)
> +{
> + return kref_put(&r->refcount, p9_req_free);
> +}
> +EXPORT_SYMBOL(p9_req_put);
> +
> /**
> * p9_tag_cleanup - cleans up tags structure and reclaims resources
> * @c: v9fs client struct
> @@ -379,7 +414,9 @@ static void p9_tag_cleanup(struct p9_client *c)
> rcu_read_lock();
> idr_for_each_entry(&c->reqs, req, id) {
> pr_info("Tag %d still in use\n", id);
> - p9_tag_remove(c, req);
> + if (p9_tag_remove(c, req) == 0)
> + pr_warn("Packet with tag %d has still references",
> + req->tc.tag);
> }
> rcu_read_unlock();
> }
> @@ -403,6 +440,7 @@ void p9_client_cb(struct p9_client *c, struct p9_req_t *req, int status)
>
> wake_up(&req->wq);
> p9_debug(P9_DEBUG_MUX, "wakeup: %d\n", req->tc.tag);
> + p9_req_put(req);
> }
> EXPORT_SYMBOL(p9_client_cb);
>
> @@ -682,6 +720,8 @@ static struct p9_req_t *p9_client_prepare_req(struct p9_client *c,
> return req;
> reterr:
> p9_tag_remove(c, req);
> + /* We have to put also the 2nd reference as it won't be used */
> + p9_req_put(req);
> return ERR_PTR(err);
> }
>
> @@ -716,6 +756,8 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>
> err = c->trans_mod->request(c, req);
> if (err < 0) {
> + /* write won't happen */
> + p9_req_put(req);
> if (err != -ERESTARTSYS && err != -EFAULT)
> c->status = Disconnected;
> goto recalc_sigpending;
> @@ -2241,7 +2283,7 @@ EXPORT_SYMBOL(p9_client_readlink);
>
> int __init p9_client_init(void)
> {
> - p9_req_cache = KMEM_CACHE(p9_req_t, 0);
> + p9_req_cache = KMEM_CACHE(p9_req_t, SLAB_TYPESAFE_BY_RCU);
> return p9_req_cache ? 0 : -ENOMEM;
> }
>
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index 20f46f13fe83..686e24e355d0 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -132,6 +132,7 @@ struct p9_conn {
> struct list_head req_list;
> struct list_head unsent_req_list;
> struct p9_req_t *req;
> + struct p9_req_t *wreq;

Why adding a wreq for write work? And I wonder we should rename req to
rreq?

> char tmp_buf[7];
> struct p9_fcall rc;
> int wpos;
> @@ -383,6 +384,7 @@ static void p9_read_work(struct work_struct *work)
> m->rc.sdata = NULL;
> m->rc.offset = 0;
> m->rc.capacity = 0;
> + p9_req_put(m->req);
> m->req = NULL;
> }
>
> @@ -472,6 +474,8 @@ static void p9_write_work(struct work_struct *work)
> m->wbuf = req->tc.sdata;
> m->wsize = req->tc.size;
> m->wpos = 0;
> + p9_req_get(req);
> + m->wreq = req;
> spin_unlock(&m->client->lock);
> }
>
> @@ -492,8 +496,11 @@ static void p9_write_work(struct work_struct *work)
> }
>
> m->wpos += err;
> - if (m->wpos == m->wsize)
> + if (m->wpos == m->wsize) {
> m->wpos = m->wsize = 0;
> + p9_req_put(m->wreq);
> + m->wreq = NULL;
> + }
>
> end_clear:
> clear_bit(Wworksched, &m->wsched);
> @@ -694,6 +701,7 @@ static int p9_fd_cancel(struct p9_client *client, struct p9_req_t *req)
> if (req->status == REQ_STATUS_UNSENT) {
> list_del(&req->req_list);
> req->status = REQ_STATUS_FLSHD;
> + p9_req_put(req);
> ret = 0;
> }
> spin_unlock(&client->lock);
> @@ -711,6 +719,7 @@ static int p9_fd_cancelled(struct p9_client *client, struct p9_req_t *req)
> spin_lock(&client->lock);
> list_del(&req->req_list);
> spin_unlock(&client->lock);
> + p9_req_put(req);
>
> return 0;
> }
> diff --git a/net/9p/trans_rdma.c b/net/9p/trans_rdma.c
> index c60655c90c9e..8cff368a11e3 100644
> --- a/net/9p/trans_rdma.c
> +++ b/net/9p/trans_rdma.c
> @@ -365,6 +365,7 @@ send_done(struct ib_cq *cq, struct ib_wc *wc)
> c->busa, c->req->tc.size,
> DMA_TO_DEVICE);
> up(&rdma->sq_sem);
> + p9_req_put(c->req);
> kfree(c);
> }
>
>

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