[PATCH] net/9p: Validate current->sighand in client.c

[Alexander Kapshuk <alexander.kapshuk@gmail.com>]

Fix rcu not being dereferenced cleanly by using the task
helpers (un)lock_task_sighand instead of spin_lock_irqsave and
spin_unlock_irqrestore to ensure current->sighand is a valid pointer as
suggested in the email referenced below.

Signed-off-by: Alexander Kapshuk <alexander.kapshuk@gmail.com>
Link: https://lore.kernel.org/lkml/20200618190807.GA20699@nautica/
---
 net/9p/client.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index fc1f3635e5dd..15f16f2baa8f 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -787,9 +787,14 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 	}
 recalc_sigpending:
 	if (sigpending) {
-		spin_lock_irqsave(&current->sighand->siglock, flags);
+		if (!lock_task_sighand(current, &flags)) {
+			pr_warn("%s (%d): current->sighand==NULL in recalc_sigpending\n",
+				__func__, task_pid_nr(current));
+			err = -ESRCH;
+			goto reterr;
+		}
 		recalc_sigpending();
-		spin_unlock_irqrestore(&current->sighand->siglock, flags);
+		unlock_task_sighand(current, &flags);
 	}
 	if (err < 0)
 		goto reterr;
@@ -869,9 +874,14 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 	}
 recalc_sigpending:
 	if (sigpending) {
-		spin_lock_irqsave(&current->sighand->siglock, flags);
+		if (!lock_task_sighand(current, &flags)) {
+			pr_warn("%s (%d): current->sighand==NULL in recalc_sigpending\n",
+				__func__, task_pid_nr(current));
+			err = -ESRCH;
+			goto reterr;
+		}
 		recalc_sigpending();
-		spin_unlock_irqrestore(&current->sighand->siglock, flags);
+		unlock_task_sighand(current, &flags);
 	}
 	if (err < 0)
 		goto reterr;
--
2.27.0