xen: privcmd: Fix possible access to a freed kirqfd instance

Nothing prevents simultaneous ioctl calls to privcmd_irqfd_assign() and
privcmd_irqfd_deassign(). If that happens, it is possible that a kirqfd
created and added to the irqfds_list by privcmd_irqfd_assign() may get
removed by another thread executing privcmd_irqfd_deassign(), while the
former is still using it after dropping the locks.

This can lead to a situation where an already freed kirqfd instance may
be accessed and cause kernel oops.

Use SRCU locking to prevent the same, as is done for the KVM
implementation for irqfds.
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/9e884af1f1f842eacbb7afc5672c8feb4dea7f3f.1718703669.git.viresh.kumar@linaro.orgSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/privcmd.c

@@ -17,6 +17,7 @@
 #include <linux/poll.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/srcu.h>
 #include <linux/string.h>
 #include <linux/workqueue.h>
 #include <linux/errno.h>
@@ -847,6 +848,7 @@ static long privcmd_ioctl_mmap_resource(struct file *file,
 /* Irqfd support */
 static struct workqueue_struct *irqfd_cleanup_wq;
 static DEFINE_SPINLOCK(irqfds_lock);
+DEFINE_STATIC_SRCU(irqfds_srcu);
 static LIST_HEAD(irqfds_list);
 
 struct privcmd_kernel_irqfd {
@@ -874,6 +876,9 @@ static void irqfd_shutdown(struct work_struct *work)
 		container_of(work, struct privcmd_kernel_irqfd, shutdown);
 	u64 cnt;
 
+	/* Make sure irqfd has been initialized in assign path */
+	synchronize_srcu(&irqfds_srcu);
+
 	eventfd_ctx_remove_wait_queue(kirqfd->eventfd, &kirqfd->wait, &cnt);
 	eventfd_ctx_put(kirqfd->eventfd);
 	kfree(kirqfd);
@@ -936,7 +941,7 @@ static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 	__poll_t events;
 	struct fd f;
 	void *dm_op;
-	int ret;
+	int ret, idx;
 
 	kirqfd = kzalloc(sizeof(*kirqfd) + irqfd->size, GFP_KERNEL);
 	if (!kirqfd)
@@ -982,6 +987,7 @@ static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 		}
 	}
 
+	idx = srcu_read_lock(&irqfds_srcu);
 	list_add_tail(&kirqfd->list, &irqfds_list);
 	spin_unlock_irqrestore(&irqfds_lock, flags);
 
@@ -993,6 +999,8 @@ static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 	if (events & EPOLLIN)
 		irqfd_inject(kirqfd);
 
+	srcu_read_unlock(&irqfds_srcu, idx);
+
 	/*
 	 * Do not drop the file until the kirqfd is fully initialized, otherwise
 	 * we might race against the EPOLLHUP.