RDMA/ucma: Protect mc during concurrent multicast leaves

Partially revert the commit mentioned in the Fixes line to make sure that
allocation and erasing multicast struct are locked.

BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529
CPU: 0 PID: 25529 Comm: syz-executor.1 Not tainted 5.16.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247
__kasan_report mm/kasan/report.c:433 [inline]
kasan_report.cold+0x83/0xdf mm/kasan/report.c:450
ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
ucma_destroy_id+0x1e6/0x280 drivers/infiniband/core/ucma.c:614
ucma_write+0x25c/0x350 drivers/infiniband/core/ucma.c:1732
vfs_write+0x28e/0xae0 fs/read_write.c:588
ksys_write+0x1ee/0x250 fs/read_write.c:643
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

Currently the xarray search can touch a concurrently freeing mc as the
xa_for_each() is not surrounded by any lock. Rather than hold the lock for
a full scan hold it only for the effected items, which is usually an empty
list.

Fixes: 95fe51096b7a ("RDMA/ucma: Remove mc_list and rely on xarray")
Link: https://lore.kernel.org/r/[e-mail]
Reported-by: [e-mail]
Suggested-by: Jason Gunthorpe <[e-mail]>
Reviewed-by: Maor Gottlieb <[e-mail]>
Signed-off-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/core/ucma.c

···
 	u64			uid;
 
 	struct list_head	list;
+	struct list_head	mc_list;
 	struct work_struct	close_work;
 };
 
···
 
 	u64			uid;
 	u8			join_state;
+	struct list_head	list;
 	struct sockaddr_storage	addr;
 };
 
···
 
 	INIT_WORK(&ctx->close_work, ucma_close_id);
 	init_completion(&ctx->comp);
+	INIT_LIST_HEAD(&ctx->mc_list);
 	/* So list_del() will work if we don't do ucma_finish_ctx() */
 	INIT_LIST_HEAD(&ctx->list);
 	ctx->file = file;
···
 
 static void ucma_cleanup_multicast(struct ucma_context *ctx)
 {
-	struct ucma_multicast *mc;
-	unsigned long index;
+	struct ucma_multicast *mc, *tmp;
 
-	xa_for_each(&multicast_table, index, mc) {
-		if (mc->ctx != ctx)
-			continue;
+	xa_lock(&multicast_table);
+	list_for_each_entry_safe(mc, tmp, &ctx->mc_list, list) {
+		list_del(&mc->list);
 		/*
 		 * At this point mc->ctx->ref is 0 so the mc cannot leave the
 		 * lock on the reader and this is enough serialization
 		 */
-		xa_erase(&multicast_table, index);
+		__xa_erase(&multicast_table, mc->id);
 		kfree(mc);
 	}
+	xa_unlock(&multicast_table);
 }
 
 static void ucma_cleanup_mc_events(struct ucma_multicast *mc)
···
 	mc->uid = cmd->uid;
 	memcpy(&mc->addr, addr, cmd->addr_size);
 
-	if (xa_alloc(&multicast_table, &mc->id, NULL, xa_limit_32b,
+	xa_lock(&multicast_table);
+	if (__xa_alloc(&multicast_table, &mc->id, NULL, xa_limit_32b,
 		     GFP_KERNEL)) {
 		ret = -ENOMEM;
 		goto err_free_mc;
 	}
+
+	list_add_tail(&mc->list, &ctx->mc_list);
+	xa_unlock(&multicast_table);
 
 	mutex_lock(&ctx->mutex);
 	ret = rdma_join_multicast(ctx->cm_id, (struct sockaddr *)&mc->addr,
···
 	mutex_unlock(&ctx->mutex);
 	ucma_cleanup_mc_events(mc);
 err_xa_erase:
-	xa_erase(&multicast_table, mc->id);
+	xa_lock(&multicast_table);
+	list_del(&mc->list);
+	__xa_erase(&multicast_table, mc->id);
 err_free_mc:
+	xa_unlock(&multicast_table);
 	kfree(mc);
 err_put_ctx:
 	ucma_put_ctx(ctx);
···
 		mc = ERR_PTR(-EINVAL);
 	else if (!refcount_inc_not_zero(&mc->ctx->ref))
 		mc = ERR_PTR(-ENXIO);
-	else
-		__xa_erase(&multicast_table, mc->id);
-	xa_unlock(&multicast_table);
 
 	if (IS_ERR(mc)) {
+		xa_unlock(&multicast_table);
 		ret = PTR_ERR(mc);
 		goto out;
 	}
+
+	list_del(&mc->list);
+	__xa_erase(&multicast_table, mc->id);
+	xa_unlock(&multicast_table);
 
 	mutex_lock(&mc->ctx->mutex);
 	rdma_leave_multicast(mc->ctx->cm_id, (struct sockaddr *) &mc->addr);