[PATCH] vmci_host: fix a race condition in vmci_host_poll() causing GPF

["Dae R. Jeong" <threeearcat@gmail.com>]

During fuzzing, a general protection fault is observed in
vmci_host_poll().

general protection fault, probably for non-canonical address 0xdffffc0000000019: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000000c8-0x00000000000000cf]
RIP: 0010:__lock_acquire+0xf3/0x5e00 kernel/locking/lockdep.c:4926
<- omitting registers ->
Call Trace:
 <TASK>
 lock_acquire+0x1a4/0x4a0 kernel/locking/lockdep.c:5672
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
 _raw_spin_lock_irqsave+0xb3/0x100 kernel/locking/spinlock.c:162
 add_wait_queue+0x3d/0x260 kernel/sched/wait.c:22
 poll_wait include/linux/poll.h:49 [inline]
 vmci_host_poll+0xf8/0x2b0 drivers/misc/vmw_vmci/vmci_host.c:174
 vfs_poll include/linux/poll.h:88 [inline]
 do_pollfd fs/select.c:873 [inline]
 do_poll fs/select.c:921 [inline]
 do_sys_poll+0xc7c/0x1aa0 fs/select.c:1015
 __do_sys_ppoll fs/select.c:1121 [inline]
 __se_sys_ppoll+0x2cc/0x330 fs/select.c:1101
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x4e/0xa0 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

Example thread interleaving that causes the general protection fault
is as follows:

CPU1 (vmci_host_poll)               CPU2 (vmci_host_do_init_context)
-----                               -----
// Read uninitialized context
context = vmci_host_dev->context;
                                    // Initialize context
                                    vmci_host_dev->context = vmci_ctx_create();
                                    vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;

if (vmci_host_dev->ct_type == VMCIOBJ_CONTEXT) {
    // Dereferencing the wrong pointer
    poll_wait(..., &context->host_context);
}

In this scenario, vmci_host_poll() reads vmci_host_dev->context first,
and then reads vmci_host_dev->ct_type to check that
vmci_host_dev->context is initialized. However, since these two reads
are not atomically executed, there is a chance of a race condition as
described above.

To fix this race condition, read vmci_host_dev->context after checking
the value of vmci_host_dev->ct_type so that vmci_host_poll() always
reads an initialized context.

Reported-by: Dae R. Jeong <threeearcat@gmail.com>
Fixes: 8bf503991f87 ("VMCI: host side driver implementation.")
Signed-off-by: Dae R. Jeong <threeearcat@gmail.com>
---
 drivers/misc/vmw_vmci/vmci_host.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/vmw_vmci/vmci_host.c b/drivers/misc/vmw_vmci/vmci_host.c
index 857b9851402a..abe79f6fd2a7 100644
--- a/drivers/misc/vmw_vmci/vmci_host.c
+++ b/drivers/misc/vmw_vmci/vmci_host.c
@@ -165,10 +165,16 @@ static int vmci_host_close(struct inode *inode, struct file *filp)
 static __poll_t vmci_host_poll(struct file *filp, poll_table *wait)
 {
 	struct vmci_host_dev *vmci_host_dev = filp->private_data;
-	struct vmci_ctx *context = vmci_host_dev->context;
+	struct vmci_ctx *context;
 	__poll_t mask = 0;
 
 	if (vmci_host_dev->ct_type == VMCIOBJ_CONTEXT) {
+		/*
+		 * Read context only if ct_type == VMCIOBJ_CONTEXT to make
+		 * sure that context is initialized
+		 */
+		context = vmci_host_dev->context;
+
 		/* Check for VMCI calls to this VM context. */
 		if (wait)
 			poll_wait(filp, &context->host_context.wait_queue,
-- 
2.17.1