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

[Vishnu Dasa]

> On Mar 30, 2023, at 4:18 AM, Dae R. Jeong <threeearcat@xxxxxxxxx> wrote:
> 
> !! External Email
> 
> On Wed, Mar 29, 2023 at 12:15:21PM +0200, Greg KH wrote:
>> On Mon, Mar 27, 2023 at 09:01:53PM +0900, Dae R. Jeong wrote:
>>> 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@xxxxxxxxx>
>>> Fixes: 8bf503991f87 ("VMCI: host side driver implementation.")
>>> Signed-off-by: Dae R. Jeong <threeearcat@xxxxxxxxx>
>> 
>> If you author and sign-off on the patch, no need for a Reported-by: as
>> that is obvious :)
> 
> For a student like me, two of my names are cooler than one... (shy smile :)...)
> 
>> And how did you test this change?
> 
> I attach a simple C program used in testing at the end of this email.
> I used the same C program to trigger the crash before applying the
> patch, and to test the patch after applying it.
> 
> Before applying this patch, I inserted msleep() to trigger the crash
> easily as follows:

Hi Dae,
Thanks for your test code and for your patience.

Your test conditions are a little far from regular operating conditions.  My
comments/questions are as follows:
- the test uses separate threads to do the ioctl and poll operations in parallel.  Normally,
  the vmx process schedules periodic poll after the host context init ioctl completes
  successfully.  Also, the vmx process does not keep calling the host context init ioctl
  once this is done.  So, the race condition shouldn't arise in the first place due to the
  way vmx works.  I understand that you are fuzzing, but just letting you know the
  normal operating conditions.
- vmx process gets the vmci device handle with elevated privileges.  I believe you ran
  the test as root as well?  Since vmx can't cause the race condition, a malicious app
  like your fuzzer would need root privileges to do the damage.
- how easy is it to hit the gpf without the msleep?

Overall, I don't think this is a "must fix" issue today, but I'm also not sure if we should
reject it either due to the simple nature of the patch.  For arm, we may need to add
a write barrier before 'vmci_host_dev->ct_type = VMCIOBJ_CONTEXT' in
vmci_host_do_init_context and a read barrier before
'context = vmci_host_dev->context' in your patch.

Thanks,
Vishnu

> 
> diff --git a/drivers/misc/vmw_vmci/vmci_host.c b/drivers/misc/vmw_vmci/vmci_host.c
> index 857b9851402a..e925f7effac4 100644
> --- a/drivers/misc/vmw_vmci/vmci_host.c
> +++ b/drivers/misc/vmw_vmci/vmci_host.c
> @@ -24,6 +24,7 @@
> #include <linux/smp.h>
> #include <linux/fs.h>
> #include <linux/io.h>
> +#include <linux/delay.h>
> 
> #include "vmci_handle_array.h"
> #include "vmci_queue_pair.h"
> @@ -168,6 +169,11 @@ static __poll_t vmci_host_poll(struct file *filp, poll_table *wait)
>        struct vmci_ctx *context = vmci_host_dev->context;
>        __poll_t mask = 0;
> 
> +       msleep(1000);
> +
> +       printk("%s, context=%px, ct_type=%d\n", __func__, context,
> +                  vmci_host_dev->ct_type);
> +
>        if (vmci_host_dev->ct_type == VMCIOBJ_CONTEXT) {
>                /* Check for VMCI calls to this VM context. */
>                if (wait)
> @@ -341,6 +347,10 @@ static int vmci_host_do_init_context(struct vmci_host_dev *vmci_host_dev,
>        }
> 
>        vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
> +
> +       printk("%s, context=%px, ct_type=%d\n", __func__,
> +                  vmci_host_dev->context, vmci_host_dev->ct_type);
> +
>        atomic_inc(&vmci_host_active_users);
> 
>        vmci_call_vsock_callback(true);
> 
> With this msleep(), the C program below can easily trigger the GPF.
> 
> Then I applied the patch that I propose, and ran the same C
> program. After applying the patch, I couldn't observe the GPF.
> 
>> thanks,
>> 
>> greg k-h
> 
> Best regards,
> Dae R. Jeong
> 
> ------------->8-----------------
> #include <fcntl.h>
> #include <poll.h>
> #include <pthread.h>
> #include <signal.h>
> #include <stdio.h>
> #include <sys/ioctl.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <unistd.h>
> 
> struct init_block {
>  int cid;
>  int flags;
> };
> 
> int fd;
> 
> void *th1(void *a) {
>  struct init_block s = {
>      .cid = 2,
>      .flags = 0,
>  };
>  ioctl(fd, 0x7a0, &s);
>  return NULL;
> }
> 
> void *th2(void *a) {
>  struct pollfd pfd = {.fd = fd};
>  poll(&pfd, 1, 20000);
>  return NULL;
> }
> 
> int test() {
> #define filename "/dev/vmci"
> 
>  fd = openat(AT_FDCWD, filename, 0x2, 0x0);
>  int var = 0x10000;
>  ioctl(fd, 0x7a7, &var);
>  pthread_t pth1, pth2;
>  pthread_create(&pth1, NULL, th1, NULL);
>  pthread_create(&pth2, NULL, th2, NULL);
>  pthread_join(pth1, NULL);
>  pthread_join(pth2, NULL);
>  return 0;
> }
> 
> int main() {
>  for (;;) {
>    pid_t pid = fork();
>    if (pid == 0) {
>      test();
>    } else {
>      usleep(100 * 1000);
>      kill(pid, SIGKILL);
>    }
>  }
> }
> 
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