usb: mon: Fix atomicity violation in mon_bin_vma_fault

In mon_bin_vma_fault():

	offset = vmf->pgoff << PAGE_SHIFT;
	if (offset >= rp->b_size)
		return VM_FAULT_SIGBUS;
	chunk_idx = offset / CHUNK_SIZE;
	pageptr = rp->b_vec[chunk_idx].pg;

The code is executed without holding any lock.

In mon_bin_vma_close():

	spin_lock_irqsave(&rp->b_lock, flags);
	rp->mmap_active--;
	spin_unlock_irqrestore(&rp->b_lock, flags);

In mon_bin_ioctl():

	spin_lock_irqsave(&rp->b_lock, flags);
	if (rp->mmap_active) {
		...
	} else {
		...
		kfree(rp->b_vec);
		rp->b_vec  = vec;
		rp->b_size = size;
		...
	}
	spin_unlock_irqrestore(&rp->b_lock, flags);

Concurrent execution of mon_bin_vma_fault() with mon_bin_vma_close() and mon_bin_ioctl() could lead to atomicity violations. mon_bin_vma_fault() accesses rp->b_size and rp->b_vec without locking, risking array out-of-bounds access or use-after-free bugs due to possible modifications in mon_bin_ioctl().

This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 6.2.

To address this issue, it is proposed to add a spin lock pair in mon_bin_vma_fault() to ensure atomicity. With this patch applied, our tool never reports the possible bug, with the kernel configuration allyesconfig for x86_64. Due to the lack of associated hardware, we cannot test the patch in runtime testing, and just verify it according to the code logic.

[1] https://sites.google.com/view/basscheck/

Fixes 19e6317d24c25 ("usb: mon: Fix a deadlock in usbmon between ...")

Cc [email protected]

Reported-by BassCheck <[email protected]>

Signed-off-by Gui-Dong Han <[email protected]>

---
v2:
* In this patch v2, we've added some information of the static analysis tool used, as per the researcher guidelines. Also, we've added a cc in the signed-off-by area, according to the stable-kernel-rules. Thank Greg KH for helpful advice.

---
 drivers/usb/mon/mon_bin.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

drivers/usb/mon/mon_bin.c +0 -0

index 9ca9305243fe..509cd1b8ff13 100644

Quoted 67 lines · Gui-Dong Han

In mon_bin_vma_fault():

	offset = vmf->pgoff << PAGE_SHIFT;
	if (offset >= rp->b_size)
		return VM_FAULT_SIGBUS;
	chunk_idx = offset / CHUNK_SIZE;
	pageptr = rp->b_vec[chunk_idx].pg;

The code is executed without holding any lock.

In mon_bin_vma_close():

	spin_lock_irqsave(&rp->b_lock, flags);
	rp->mmap_active--;
	spin_unlock_irqrestore(&rp->b_lock, flags);

In mon_bin_ioctl():

	spin_lock_irqsave(&rp->b_lock, flags);
	if (rp->mmap_active) {
		...
	} else {
		...
		kfree(rp->b_vec);
		rp->b_vec  = vec;
		rp->b_size = size;
		...
	}
	spin_unlock_irqrestore(&rp->b_lock, flags);

Concurrent execution of mon_bin_vma_fault() with mon_bin_vma_close() and mon_bin_ioctl() could lead to atomicity violations. mon_bin_vma_fault() accesses rp->b_size and rp->b_vec without locking, risking array out-of-bounds access or use-after-free bugs due to possible modifications in mon_bin_ioctl().

This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 6.2.

To address this issue, it is proposed to add a spin lock pair in mon_bin_vma_fault() to ensure atomicity. With this patch applied, our tool never reports the possible bug, with the kernel configuration allyesconfig for x86_64. Due to the lack of associated hardware, we cannot test the patch in runtime testing, and just verify it according to the code logic.

[1] https://sites.google.com/view/basscheck/

Fixes 19e6317d24c25 ("usb: mon: Fix a deadlock in usbmon between ...")

Cc [email protected]

Reported-by BassCheck <[email protected]>

Signed-off-by Gui-Dong Han <[email protected]>

---
v2:
* In this patch v2, we've added some information of the static analysis tool used, as per the researcher guidelines. Also, we've added a cc in the signed-off-by area, according to the stable-kernel-rules. Thank Greg KH for helpful advice.

---
  drivers/usb/mon/mon_bin.c | 8 ++++++--
  1 file changed, 6 insertions(+), 2 deletions(-)

drivers/usb/mon/mon_bin.c +0 -0

index 9ca9305243fe..509cd1b8ff13 100644

drivers/usb/mon/mon_bin.c static vm_fault_t mon_bin_vma_fault(struct vm_fault *vmf) · line 1254

Quoted 6 lines

drivers/usb/mon/mon_bin.c +2 -1

@@ -1250,12 +1250,16 @@ static vm_fault_t mon_bin_vma_fault(struct vm_fault *vmf)
  	struct mon_reader_bin *rp = vmf->vma->vm_private_data;
  	unsigned long offset, chunk_idx;
  	struct page *pageptr;
-
+	unsigned long flags;
+	spin_lock_irqsave(&rp->b_lock, flags);

Nit, you still need the blank line before spin_lock_irqsave() here, right?

drivers/usb/mon/mon_bin.c static vm_fault_t mon_bin_vma_fault(struct vm_fault *vmf) · line 1264

Quoted 11 lines

drivers/usb/mon/mon_bin.c +4 -1

  	offset = vmf->pgoff << PAGE_SHIFT;
-	if (offset >= rp->b_size)
+	if (offset >= rp->b_size) {
+		spin_unlock_irqrestore(&rp->b_lock, flags);
  		return VM_FAULT_SIGBUS;
+	}
  	chunk_idx = offset / CHUNK_SIZE;
  	pageptr = rp->b_vec[chunk_idx].pg;
+	spin_unlock_irqrestore(&rp->b_lock, flags);
  	get_page(pageptr);
  	vmf->page = pageptr;

Shouldn't the unlock go here, not 2 lines above as you are still modifying things touched by rp.

thanks,

greg k-h