[PATCH v3] usb: mon: Fix atomicity violation in mon_bin_vma_fault

[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: 19e6317d24c2 ("usb: mon: Fix a deadlock in usbmon between ...")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx>
---
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.
---
v3:
* In this patch v3, we've added a necessary blank line and adjusted the
position of spin_unlock_irqrestore() following Greg KH's suggestions.
  Thank Greg KH for helpful advice.
---
 drivers/usb/mon/mon_bin.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index 9ca9305243fe..fbc1a9c0b345 100644
--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -1250,14 +1250,19 @@ 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);
 	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;
 	get_page(pageptr);
 	vmf->page = pageptr;
+	spin_unlock_irqrestore(&rp->b_lock, flags);
 	return 0;
 }
 
-- 
2.34.1