[PATCH] usb: mon: make mmapped memory read only

[PATCH] usb: mon: make mmapped memory read only

[Tadeusz Struk]

Syzbot found an issue in usbmon where it can corrupt monitor
internal memory causing the usbmon to crash with segfault,
UAF, etc. The reproducer mmaps the /dev/usbmon memory to userspace
and overwrites it with arbitrary data, which causes the issues.
To prevent that explicitly clear the VM_WRITE flag in mon_bin_mmap().

Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: 6f23ee1fefdc ("USB: add binary API to usbmon")
Link: https://syzkaller.appspot.com/bug?id=2eb1f35d6525fa4a74d75b4244971e5b1411c95a
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
---
 drivers/usb/mon/mon_bin.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index f48a23adbc35..f452fc03093c 100644
--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -1268,6 +1268,7 @@ static int mon_bin_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	/* don't do anything here: "fault" will set up page table entries */
 	vma->vm_ops = &mon_bin_vm_ops;
+	vma->vm_flags &= ~VM_WRITE;
 	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = filp->private_data;
 	mon_bin_vma_open(vma);
-- 
2.37.3

Re: [PATCH] usb: mon: make mmapped memory read only

[Tadeusz Struk]

On 9/16/22 15:47, Tadeusz Struk wrote:
> Syzbot found an issue in usbmon where it can corrupt monitor
> internal memory causing the usbmon to crash with segfault,
> UAF, etc. The reproducer mmaps the /dev/usbmon memory to userspace
> and overwrites it with arbitrary data, which causes the issues.
> To prevent that explicitly clear the VM_WRITE flag in mon_bin_mmap().
> 
> Cc:linux-usb@vger.kernel.org
> Cc:linux-kernel@vger.kernel.org
> Cc:stable@vger.kernel.org
> Fixes: 6f23ee1fefdc ("USB: add binary API to usbmon")
> Link:https://syzkaller.appspot.com/bug?id=2eb1f35d6525fa4a74d75b4244971e5b1411c95a
> Signed-off-by: Tadeusz Struk<tadeusz.struk@linaro.org>

I forgot to add:
Reported-by: syzbot+23f57c5ae902429285d7@syzkaller.appspotmail.com

-- 
Thanks,
Tadeusz

Re: [PATCH] usb: mon: make mmapped memory read only

[Dmitry Vyukov]

Hi Tadeusz,

Looking at places like these:
https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/qib/qib_file_ops.c#L736
https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/mlx5/main.c#L2088
I think we also need to remove VM_MAYWRITE, otherwise it's still
possible to turn it into a writable mapping with mprotect.

It's also probably better to return an error if VM_WRITE (or VM_EXEC?) is set
rather than silently fix it up.

Re: [PATCH] usb: mon: make mmapped memory read only

[Dmitry Vyukov]

On Mon, 19 Sept 2022 at 06:25, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> Hi Tadeusz,
>
> Looking at places like these:
> https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/qib/qib_file_ops.c#L736
> https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/mlx5/main.c#L2088
> I think we also need to remove VM_MAYWRITE, otherwise it's still
> possible to turn it into a writable mapping with mprotect.
>
> It's also probably better to return an error if VM_WRITE (or VM_EXEC?) is set
> rather than silently fix it up.


The credit for the VM_MAYWRITE suggestion goes to the PaX Team.

Suggested-by: PaX Team <pageexec@freemail.hu>

Re: [PATCH] usb: mon: make mmapped memory read only

[Tadeusz Struk]

Hi Dmitry,
On 9/18/22 21:25, Dmitry Vyukov wrote:
> Hi Tadeusz,
> 
> Looking at places like these:
> https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/qib/qib_file_ops.c#L736
> https://elixir.bootlin.com/linux/v6.0-rc5/source/drivers/infiniband/hw/mlx5/main.c#L2088
> I think we also need to remove VM_MAYWRITE, otherwise it's still
> possible to turn it into a writable mapping with mprotect.
> 
> It's also probably better to return an error if VM_WRITE (or VM_EXEC?) is set
> rather than silently fix it up.

Yes, I think that returning an error will make more sense here.
I don't think we need to worry about VM_EXEC. Even if userspace will try to execute
from the mmaped location it won't work. It will probably crash the application without
causing any harm to the kernel.
I will update the patch and send a v2 soon.

-- 
Thanks,
Tadeusz

[PATCH v2] usb: mon: make mmapped memory read only

[Tadeusz Struk]

Syzbot found an issue in usbmon module, where the user space client
can corrupt the monitor's internal memory, causing the usbmon module
to crash the kernel with segfault, UAF, etc.
The reproducer mmaps the /dev/usbmon memory to user space, and
overwrites it with arbitrary data, which causes all kinds of issues.
Return an -EPERM error from mon_bin_mmap() if the flag VM_WRTIE is set.
Also clear VM_MAYWRITE to make it impossible to change it to writable
later.

Cc: "Dmitry Vyukov" <dvyukov@google.com>
Cc: <linux-usb@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Fixes: 6f23ee1fefdc ("USB: add binary API to usbmon")

For the VM_MAYWRITE part:
Suggested-by: PaX Team <pageexec@freemail.hu>

Link: https://syzkaller.appspot.com/bug?id=2eb1f35d6525fa4a74d75b4244971e5b1411c95a
Reported-by: syzbot+23f57c5ae902429285d7@syzkaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
---
v2:
   Return an error instead of quietly clearing the flag,
   when VM_WRTIE is set. Also clear VM_MAYWRITE.
---
 drivers/usb/mon/mon_bin.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index f48a23adbc35..094e812e9e69 100644
--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -1268,6 +1268,11 @@ static int mon_bin_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	/* don't do anything here: "fault" will set up page table entries */
 	vma->vm_ops = &mon_bin_vm_ops;
+
+	if (vma->vm_flags & VM_WRITE)
+		return -EPERM;
+
+	vma->vm_flags &= ~VM_MAYWRITE;
 	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = filp->private_data;
 	mon_bin_vma_open(vma);
-- 
2.37.3