[PATCH] usb: require FMODE_WRITE for usbdev_mmap()

[Jann Horn <jannh@google.com>]

usbdev_mmap() creates VMAs which can only be used through
usbdev_do_ioctl(), which requires FMODE_WRITE; so usbdev_mmap() is only
useful with FMODE_WRITE.

On typical Linux systems, files at /dev/bus/usb/*/* are mode 0664, so
UIDs without any special privileges can't use usbdev_do_ioctl(), but
they can still execute the usbdev_mmap() codepath.

Check for FMODE_WRITE in usbdev_mmap() to reduce attack surface a little
bit.

Signed-off-by: Jann Horn <jannh@google.com>
---
 drivers/usb/core/devio.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index 3beb6a862e80..5363468a282f 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -238,6 +238,9 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
 	dma_addr_t dma_handle = DMA_MAPPING_ERROR;
 	int ret;
 
+	if (!(file->f_mode & FMODE_WRITE))
+		return -EPERM;
+
 	ret = usbfs_increase_memory_usage(size + sizeof(struct usb_memory));
 	if (ret)
 		goto error;

---
base-commit: eca631b8fe808748d7585059c4307005ca5c5820
change-id: 20241016-usbdev-mmap-require-write-7e56d528d062
-- 
Jann Horn <jannh@google.com>