[PATCH 2/2] USB: sisusbvga: Fix NULL pointer dereference in sisusb_read

[Vasiliy Kovalev <kovalev@altlinux.org>]

sisusb_read() passes the user-supplied buffer pointer as 'userbuffer' to
sisusb_read_mem_bulk() in two branches:

    /* VRAM path */
    errno = sisusb_read_mem_bulk(sisusb, address,
            NULL, count, buffer, &bytes_read);

    /* MMIO path */
    errno = sisusb_read_mem_bulk(sisusb, address,
            NULL, count, buffer, &bytes_read);

If buffer == NULL (e.g. read(fd, NULL, count) from userspace), both calls
reach sisusb_read_mem_bulk() with kernbuffer=NULL and userbuffer=NULL.
The condition:

    if (userbuffer)

evaluates to false, the kernbuffer path is taken, and the subsequent
dereference:

    swap32 = *((u32 *)kernbuffer);

panics the kernel:

  Oops: general protection fault, probably for non-canonical
        address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  CPU: 3 UID: 0 PID: 370 Comm: sisusbvga-fops- Not tainted 6.19.0-next-20260217 #1
  RIP: 0010:sisusb_read_mem_bulk.constprop.0 (drivers/usb/misc/sisusbvga/sisusbvga.c:1171)
  Call Trace:
   <TASK>
   __pfx_sisusb_read_mem_bulk.constprop.0 (drivers/usb/misc/sisusbvga/sisusbvga.c:1092)
   sisusb_read (drivers/usb/misc/sisusbvga/sisusbvga.c:2396)
   vfs_read (fs/read_write.c:572)
   ksys_read (fs/read_write.c:718)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
   RIP: 0033:0x7f335af3fefc
   </TASK>

Add a NULL check after the existing sanity checks, before the first
branch, to guard both the VRAM and the MMIO paths. Release the mutex
before returning, consistent with the existing -ENODEV path above.

Found by Linux Verification Center (linuxtesting.org) with Svace.
Tested with 'USB Gadget Tests'[1]:

$ TEST=sisusbvga-fops-svace-null-deref
$ echo $TEST > tests/list.txt && make && sudo ./check.sh

[1] Link: https://github.com/kovalev0/usb-gadget-tests
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: <stable@vger.kernel.org>
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
---
 drivers/usb/misc/sisusbvga/sisusbvga.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/usb/misc/sisusbvga/sisusbvga.c b/drivers/usb/misc/sisusbvga/sisusbvga.c
index 89d566d192aa..e14deb1955d9 100644
--- a/drivers/usb/misc/sisusbvga/sisusbvga.c
+++ b/drivers/usb/misc/sisusbvga/sisusbvga.c
@@ -2319,6 +2319,11 @@ static ssize_t sisusb_read(struct file *file, char __user *buffer,
 		return -ENODEV;
 	}
 
+	if (!buffer) {
+		mutex_unlock(&sisusb->lock);
+		return -EFAULT;
+	}
+
 	if ((*ppos) >= SISUSB_PCI_PSEUDO_IOPORTBASE &&
 			(*ppos) <  SISUSB_PCI_PSEUDO_IOPORTBASE + 128) {
 
-- 
2.50.1