ALSA: asihpi: fix a potential double-fetch bug when copying puhm

The hm->h.size is intended to hold the actual size of the hm struct
that is copied from userspace and should always be <= sizeof(*hm).

However, after copy_from_user(hm, puhm, hm->h.size), since userspace
process has full control over the memory region pointed by puhm, it is
possible that the value of hm->h.size is different from what is fetched-in
previously (get_user(hm->h.size, (u16 __user *)puhm)). In other words,
hm->h.size is overriden and the relation between hm->h.size and the hm
struct is broken.

This patch proposes to use a seperate variable, msg_size, to hold
the value of the first fetch and override hm->h.size to msg_size
after the second fetch to maintain the relation.

Signed-off-by: Meng Xu <mengxu.gatech@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/pci/asihpi/hpioctl.c b/sound/pci/asihpi/hpioctl.c
index 7e3aa50..5badd08 100644 (file)
--- a/sound/pci/asihpi/hpioctl.c
+++ b/sound/pci/asihpi/hpioctl.c
@@ -103,6 +103,7 @@ long asihpi_hpi_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
        void __user *puhr;
        union hpi_message_buffer_v1 *hm;
        union hpi_response_buffer_v1 *hr;
+       u16 msg_size;
        u16 res_max_size;
        u32 uncopied_bytes;
        int err = 0;
@@ -127,22 +128,25 @@ long asihpi_hpi_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
        }
 
        /* Now read the message size and data from user space.  */
-       if (get_user(hm->h.size, (u16 __user *)puhm)) {
+       if (get_user(msg_size, (u16 __user *)puhm)) {
                err = -EFAULT;
                goto out;
        }
-       if (hm->h.size > sizeof(*hm))
-               hm->h.size = sizeof(*hm);
+       if (msg_size > sizeof(*hm))
+               msg_size = sizeof(*hm);
 
        /* printk(KERN_INFO "message size %d\n", hm->h.wSize); */
 
-       uncopied_bytes = copy_from_user(hm, puhm, hm->h.size);
+       uncopied_bytes = copy_from_user(hm, puhm, msg_size);
        if (uncopied_bytes) {
                HPI_DEBUG_LOG(ERROR, "uncopied bytes %d\n", uncopied_bytes);
                err = -EFAULT;
                goto out;
        }
 
+       /* Override h.size in case it is changed between two userspace fetches */
+       hm->h.size = msg_size;
+
        if (get_user(res_max_size, (u16 __user *)puhr)) {
                err = -EFAULT;
                goto out;