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

From: Meng Xu
Date: Tue Sep 19 2017 - 01:22:16 EST

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@xxxxxxxxx>
sound/pci/asihpi/hpioctl.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/sound/pci/asihpi/hpioctl.c b/sound/pci/asihpi/hpioctl.c
index 7e3aa50..5badd08 100644
--- 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;