[PATCH v2] iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read

[Kai Aizen <kai.aizen.dev@gmail.com>]

The bound-check in iommufd_veventq_fops_read() for the normal vEVENT
path uses sizeof(hdr) where the surrounding code uses sizeof(*hdr):

	if (!vevent_for_lost_events_header(cur) &&
	    sizeof(hdr) + cur->data_len > count - done) {

hdr is declared as struct iommufd_vevent_header *, so sizeof(hdr)
evaluates to the size of the pointer.  Surrounding code uses
sizeof(*hdr) consistently:

	if (done >= count || sizeof(*hdr) > count - done) {
	...
	if (copy_to_user(buf + done, hdr, sizeof(*hdr))) {
	...
	done += sizeof(*hdr);

struct iommufd_vevent_header is currently 8 bytes (two __u32 fields,
flags and sequence), so on 64-bit (sizeof(void *) == 8) the two
expressions happen to be equal and the check works as intended.

On 32-bit (sizeof(void *) == 4) the check under-counts the header by
4 bytes: a vEVENT whose data_len causes 8 + cur->data_len to exceed
count - done while 4 + cur->data_len does not will pass the check,
then the loop will copy_to_user 8 bytes of header followed by data_len
bytes of payload, writing past the user-supplied buffer.

It is also a latent bug for any future expansion of struct
iommufd_vevent_header beyond sizeof(void *) on 64-bit; the check
should not depend on the type happening to match the host pointer
width.

Use sizeof(*hdr) to match the rest of the function and the actual
amount that will be copied.

Fixes: e36ba5ab808e ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Cc: stable@vger.kernel.org
Reported-by: Kai Aizen <kai.aizen.dev@gmail.com>
Signed-off-by: Kai Aizen <kai.aizen.dev@gmail.com>
---
v2: fix From/Signed-off-by to use real name and email address.
---
 drivers/iommu/iommufd/eventq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/iommufd/eventq.c b/drivers/iommu/iommufd/eventq.c
index 710eef0b6..78689fb52 100644
--- a/drivers/iommu/iommufd/eventq.c
+++ b/drivers/iommu/iommufd/eventq.c
@@ -321,7 +321,7 @@ static ssize_t iommufd_veventq_fops_read(struct file *filep, char __user *buf,
 
 		/* If being a normal vEVENT, validate against the full size */
 		if (!vevent_for_lost_events_header(cur) &&
-		    sizeof(hdr) + cur->data_len > count - done) {
+		    sizeof(*hdr) + cur->data_len > count - done) {
 			iommufd_veventq_deliver_restore(veventq, cur);
 			break;
 		}
-- 
2.43.0