s390/sclp_ctl: fix potential information leak with /dev/sclp

The sclp_ctl_ioctl_sccb function uses two copy_from_user calls to
retrieve the sclp request from user space. The first copy_from_user
fetches the length of the request which is stored in the first two
bytes of the request. The second copy_from_user gets the complete
sclp request, but this copies the length field a second time.
A malicious user may have changed the length in the meantime.

Reported-by: Pengfei Wang <wpengfeinudt@gmail.com>
Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/drivers/s390/char/sclp_ctl.c b/drivers/s390/char/sclp_ctl.c
index 648cb86afd427776abc0d109f0982284d094699a..ea607a4a1bddaf3e41165aebed1fd787b87d754e 100644 (file)
--- a/drivers/s390/char/sclp_ctl.c
+++ b/drivers/s390/char/sclp_ctl.c
@@ -56,6 +56,7 @@ static int sclp_ctl_ioctl_sccb(void __user *user_area)
 {
        struct sclp_ctl_sccb ctl_sccb;
        struct sccb_header *sccb;
+       unsigned long copied;
        int rc;
 
        if (copy_from_user(&ctl_sccb, user_area, sizeof(ctl_sccb)))
@@ -65,14 +66,15 @@ static int sclp_ctl_ioctl_sccb(void __user *user_area)
        sccb = (void *) get_zeroed_page(GFP_KERNEL | GFP_DMA);
        if (!sccb)
                return -ENOMEM;
-       if (copy_from_user(sccb, u64_to_uptr(ctl_sccb.sccb), sizeof(*sccb))) {
+       copied = PAGE_SIZE -
+               copy_from_user(sccb, u64_to_uptr(ctl_sccb.sccb), PAGE_SIZE);
+       if (offsetof(struct sccb_header, length) +
+           sizeof(sccb->length) > copied || sccb->length > copied) {
                rc = -EFAULT;
                goto out_free;
        }
-       if (sccb->length > PAGE_SIZE || sccb->length < 8)
-               return -EINVAL;
-       if (copy_from_user(sccb, u64_to_uptr(ctl_sccb.sccb), sccb->length)) {
-               rc = -EFAULT;
+       if (sccb->length < 8) {
+               rc = -EINVAL;
                goto out_free;
        }
        rc = sclp_sync_request(ctl_sccb.cmdw, sccb);