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Subject[PATCH v2 3/3] xen/privcmd: add IOCTL_PRIVCMD_RESTRICT
Date
The purpose if this ioctl is to allow a user of privcmd to restrict its
operation such that it will no longer service arbitrary hypercalls via
IOCTL_PRIVCMD_HYPERCALL, and will check for a matching domid when
servicing IOCTL_PRIVCMD_DM_OP. The aim of this is to limit the attack
surface for a compromised device model.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>

v2:
- Make sure that a restriction cannot be cleared
---
drivers/xen/privcmd.c | 67 +++++++++++++++++++++++++++++++++++++++++++---
include/uapi/xen/privcmd.h | 2 ++
2 files changed, 65 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index d5cf042..e372aae 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -44,16 +44,25 @@ MODULE_LICENSE("GPL");

#define PRIV_VMA_LOCKED ((void *)1)

+struct privcmd_data {
+ domid_t domid;
+};
+
static int privcmd_vma_range_is_mapped(
struct vm_area_struct *vma,
unsigned long addr,
unsigned long nr_pages);

-static long privcmd_ioctl_hypercall(void __user *udata)
+static long privcmd_ioctl_hypercall(struct file *file, void __user *udata)
{
+ struct privcmd_data *data = file->private_data;
struct privcmd_hypercall hypercall;
long ret;

+ /* Disallow arbitrary hypercalls if restricted */
+ if (data->domid != DOMID_INVALID)
+ return -EPERM;
+
if (copy_from_user(&hypercall, udata, sizeof(hypercall)))
return -EFAULT;

@@ -591,8 +600,9 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
}
}

-static long privcmd_ioctl_dm_op(void __user *udata)
+static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
{
+ struct privcmd_data *data = file->private_data;
struct privcmd_dm_op kdata;
struct privcmd_dm_op_buf *kbufs;
unsigned int nr_pages = 0;
@@ -604,6 +614,10 @@ static long privcmd_ioctl_dm_op(void __user *udata)
if (copy_from_user(&kdata, udata, sizeof(kdata)))
return -EFAULT;

+ /* If restriction is in place, check the domid matches */
+ if (data->domid != DOMID_INVALID && data->domid != kdata.dom)
+ return -EPERM;
+
if (kdata.num == 0)
return 0;

@@ -682,6 +696,23 @@ static long privcmd_ioctl_dm_op(void __user *udata)
return rc;
}

+static long privcmd_ioctl_restrict(struct file *file, void __user *udata)
+{
+ struct privcmd_data *data = file->private_data;
+ domid_t dom;
+
+ if (copy_from_user(&dom, udata, sizeof(dom)))
+ return -EFAULT;
+
+ /* Set restriction to the specified domain, or check it matches */
+ if (data->domid == DOMID_INVALID)
+ data->domid = dom;
+ else if (data->domid != dom)
+ return -EINVAL;
+
+ return 0;
+}
+
static long privcmd_ioctl(struct file *file,
unsigned int cmd, unsigned long data)
{
@@ -690,7 +721,7 @@ static long privcmd_ioctl(struct file *file,

switch (cmd) {
case IOCTL_PRIVCMD_HYPERCALL:
- ret = privcmd_ioctl_hypercall(udata);
+ ret = privcmd_ioctl_hypercall(file, udata);
break;

case IOCTL_PRIVCMD_MMAP:
@@ -706,7 +737,11 @@ static long privcmd_ioctl(struct file *file,
break;

case IOCTL_PRIVCMD_DM_OP:
- ret = privcmd_ioctl_dm_op(udata);
+ ret = privcmd_ioctl_dm_op(file, udata);
+ break;
+
+ case IOCTL_PRIVCMD_RESTRICT:
+ ret = privcmd_ioctl_restrict(file, udata);
break;

default:
@@ -716,6 +751,28 @@ static long privcmd_ioctl(struct file *file,
return ret;
}

+static int privcmd_open(struct inode *ino, struct file *file)
+{
+ struct privcmd_data *data = kzalloc(sizeof(*data), GFP_KERNEL);
+
+ if (!data)
+ return -ENOMEM;
+
+ /* DOMID_INVALID implies no restriction */
+ data->domid = DOMID_INVALID;
+
+ file->private_data = data;
+ return 0;
+}
+
+static int privcmd_release(struct inode *ino, struct file *file)
+{
+ struct privcmd_data *data = file->private_data;
+
+ kfree(data);
+ return 0;
+}
+
static void privcmd_close(struct vm_area_struct *vma)
{
struct page **pages = vma->vm_private_data;
@@ -784,6 +841,8 @@ static int privcmd_vma_range_is_mapped(
const struct file_operations xen_privcmd_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = privcmd_ioctl,
+ .open = privcmd_open,
+ .release = privcmd_release,
.mmap = privcmd_mmap,
};
EXPORT_SYMBOL_GPL(xen_privcmd_fops);
diff --git a/include/uapi/xen/privcmd.h b/include/uapi/xen/privcmd.h
index f8c5d75..63ee95c 100644
--- a/include/uapi/xen/privcmd.h
+++ b/include/uapi/xen/privcmd.h
@@ -111,5 +111,7 @@ struct privcmd_dm_op {
_IOC(_IOC_NONE, 'P', 4, sizeof(struct privcmd_mmapbatch_v2))
#define IOCTL_PRIVCMD_DM_OP \
_IOC(_IOC_NONE, 'P', 5, sizeof(struct privcmd_dm_op))
+#define IOCTL_PRIVCMD_RESTRICT \
+ _IOC(_IOC_NONE, 'P', 6, sizeof(domid_t))

#endif /* __LINUX_PUBLIC_PRIVCMD_H__ */
--
2.1.4
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