xen-pciback: limit guest control of command register

Otherwise the guest can abuse that control to cause e.g. PCIe
Unsupported Request responses by disabling memory and/or I/O decoding
and subsequently causing (CPU side) accesses to the respective address
ranges, which (depending on system configuration) may be fatal to the
host.

Note that to alter any of the bits collected together as
PCI_COMMAND_GUEST permissive mode is now required to be enabled
globally or on the specific device.

This is CVE-2015-2150 / XSA-120.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

3 files changed, 51 insertions, 14 deletions

diff --git a/drivers/xen/xen-pciback/conf_space.c b/drivers/xen/xen-pciback/conf_space.c
index 46ae0f9f02adcc..75fe3d466515a0 100644
--- a/drivers/xen/xen-pciback/conf_space.c
+++ b/drivers/xen/xen-pciback/conf_space.c
@@ -16,7 +16,7 @@
 #include "conf_space.h"
 #include "conf_space_quirks.h"
 
-static bool permissive;
+bool permissive;
 module_param(permissive, bool, 0644);
 
 /* This is where xen_pcibk_read_config_byte, xen_pcibk_read_config_word,
diff --git a/drivers/xen/xen-pciback/conf_space.h b/drivers/xen/xen-pciback/conf_space.h
index e56c934ad137be..2e1d73d1d5d093 100644
--- a/drivers/xen/xen-pciback/conf_space.h
+++ b/drivers/xen/xen-pciback/conf_space.h
@@ -64,6 +64,8 @@ struct config_field_entry {
 	void *data;
 };
 
+extern bool permissive;
+
 #define OFFSET(cfg_entry) ((cfg_entry)->base_offset+(cfg_entry)->field->offset)
 
 /* Add fields to a device - the add_fields macro expects to get a pointer to
diff --git a/drivers/xen/xen-pciback/conf_space_header.c b/drivers/xen/xen-pciback/conf_space_header.c
index c5ee82587e8cc3..2d7369391472fd 100644
--- a/drivers/xen/xen-pciback/conf_space_header.c
+++ b/drivers/xen/xen-pciback/conf_space_header.c
@@ -11,6 +11,10 @@
 #include "pciback.h"
 #include "conf_space.h"
 
+struct pci_cmd_info {
+	u16 val;
+};
+
 struct pci_bar_info {
 	u32 val;
 	u32 len_val;
@@ -20,22 +24,36 @@ struct pci_bar_info {
 #define is_enable_cmd(value) ((value)&(PCI_COMMAND_MEMORY|PCI_COMMAND_IO))
 #define is_master_cmd(value) ((value)&PCI_COMMAND_MASTER)
 
-static int command_read(struct pci_dev *dev, int offset, u16 *value, void *data)
+/* Bits guests are allowed to control in permissive mode. */
+#define PCI_COMMAND_GUEST (PCI_COMMAND_MASTER|PCI_COMMAND_SPECIAL| \
+			   PCI_COMMAND_INVALIDATE|PCI_COMMAND_VGA_PALETTE| \
+			   PCI_COMMAND_WAIT|PCI_COMMAND_FAST_BACK)
+
+static void *command_init(struct pci_dev *dev, int offset)
 {
-	int i;
-	int ret;
-
-	ret = xen_pcibk_read_config_word(dev, offset, value, data);
-	if (!pci_is_enabled(dev))
-		return ret;
-
-	for (i = 0; i < PCI_ROM_RESOURCE; i++) {
-		if (dev->resource[i].flags & IORESOURCE_IO)
-			*value |= PCI_COMMAND_IO;
-		if (dev->resource[i].flags & IORESOURCE_MEM)
-			*value |= PCI_COMMAND_MEMORY;
+	struct pci_cmd_info *cmd = kmalloc(sizeof(*cmd), GFP_KERNEL);
+	int err;
+
+	if (!cmd)
+		return ERR_PTR(-ENOMEM);
+
+	err = pci_read_config_word(dev, PCI_COMMAND, &cmd->val);
+	if (err) {
+		kfree(cmd);
+		return ERR_PTR(err);
 	}
 
+	return cmd;
+}
+
+static int command_read(struct pci_dev *dev, int offset, u16 *value, void *data)
+{
+	int ret = pci_read_config_word(dev, offset, value);
+	const struct pci_cmd_info *cmd = data;
+
+	*value &= PCI_COMMAND_GUEST;
+	*value |= cmd->val & ~PCI_COMMAND_GUEST;
+
 	return ret;
 }
 
@@ -43,6 +61,8 @@ static int command_write(struct pci_dev *dev, int offset, u16 value, void *data)
 {
 	struct xen_pcibk_dev_data *dev_data;
 	int err;
+	u16 val;
+	struct pci_cmd_info *cmd = data;
 
 	dev_data = pci_get_drvdata(dev);
 	if (!pci_is_enabled(dev) && is_enable_cmd(value)) {
@@ -83,6 +103,19 @@ static int command_write(struct pci_dev *dev, int offset, u16 value, void *data)
 		}
 	}
 
+	cmd->val = value;
+
+	if (!permissive && (!dev_data || !dev_data->permissive))
+		return 0;
+
+	/* Only allow the guest to control certain bits. */
+	err = pci_read_config_word(dev, offset, &val);
+	if (err || val == value)
+		return err;
+
+	value &= PCI_COMMAND_GUEST;
+	value |= val & ~PCI_COMMAND_GUEST;
+
 	return pci_write_config_word(dev, offset, value);
 }
 
@@ -282,6 +315,8 @@ static const struct config_field header_common[] = {
 	{
 	 .offset    = PCI_COMMAND,
 	 .size      = 2,
+	 .init      = command_init,
+	 .release   = bar_release,
 	 .u.w.read  = command_read,
 	 .u.w.write = command_write,
 	},