[PATCH 3.16 202/366] scsi: sg: mitigate read/write abuse

[Ben Hutchings]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

commit 26b5b874aff5659a7e26e5b1997e3df2c41fa7fd upstream.

As Al Viro noted in commit 128394eff343 ("sg_write()/bsg_write() is not fit
to be called under KERNEL_DS"), sg improperly accesses userspace memory
outside the provided buffer, permitting kernel memory corruption via
splice().  But it doesn't just do it on ->write(), also on ->read().

As a band-aid, make sure that the ->read() and ->write() handlers can not
be called in weird contexts (kernel context or credentials different from
file opener), like for ib_safe_file_access().

If someone needs to use these interfaces from different security contexts,
a new interface should be written that goes through the ->ioctl() handler.

I've mostly copypasted ib_safe_file_access() over as sg_safe_file_access()
because I couldn't find a good common header - please tell me if you know a
better way.

[mkp: s/_safe_/_check_/]

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
Signed-off-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
[bwh: Backported to 3.16: open-code uaccess_kernel()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/scsi/sg.c | 42 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -52,6 +52,7 @@ static int sg_version_num = 30534;	/* 2
 #include <linux/blktrace_api.h>
 #include <linux/mutex.h>
 #include <linux/ratelimit.h>
+#include <linux/cred.h> /* for sg_check_file_access() */
 
 #include "scsi.h"
 #include <scsi/scsi_dbg.h>
@@ -215,6 +216,33 @@ static void sg_put_dev(Sg_device *sdp);
 #define SZ_SG_IOVEC sizeof(sg_iovec_t)
 #define SZ_SG_REQ_INFO sizeof(sg_req_info_t)
 
+/*
+ * The SCSI interfaces that use read() and write() as an asynchronous variant of
+ * ioctl(..., SG_IO, ...) are fundamentally unsafe, since there are lots of ways
+ * to trigger read() and write() calls from various contexts with elevated
+ * privileges. This can lead to kernel memory corruption (e.g. if these
+ * interfaces are called through splice()) and privilege escalation inside
+ * userspace (e.g. if a process with access to such a device passes a file
+ * descriptor to a SUID binary as stdin/stdout/stderr).
+ *
+ * This function provides protection for the legacy API by restricting the
+ * calling context.
+ */
+static int sg_check_file_access(struct file *filp, const char *caller)
+{
+	if (filp->f_cred != current_real_cred()) {
+		pr_err_once("%s: process %d (%s) changed security contexts after opening file descriptor, this is not allowed.\n",
+			caller, task_tgid_vnr(current), current->comm);
+		return -EPERM;
+	}
+	if (unlikely(segment_eq(get_fs(), KERNEL_DS))) {
+		pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
+			caller, task_tgid_vnr(current), current->comm);
+		return -EACCES;
+	}
+	return 0;
+}
+
 static int sg_allow_access(struct file *filp, unsigned char *cmd)
 {
 	struct sg_fd *sfp = filp->private_data;
@@ -382,6 +410,14 @@ sg_read(struct file *filp, char __user *
 	struct sg_header *old_hdr = NULL;
 	int retval = 0;
 
+	/*
+	 * This could cause a response to be stranded. Close the associated
+	 * file descriptor to free up any resources being held.
+	 */
+	retval = sg_check_file_access(filp, __func__);
+	if (retval)
+		return retval;
+
 	if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
 		return -ENXIO;
 	SCSI_LOG_TIMEOUT(3, printk("sg_read: %s, count=%d\n",
@@ -567,9 +603,11 @@ sg_write(struct file *filp, const char _
 	struct sg_header old_hdr;
 	sg_io_hdr_t *hp;
 	unsigned char cmnd[MAX_COMMAND_SIZE];
+	int retval;
 
-	if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
-		return -EINVAL;
+	retval = sg_check_file_access(filp, __func__);
+	if (retval)
+		return retval;
 
 	if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
 		return -ENXIO;