On 2018-06-21 08:56 PM, Jann Horn wrote:
On Thu, Jun 21, 2018 at 6:53 PM Douglas Gilbert <dgilbert@xxxxxxxxxxxx> wrote:
On 2018-06-21 05:18 PM, Jann Horn wrote:
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.
The duplicate pr_err_once() calls are so that each of them fires once;
otherwise, this would probably have to be a macro.
changed in v2:
- remove the bsg parts per Christoph Hellwig's request
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
drivers/scsi/sg.c | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 53ae52dbff84..51b685192646 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -51,6 +51,7 @@ static int sg_version_num = 30536; /* 2 digits for each component */
#include <linux/atomic.h>
#include <linux/ratelimit.h>
#include <linux/uio.h>
+#include <linux/cred.h> /* for sg_safe_file_access() */
#include "scsi.h"
#include <scsi/scsi_dbg.h>
@@ -209,6 +210,23 @@ static void sg_device_destroy(struct kref *kref);
sdev_prefix_printk(prefix, (sdp)->device, \
(sdp)->disk->disk_name, fmt, ##a)
+/*
+ * 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 inline bool sg_safe_file_access(struct file *filp)
+{
+ return filp->f_cred == current_cred() && !uaccess_kernel();
+}
+
static int sg_allow_access(struct file *filp, unsigned char *cmd)
{
struct sg_fd *sfp = filp->private_data;
@@ -393,6 +411,12 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
struct sg_header *old_hdr = NULL;
int retval = 0;
+ if (!sg_safe_file_access(filp)) {
+ pr_err_once("%s: process %d (%s) changed security contexts after opening file descriptor, this is not allowed.\n",
+ __func__, task_tgid_vnr(current), current->comm);
+ return -EINVAL;
The error message and returned code apply to the
(filp->f_cred == current_cred()) case, not so much to !uaccess_kernel().
While on the error path could you not break out the !uaccess_kernel()
with an appropriate error message and a return code of -EACCES ? Perhaps
a message is unneeded since EACCES is clear.
Not that wild about EINVAL either since it suggests (to me) a "front end"
error (e.g. associated with a badly formed request). How about EPERM for
the changing credentials case.
I used EINVAL since infiniband uses that error case, but I see how it
would be a relatively confusing error code in the context of an sg
device - I agree that EACCES and EPERM might be a better fit here.
I'll adjust the patch.
However, shouldn't it be EPERM in the uaccess_kernel() case and EACCES
in the filp->f_cred!=current_cred() case (instead of the other way
around)?
NO!
See 'man errno':
EACCES Permission denied
EPERM Operation not permitted
Someone was drinking when they chose those abbreviations or had a perverse
sense of humour. It might also explain why some folks say "access denied"
rather than "permission denied".
So if the process/user doesn't have root permissions and they are required,
that should generate an EACCES errno (and no action taken).
Doug Gilbert
And could I suggest a comment in the code along these lines:
/*
* This could cause a response to be stranded. Close the associated
* file descriptor to free up any resources being held.
*/
You mean, as advice to users of this interface, telling them to
close() the FD if they get an error code from read()?
+ }
+
if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
return -ENXIO;
SCSI_LOG_TIMEOUT(3, sg_printk(KERN_INFO, sdp,
@@ -581,8 +605,11 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
sg_io_hdr_t *hp;
unsigned char cmnd[SG_MAX_CDB_SIZE];
- if (unlikely(uaccess_kernel()))
+ if (!sg_safe_file_access(filp)) {
+ pr_err_once("%s: process %d (%s) changed security contexts after opening file descriptor, this is not allowed.\n",
+ __func__, task_tgid_vnr(current), current->comm);
return -EINVAL;
Same comments as above.
Doug Gilbert
+ }
if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
return -ENXIO;
