[PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release

Jann Horn
2018-06-15T15:24:00Z

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

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().
Also, completely prevent user memory accesses from ->release().

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
scsi_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.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
---

I'm CC-ing security@ on this patch in case someone cares a lot, but
since you already need to have some pretty high privileges to use these
devices in the first place, I think this can be handled publicly.

In case anyone is interested in how I found these: I was looking at a
reverse callgraph of __might_fault and spotted the ->release handler of
block/bsg.c in there.

block/bsg-lib.c | 5 ++++-
block/bsg.c | 29 +++++++++++++++++++++--------
drivers/scsi/sg.c | 11 ++++++++++-
include/linux/bsg.h | 3 ++-
include/scsi/scsi_cmnd.h | 19 +++++++++++++++++++
5 files changed, 56 insertions(+), 11 deletions(-)

diff --git a/block/bsg-lib.c b/block/bsg-lib.c
index 9419def8c017..cf5d4fdddbeb 100644
--- a/block/bsg-lib.c
+++ b/block/bsg-lib.c
@@ -53,7 +53,8 @@ static int bsg_transport_fill_hdr(struct request *rq, struct sg_io_v4 *hdr,
return 0;
}

-static int bsg_transport_complete_rq(struct request *rq, struct sg_io_v4 *hdr)
+static int bsg_transport_complete_rq(struct request *rq, struct sg_io_v4 *hdr,
+ bool cleaning_up)
{
struct bsg_job *job = blk_mq_rq_to_pdu(rq);
int ret = 0;
@@ -79,6 +80,8 @@ static int bsg_transport_complete_rq(struct request *rq, struct sg_io_v4 *hdr)
if (job->reply_len && hdr->response) {
int len = min(hdr->max_response_len, job->reply_len);

+ if (unlikely(cleaning_up))
+ ret = -EINVAL;
if (copy_to_user(uptr64(hdr->response), job->reply, len))
ret = -EFAULT;
else
diff --git a/block/bsg.c b/block/bsg.c
index 132e657e2d91..e64ef807d2d0 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -159,7 +159,8 @@ static int bsg_scsi_fill_hdr(struct request *rq, struct sg_io_v4 *hdr,
return 0;
}

-static int bsg_scsi_complete_rq(struct request *rq, struct sg_io_v4 *hdr)
+static int bsg_scsi_complete_rq(struct request *rq, struct sg_io_v4 *hdr,
+ bool cleaning_up)
{
struct scsi_request *sreq = scsi_req(rq);
int ret = 0;
@@ -179,7 +180,9 @@ static int bsg_scsi_complete_rq(struct request *rq, struct sg_io_v4 *hdr)
int len = min_t(unsigned int, hdr->max_response_len,
sreq->sense_len);

- if (copy_to_user(uptr64(hdr->response), sreq->sense, len))
+ if (cleaning_up)
+ ret = -EINVAL;
+ else if (copy_to_user(uptr64(hdr->response), sreq->sense, len))
ret = -EFAULT;
else
hdr->response_len = len;
@@ -383,11 +386,12 @@ static struct bsg_command *bsg_get_done_cmd(struct bsg_device *bd)
}

static int blk_complete_sgv4_hdr_rq(struct request *rq, struct sg_io_v4 *hdr,
- struct bio *bio, struct bio *bidi_bio)
+ struct bio *bio, struct bio *bidi_bio,
+ bool cleaning_up)
{
int ret;

- ret = rq->q->bsg_dev.ops->complete_rq(rq, hdr);
+ ret = rq->q->bsg_dev.ops->complete_rq(rq, hdr, cleaning_up);

if (rq->next_rq) {
blk_rq_unmap_user(bidi_bio);
@@ -453,7 +457,7 @@ static int bsg_complete_all_commands(struct bsg_device *bd)
break;

tret = blk_complete_sgv4_hdr_rq(bc->rq, &bc->hdr, bc->bio,
- bc->bidi_bio);
+ bc->bidi_bio, true);
if (!ret)
ret = tret;

@@ -488,7 +492,7 @@ __bsg_read(char __user *buf, size_t count, struct bsg_device *bd,
* bsg_complete_work() cannot do that for us
*/
ret = blk_complete_sgv4_hdr_rq(bc->rq, &bc->hdr, bc->bio,
- bc->bidi_bio);
+ bc->bidi_bio, false);

if (copy_to_user(buf, &bc->hdr, sizeof(bc->hdr)))
ret = -EFAULT;
@@ -532,6 +536,12 @@ bsg_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
int ret;
ssize_t bytes_read;

+ if (!scsi_safe_file_access(file)) {
+ 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;
+ }
+
bsg_dbg(bd, "read %zd bytes\n", count);

bsg_set_block(bd, file);
@@ -608,8 +618,11 @@ bsg_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)

bsg_dbg(bd, "write %zd bytes\n", count);

- if (unlikely(uaccess_kernel()))
+ if (!scsi_safe_file_access(file)) {
+ 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;
+ }

bsg_set_block(bd, file);

@@ -859,7 +872,7 @@ static long bsg_ioctl(struct file *file, unsigned int cmd, unsigned long arg)

at_head = (0 == (hdr.flags & BSG_FLAG_Q_AT_TAIL));
blk_execute_rq(bd->queue, NULL, rq, at_head);
- ret = blk_complete_sgv4_hdr_rq(rq, &hdr, bio, bidi_bio);
+ ret = blk_complete_sgv4_hdr_rq(rq, &hdr, bio, bidi_bio, false);

if (copy_to_user(uarg, &hdr, sizeof(hdr)))
return -EFAULT;
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 53ae52dbff84..997e06a22527 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -393,6 +393,12 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
struct sg_header *old_hdr = NULL;
int retval = 0;

+ if (!scsi_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;
+ }
+
if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
return -ENXIO;
SCSI_LOG_TIMEOUT(3, sg_printk(KERN_INFO, sdp,
@@ -581,8 +587,11 @@ 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 (!scsi_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;
+ }

if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
return -ENXIO;
diff --git a/include/linux/bsg.h b/include/linux/bsg.h
index dac37b6e00ec..c22bc359552a 100644
--- a/include/linux/bsg.h
+++ b/include/linux/bsg.h
@@ -11,7 +11,8 @@ struct bsg_ops {
int (*check_proto)(struct sg_io_v4 *hdr);
int (*fill_hdr)(struct request *rq, struct sg_io_v4 *hdr,
fmode_t mode);
- int (*complete_rq)(struct request *rq, struct sg_io_v4 *hdr);
+ int (*complete_rq)(struct request *rq, struct sg_io_v4 *hdr,
+ bool cleaning_up);
void (*free_rq)(struct request *rq);
};

diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
index aaf1e971c6a3..d22118a38aa4 100644
--- a/include/scsi/scsi_cmnd.h
+++ b/include/scsi/scsi_cmnd.h
@@ -8,6 +8,8 @@
#include <linux/types.h>
#include <linux/timer.h>
#include <linux/scatterlist.h>
+#include <linux/cred.h> /* for scsi_safe_file_access() */
+#include <linux/fs.h> /* for scsi_safe_file_access() */
#include <scsi/scsi_device.h>
#include <scsi/scsi_request.h>

@@ -363,4 +365,21 @@ static inline unsigned scsi_transfer_length(struct scsi_cmnd *scmd)
return xfer_len;
}

+/*
+ * 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 scsi_safe_file_access(struct file *filp)
+{
+ return filp->f_cred == current_cred() && !uaccess_kernel();
+}
+
#endif /* _SCSI_SCSI_CMND_H */
--
2.18.0.rc1.244.gcf134e6275-goog