Re: [PATCH v2] sg: mitigate read/write abuse

[Douglas Gilbert]

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.

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.
 */

> +	}
> +
>   	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;