Re: [PATCH v3] sg: mitigate read/write abuse

[Douglas Gilbert <dgilbert@xxxxxxxxxxxx>]

On 2018-06-25 04:25 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.
>
> changed in v2:
>   - remove the bsg parts per Christoph Hellwig's request
>
> changed in v3:
>   - move error messages into helper function
>   - use two different error messages and return values (Douglas Gilbert)
>   - add comment on stranded responses (Douglas Gilbert)
>   - use current_real_cred() instead of current_cred() (so that
>     override_creds() can't bypass this check)
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>   drivers/scsi/sg.c | 42 ++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index 53ae52dbff84..4f4e88ca8213 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() */

s/_safe_/_check_/

>   
>   #include "scsi.h"
>   #include <scsi/scsi_dbg.h>
> @@ -209,6 +210,33 @@ 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 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 (uaccess_kernel()) {
> +		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;
> @@ -393,6 +421,14 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
>   	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, sg_printk(KERN_INFO, sdp,
> @@ -580,9 +616,11 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
>   	struct sg_header old_hdr;
>   	sg_io_hdr_t *hp;
>   	unsigned char cmnd[SG_MAX_CDB_SIZE];
> +	int retval;
>   
> -	if (unlikely(uaccess_kernel()))
> -		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;

If you need to make any other changes to this patch, then you could fix
that typo above.

Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>