[v2] sg: mitigate read/write abuse
diff mbox

Message ID 20180621151809.10921-1-jannh@google.com
State New
Headers show

Commit Message

Jann Horn June 21, 2018, 3:18 p.m. UTC
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@vger.kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
---
 drivers/scsi/sg.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

Comments

Douglas Gilbert June 21, 2018, 4:53 p.m. UTC | #1
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@vger.kernel.org>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>   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;
>
Jann Horn June 21, 2018, 6:56 p.m. UTC | #2
On Thu, Jun 21, 2018 at 6:53 PM Douglas Gilbert <dgilbert@interlog.com> 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@vger.kernel.org>
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> >   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)?

> 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;
> >
>
Douglas Gilbert June 23, 2018, 10:05 p.m. UTC | #3
On 2018-06-21 08:56 PM, Jann Horn wrote:
> On Thu, Jun 21, 2018 at 6:53 PM Douglas Gilbert <dgilbert@interlog.com> 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@vger.kernel.org>
>>> Signed-off-by: Jann Horn <jannh@google.com>
>>> ---
>>>    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;
>>>
>>
>
Andy Lutomirski June 25, 2018, 12:46 a.m. UTC | #4
On Sat, Jun 23, 2018 at 3:06 PM Douglas Gilbert <dgilbert@interlog.com> wrote:
>
> On 2018-06-21 08:56 PM, Jann Horn wrote:
> > On Thu, Jun 21, 2018 at 6:53 PM Douglas Gilbert <dgilbert@interlog.com> 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@vger.kernel.org>
> >>> Signed-off-by: Jann Horn <jannh@google.com>
> >>> ---
> >>>    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
>

Usually EPERM means the caller doesn't have access and EACCES means
the fd doesn't have access.

What we really want is -EDRIVERISAPIECEOFCRAP.

--Andy

Patch
diff mbox

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;
+	}
+
 	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;
+	}
 
 	if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
 		return -ENXIO;