Message-ID: <06d662b8-89f1-458f-bc09-519c9442c63d@interlog.com>
Date:   Sun, 24 Jun 2018 00:05:48 +0200
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     jejb@...ux.vnet.ibm.com, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        kernel list <linux-kernel@...r.kernel.org>, axboe@...nel.dk,
        fujita.tomonori@....ntt.co.jp,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        security@...nel.org, bblock@...ux.vnet.ibm.com
Subject: Re: [PATCH v2] sg: mitigate read/write abuse

On 2018-06-21 08:56 PM, Jann Horn wrote:
> On Thu, Jun 21, 2018 at 6:53 PM Douglas Gilbert <dgilbert@...erlog.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@...r.kernel.org>
>>> Signed-off-by: Jann Horn <jannh@...gle.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;
>>>
>>
> 

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