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SubjectRe: [PATCH] sg: Fix a double-fetch bug in drivers/scsi/sg.c
On Wed, Jun 05, 2019 at 01:07:25PM -0400, Douglas Gilbert wrote:
> On 2019-06-05 2:00 a.m., Jiri Slaby wrote:
> >On 23. 05. 19, 4:38, Gen Zhang wrote:
> >>In sg_write(), the opcode of the command is fetched the first time from
> >>the userspace by __get_user(). Then the whole command, the opcode
> >>included, is fetched again from userspace by __copy_from_user().
> >>However, a malicious user can change the opcode between the two fetches.
> >>This can cause inconsistent data and potential errors as cmnd is used in
> >>the following codes.
> >>
> >>Thus we should check opcode between the two fetches to prevent this.
> >>
> >>Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
> >>---
> >>diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> >>index d3f1531..a2971b8 100644
> >>--- a/drivers/scsi/sg.c
> >>+++ b/drivers/scsi/sg.c
> >>@@ -694,6 +694,8 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
> >> hp->flags = input_size; /* structure abuse ... */
> >> hp->pack_id = old_hdr.pack_id;
> >> hp->usr_ptr = NULL;
> >>+ if (opcode != cmnd[0])
> >>+ return -EINVAL;
> >
> >Isn't it too early to check cmnd which is copied only here:
> >
> >> if (__copy_from_user(cmnd, buf, cmd_size))
> >> return -EFAULT;
> >> /*
> >>---
> >>
>
> Hi,
> Yes, it is too early. It needs to be after that __copy_from_user(cmnd,
> buf, cmd_size) call.
Yes, it is.
>
> To put this in context, this is a very old interface; dating from 1992
> and deprecated for almost 20 years. The fact that the first byte of
> the SCSI cdb needs to be read first to work out that size of the
> following SCSI command and optionally the offset of a data-out
> buffer that may follow the command; is one reason why that interface
> was replaced. Also the implementation did not handle SCSI variable
> length cdb_s.
>
> Then there is the question of whether this double-fetch is exploitable?
> I cannot think of an example, but there might be (e.g. turning a READ
> command into a WRITE). But the "double-fetch" issue may be more wide
> spread. The replacement interface passes the command and data-in/-out as
> pointers while their corresponding lengths are placed in the newer
> interface structure. This assumes that the cdb and data-out won't
> change in the user space between when the write(2) is called and
> before or while the driver, using those pointers, reads the data.
> All drivers that use pointers to pass data have this "feature".
>
> Also I'm looking at this particular double-fetch from the point of view
> of the driver rewrite I have done and is currently in the early stages
> of review [linux-scsi list: "[PATCH 00/19] sg: v4 interface, rq sharing
> + multiple rqs"] and this problem is more difficult to fix since the
> full cdb read is delayed to a common point further along the submit
> processing path. To detect a change in cbd[0] my current code would
> need to be altered to carry cdb[0] through to that common point. So
> is it worth it for such an old, deprecated and replaced interface??
> What cdb/user_permissions checking that is done, is done _after_
> the full cdb is read. So trying to get around a user exclusion of
> say WRITE(10) by first using the first byte of READ(10), won't succeed.
>
> Doug Gilbert
>
Thanks for your explaination. I guess this patch should be dropped for
the above reasons.

Thanks
Gen

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