On 7/15/2022 11:46 AM, Paul Moore wrote:

On Thu, Jul 14, 2022 at 9:00 PM Luis Chamberlain [off-list ref] wrote:

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On Wed, Jul 13, 2022 at 11:00:42PM -0400, Paul Moore wrote:

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On Wed, Jul 13, 2022 at 8:05 PM Luis Chamberlain [off-list ref] wrote:

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io-uring cmd support was added through ee692a21e9bf ("fs,io_uring:
add infrastructure for uring-cmd"), this extended the struct
file_operations to allow a new command which each subsystem can use
to enable command passthrough. Add an LSM specific for the command
passthrough which enables LSMs to inspect the command details.

This was discussed long ago without no clear pointer for something
conclusive, so this enables LSMs to at least reject this new file
operation.

[0] https://lkml.kernel.org/r/8adf55db-7bab-f59d-d612-ed906b948d19@schaufler-ca.com

[NOTE: I now see that the IORING_OP_URING_CMD has made it into the
v5.19-rcX releases, I'm going to be honest and say that I'm
disappointed you didn't post the related LSM additions

It does not mean I didn't ask for them too.

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until
v5.19-rc6, especially given our earlier discussions.]

And hence since I don't see it either, it's on us now.

It looks like I owe you an apology, Luis.  While my frustration over
io_uring remains, along with my disappointment that the io_uring
developers continue to avoid discussing access controls with the LSM
community, you are not the author of the IORING_OP_URING_CMD.   You
are simply trying to do the right thing by adding the necessary LSM
controls and in my confusion I likely caused you a bit of frustration;
I'm sorry for that.

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As important as I think LSMs are, I cannot convince everyone
to take them as serious as I do.

Yes, I think a lot of us are familiar with that feeling unfortunately :/

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While the earlier discussion may not have offered a detailed approach
on how to solve this, I think it was rather conclusive in that the
approach used then (and reproduced here) did not provide enough
context to the LSMs to be able to make a decision.

Right...

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There were similar
concerns when it came to auditing the command passthrough.  It appears
that most of my concerns in the original thread still apply to this
patch.

Given the LSM hook in this patch, it is very difficult (impossible?)
to determine the requested operation as these command opcodes are
device/subsystem specific.  The unfortunate result is that the LSMs
are likely going to either allow all, or none, of the commands for a
given device/subsystem, and I think we can all agree that is not a
good idea.

That is the critical bit of feedback on this patch, but there is more
feedback inline below.

Given a clear solution is not easily tangible at this point
I was hoping perhaps at least the abilility to enable LSMs to
reject uring-cmd would be better than nothing at this point.

Without any cooperation from the io_uring developers, that is likely
what we will have to do.  I know there was a lot of talk about this
functionality not being like another ioctl(), but from a LSM
perspective I think that is how we will need to treat it.

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Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 include/linux/lsm_hook_defs.h | 1 +
 include/linux/lsm_hooks.h     | 3 +++
 include/linux/security.h      | 5 +++++
 io_uring/uring_cmd.c          | 5 +++++
 security/security.c           | 4 ++++
 5 files changed, 18 insertions(+)

...

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diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index 0a421ed51e7e..5e666aa7edb8 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c

@@ -3,6 +3,7 @@
 #include <linux/errno.h>
 #include <linux/file.h>
 #include <linux/io_uring.h>
+#include <linux/security.h>

 #include <uapi/linux/io_uring.h>

@@ -82,6 +83,10 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
        struct file *file = req->file;
        int ret;

+       ret = security_uring_cmd(ioucmd);
+       if (ret)
+               return ret;
+
        if (!req->file->f_op->uring_cmd)
                return -EOPNOTSUPP;

In order to be consistent with most of the other LSM hooks, the
'req->file->f_op->uring_cmd' check should come before the LSM hook
call.

Sure.

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The general approach used in most places is to first validate
the request and do any DAC based access checks before calling into the
LSM.

OK.

Let me know how you'd like to proceed given our current status.

Well, we're at -rc6 right now which means IORING_OP_URING_CMD is
happening and it's unlikely the LSM folks are going to be able to
influence the design/implementation much at this point so we have to
do the best we can.  Given the existing constraints, I think your
patch is reasonable (although please do shift the hook call site down
a bit as discussed above), we just need to develop the LSM
implementations to go along with it.

Luis, can you respin and resend the patch with the requested changes?

Casey, it looks like Smack and SELinux are the only LSMs to implement
io_uring access controls.  Given the hook that Luis developed in this
patch, could you draft a patch for Smack to add the necessary checks?

Yes. I don't think it will be anything more sophisticated than the
existing "Brutalist" Smack support. It will also be tested to the
limited extent my resources and understanding of io_uring allow.

I am seriously concerned that without better integration between
LSM and io_uring development I'm going to end up in the same place
that led to Al Viro's comment regarding the Smack fcntl hooks:

	"I think I have an adequate flame, but it won't fit
	the maillist size limit..."

That came about because my understanding of fnctl() was incomplete.
I know a lot more about fnctl than I do about io_uring. I would
really like io_uring to work well in a Smack environment. It saddens
me that there isn't any reciporicol interest. But enough whinging.
On to the patch.

I'll do the same for SELinux.  My initial thinking is that all we can
really do is check the access between the creds on the current task
(any overrides will have already taken place by the time the LSM hook
is called) with the io_uring_cmd:file label/creds; we won't be able to
provide much permission granularity for all the reasons previously
discussed, but I suspect that will be more of a SELinux problem than a
Smack problem (although I suspect Smack will need to treat this as
both a read and a write, which is likely less than ideal).

I think it's doubtful we will have all of this ready and tested in
time for v5.19, but I think we can have it ready shortly after that
and I'll mark all of the patches for -stable when I send them to
Linus.

I also think we should mark the patches with a 'Fixes:' line that
points at the IORING_OP_URING_CMD commit, ee692a21e9bf ("fs,io_uring:
add infrastructure for uring-cmd").

How does that sound to everyone?