Re: [PATCH 03/17] io_uring: add infra and support for IORING_OP_URING_CMD

Casey Schaufler
2022-03-14T18:05:59Z

On 3/14/2022 9:32 AM, Luis Chamberlain wrote:
> On Mon, Mar 14, 2022 at 09:25:35AM -0700, Casey Schaufler wrote:
>> On 3/11/2022 9:11 AM, Luis Chamberlain wrote:
>>> On Thu, Mar 10, 2022 at 07:43:04PM -0700, Jens Axboe wrote:
>>>> On 3/10/22 6:51 PM, Luis Chamberlain wrote:
>>>>> On Tue, Mar 08, 2022 at 08:50:51PM +0530, Kanchan Joshi wrote:
>>>>>> From: Jens Axboe <axboe@kernel.dk>
>>>>>>
>>>>>> This is a file private kind of request. io_uring doesn't know what's
>>>>>> in this command type, it's for the file_operations->async_cmd()
>>>>>> handler to deal with.
>>>>>>
>>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
>>>>>> ---
>>>>> <-- snip -->
>>>>>
>>>>>> +static int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
>>>>>> +{
>>>>>> + struct file *file = req->file;
>>>>>> + int ret;
>>>>>> + struct io_uring_cmd *ioucmd = &req->uring_cmd;
>>>>>> +
>>>>>> + ioucmd->flags |= issue_flags;
>>>>>> + ret = file->f_op->async_cmd(ioucmd);
>>>>> I think we're going to have to add a security_file_async_cmd() check
>>>>> before this call here. Because otherwise we're enabling to, for
>>>>> example, bypass security_file_ioctl() for example using the new
>>>>> iouring-cmd interface.
>>>>>
>>>>> Or is this already thought out with the existing security_uring_*() stuff?
>>>> Unless the request sets .audit_skip, it'll be included already in terms
>>>> of logging.
>>> Neat.
>>>
>>>> But I'd prefer not to lodge this in with ioctls, unless
>>>> we're going to be doing actual ioctls.
>>> Oh sure, I have been an advocate to ensure folks don't conflate async_cmd
>>> with ioctl. However it *can* enable subsystems to enable ioctl
>>> passthrough, but each of those subsystems need to vet for this on their
>>> own terms. I'd hate to see / hear some LSM surprises later.
>>>
>>>> But definitely something to keep in mind and make sure that we're under
>>>> the right umbrella in terms of auditing and security.
>>> Paul, how about something like this for starters (and probably should
>>> be squashed into this series so its not a separate commit) ?
>>>
>>> >From f3ddbe822374cc1c7002bd795c1ae486d370cbd1 Mon Sep 17 00:00:00 2001
>>> From: Luis Chamberlain <mcgrof@kernel.org>
>>> Date: Fri, 11 Mar 2022 08:55:50 -0800
>>> Subject: [PATCH] lsm,io_uring: add LSM hooks to for the new async_cmd file op
>>>
>>> io-uring is extending 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.
>>>
>>> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
>>> ---
>>> fs/io_uring.c | 5 +++++
>>> include/linux/lsm_hook_defs.h | 1 +
>>> include/linux/lsm_hooks.h | 3 +++
>>> include/linux/security.h | 5 +++++
>>> security/security.c | 4 ++++
>>> 5 files changed, 18 insertions(+)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 3f6eacc98e31..1c4e6b2cb61a 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -4190,6 +4190,11 @@ static int io_uring_cmd_prep(struct io_kiocb *req,
>>> struct io_ring_ctx *ctx = req->ctx;
>>> struct io_uring_cmd *ioucmd = &req->uring_cmd;
>>> u32 ucmd_flags = READ_ONCE(sqe->uring_cmd_flags);
>>> + int ret;
>>> +
>>> + ret = security_uring_async_cmd(ioucmd);
>>> + if (ret)
>>> + return ret;
>>> if (!req->file->f_op->async_cmd)
>>> return -EOPNOTSUPP;
>>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>>> index 819ec92dc2a8..4a20f8e6b295 100644
>>> --- a/include/linux/lsm_hook_defs.h
>>> +++ b/include/linux/lsm_hook_defs.h
>>> @@ -404,4 +404,5 @@ LSM_HOOK(int, 0, perf_event_write, struct perf_event *event)
>>> #ifdef CONFIG_IO_URING
>>> LSM_HOOK(int, 0, uring_override_creds, const struct cred *new)
>>> LSM_HOOK(int, 0, uring_sqpoll, void)
>>> +LSM_HOOK(int, 0, uring_async_cmd, struct io_uring_cmd *ioucmd)
>>> #endif /* CONFIG_IO_URING */
>>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>>> index 3bf5c658bc44..21b18cf138c2 100644
>>> --- a/include/linux/lsm_hooks.h
>>> +++ b/include/linux/lsm_hooks.h
>>> @@ -1569,6 +1569,9 @@
>>> * Check whether the current task is allowed to spawn a io_uring polling
>>> * thread (IORING_SETUP_SQPOLL).
>>> *
>>> + * @uring_async_cmd:
>>> + * Check whether the file_operations async_cmd is allowed to run.
>>> + *
>>> */
>>> union security_list_options {
>>> #define LSM_HOOK(RET, DEFAULT, NAME, ...) RET (*NAME)(__VA_ARGS__);
>>> diff --git a/include/linux/security.h b/include/linux/security.h
>>> index 6d72772182c8..4d7f72813d75 100644
>>> --- a/include/linux/security.h
>>> +++ b/include/linux/security.h
>>> @@ -2041,6 +2041,7 @@ static inline int security_perf_event_write(struct perf_event *event)
>>> #ifdef CONFIG_SECURITY
>>> extern int security_uring_override_creds(const struct cred *new);
>>> extern int security_uring_sqpoll(void);
>>> +extern int security_uring_async_cmd(struct io_uring_cmd *ioucmd);
>>> #else
>>> static inline int security_uring_override_creds(const struct cred *new)
>>> {
>>> @@ -2050,6 +2051,10 @@ static inline int security_uring_sqpoll(void)
>>> {
>>> return 0;
>>> }
>>> +static inline int security_uring_async_cmd(struct io_uring_cmd *ioucmd)
>>> +{
>>> + return 0;
>>> +}
>>> #endif /* CONFIG_SECURITY */
>>> #endif /* CONFIG_IO_URING */
>>> diff --git a/security/security.c b/security/security.c
>>> index 22261d79f333..ef96be2f953a 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -2640,4 +2640,8 @@ int security_uring_sqpoll(void)
>>> {
>>> return call_int_hook(uring_sqpoll, 0);
>>> }
>>> +int security_uring_async_cmd(struct io_uring_cmd *ioucmd)
>>> +{
>>> + return call_int_hook(uring_async_cmd, 0, ioucmd);
>> I don't have a good understanding of what information is in ioucmd.
>> I am afraid that there may not be enough for a security module to
>> make appropriate decisions in all cases. I am especially concerned
>> about the modules that use path hooks, but based on the issues we've
>> always had with ioctl and the like I fear for all cases.
> As Paul pointed out, this particular LSM hook would not be needed if we can
> somehow ensure users of the cmd path use their respective LSMs there. It
> is not easy to force users to have the LSM hook to be used, one idea
> might be to have a registration mechanism which allows users to also
> specify the LSM hook, but these can vary in arguments, so perhaps then
> what is needed is the LSM type in enum form, and internally we have a
> mapping of these. That way we slowly itemize which cmds we *do* allow
> for, thus vetting at the same time a respective LSM hook. Thoughts?

tl;dr - Yuck.

I don't see how your registration mechanism would be easier than
getting "users of the cmd path" to use the LSM mechanism the way
everyone else does. What it would do is pass responsibility for
dealing with LSM to the io_uring core team. Experience has shown
that dealing with the security issues after the fact is much
harder than doing it up front, even when developers wail about
the burden. Sure, LSM is an unpleasant interface/mechanism, but
so is locking, and no one gets away without addressing that.
My $0.02.

>
> Luis