diff mbox series

[RFC,7/9] lsm,io_uring: add LSM hooks to io_uring

Message ID 162163382536.8379.3124023175473604584.stgit@sifl (mailing list archive)
State New
Headers show
Series Add LSM access controls and auditing to io_uring | expand

Commit Message

Paul Moore May 21, 2021, 9:50 p.m. UTC
WARNING - This is a work in progress, this patch, including the
description, may be incomplete or even incorrect.  You have been
warned.

A full expalantion of io_uring is beyond the scope of this commit
description, but in summary it is an asynchronous I/O mechanism
which allows for I/O requests and the resulting data to be queued
in memory mapped "rings" which are shared between the kernel and
userspace.  Optionally, io_uring offers the ability for applications
to spawn kernel threads to dequeue I/O requests from the ring and
submit the requests in the kernel, helping to minimize the syscall
overhead.  Rings are accessed in userspace by memory mapping a file
descriptor provided by the io_uring_setup(2), and can be shared
between applications as one might do with any open file descriptor.
Finally, process credentials can be registered with a given ring
and any process with access to that ring can submit I/O requests
using any of the registered credentials.

While the io_uring functionality is widely recognized as offering a
vastly improved, and high performing asynchronous I/O mechanism, its
ability to allow processes to submit I/O requests with credentials
other than its own presents a challenge to LSMs.  When a process
creates a new io_uring ring the ring's credentials are inhertied
from the calling process; if this ring is shared with another
process operating with different credentials there is the potential
to bypass the LSMs security policy.  Similarly, registering
credentials with a given ring allows any process with access to that
ring to submit I/O requests with those credentials.

In an effort to allow LSMs to apply security policy to io_uring I/O
operations, this patch adds two new LSM hooks.  These hooks, in
conjunction with the LSM anonymous inode support previously
submitted, allow an LSM to apply access control policy to the
sharing of io_uring rings as well as any io_uring credential changes
requested by a process.

The new LSM hooks are described below:

 * int security_uring_override_creds(cred)
   Controls if the current task, executing an io_uring operation,
   is allowed to override it's credentials with @cred.  In cases
   where the current task is a user application, the current
   credentials will be those of the user application.  In cases
   where the current task is a kernel thread servicing io_uring
   requests the current credentials will be those of the io_uring
   ring (inherited from the process that created the ring).

 * int security_uring_sqpoll(void)
   Controls if the current task is allowed to create an io_uring
   polling thread (IORING_SETUP_SQPOLL).  Without a SQPOLL thread
   in the kernel processes must submit I/O requests via
   io_uring_enter(2) which allows us to compare any requested
   credential changes against the application making the request.
   With a SQPOLL thread, we can no longer compare requested
   credential changes against the application making the request,
   the comparison is made against the ring's credentials.

Signed-off-by: Paul Moore <paul@paul-moore.com>
---
 fs/io_uring.c                 |   10 ++++++++++
 include/linux/lsm_hook_defs.h |    5 +++++
 include/linux/lsm_hooks.h     |   13 +++++++++++++
 include/linux/security.h      |   16 ++++++++++++++++
 security/security.c           |   12 ++++++++++++
 5 files changed, 56 insertions(+)

Comments

Stefan Metzmacher May 26, 2021, 2:48 p.m. UTC | #1
Hi Paul,

>  #define CREATE_TRACE_POINTS
>  #include <trace/events/io_uring.h>
> @@ -6537,6 +6538,11 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
>  		if (!req->work.creds)
>  			return -EINVAL;
>  		get_cred(req->work.creds);
> +		ret = security_uring_override_creds(req->work.creds);
> +		if (ret) {
> +			put_cred(req->work.creds);
> +			return ret;
> +		}

Why are you calling this per requests, shouldn't this be done in
io_register_personality()?

I'm also not sure if this really gains anything as io_register_personality()
only captures the value of get_current_cred(), so the process already has changed to
the credentials (at least once for the io_uring_register(IORING_REGISTER_PERSONALITY)
call).

metze
Paul Moore May 26, 2021, 8:45 p.m. UTC | #2
On Wed, May 26, 2021 at 10:48 AM Stefan Metzmacher <metze@samba.org> wrote:
>
> Hi Paul,

Hi Stefan.

> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/io_uring.h>
> > @@ -6537,6 +6538,11 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
> >               if (!req->work.creds)
> >                       return -EINVAL;
> >               get_cred(req->work.creds);
> > +             ret = security_uring_override_creds(req->work.creds);
> > +             if (ret) {
> > +                     put_cred(req->work.creds);
> > +                     return ret;
> > +             }
>
> Why are you calling this per requests, shouldn't this be done in
> io_register_personality()?

Generally speaking it is more interesting to see when user alice tries
to impersonate bob and not when bob registers his ID as available to
use by others.  We could always add a LSM hook to control when bob
registers his ID, but I think the impersonation is the critical code
path.

However, if I'm misunderstanding how this works in io_uring please correct me.

> I'm also not sure if this really gains anything as io_register_personality()
> only captures the value of get_current_cred(), so the process already has changed to
> the credentials (at least once for the io_uring_register(IORING_REGISTER_PERSONALITY)
> call).
>
> metze
diff mbox series

Patch

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6ff769c9b7d3..d18a594c4c6e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -79,6 +79,7 @@ 
 #include <linux/pagemap.h>
 #include <linux/io_uring.h>
 #include <linux/audit.h>
+#include <linux/security.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/io_uring.h>
@@ -6537,6 +6538,11 @@  static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		if (!req->work.creds)
 			return -EINVAL;
 		get_cred(req->work.creds);
+		ret = security_uring_override_creds(req->work.creds);
+		if (ret) {
+			put_cred(req->work.creds);
+			return ret;
+		}
 	}
 	state = &ctx->submit_state;
 
@@ -7963,6 +7969,10 @@  static int io_sq_offload_create(struct io_ring_ctx *ctx,
 		struct io_sq_data *sqd;
 		bool attached;
 
+		ret = security_uring_sqpoll();
+		if (ret)
+			return ret;
+
 		sqd = io_get_sq_data(p, &attached);
 		if (IS_ERR(sqd)) {
 			ret = PTR_ERR(sqd);
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 04c01794de83..88971b3da3c0 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -403,3 +403,8 @@  LSM_HOOK(void, LSM_RET_VOID, perf_event_free, struct perf_event *event)
 LSM_HOOK(int, 0, perf_event_read, struct perf_event *event)
 LSM_HOOK(int, 0, perf_event_write, struct perf_event *event)
 #endif /* CONFIG_PERF_EVENTS */
+
+#ifdef CONFIG_IO_URING
+LSM_HOOK(int, 0, uring_override_creds, const struct cred *new)
+LSM_HOOK(int, 0, uring_sqpoll, void)
+#endif /* CONFIG_IO_URING */
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 5c4c5c0602cb..0eb0ae95c4c4 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1557,6 +1557,19 @@ 
  * 	Read perf_event security info if allowed.
  * @perf_event_write:
  * 	Write perf_event security info if allowed.
+ *
+ * Security hooks for io_uring
+ *
+ * @uring_override_creds:
+ *      Check if the current task, executing an io_uring operation, is allowed
+ *      to override it's credentials with @new.
+ *
+ *      @new: the new creds to use
+ *
+ * @uring_sqpoll:
+ *      Check whether the current task is allowed to spawn a io_uring polling
+ *      thread (IORING_SETUP_SQPOLL).
+ *
  */
 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 06f7c50ce77f..263a744c839f 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -2037,4 +2037,20 @@  static inline int security_perf_event_write(struct perf_event *event)
 #endif /* CONFIG_SECURITY */
 #endif /* CONFIG_PERF_EVENTS */
 
+#ifdef CONFIG_IO_URING
+#ifdef CONFIG_SECURITY
+extern int security_uring_override_creds(const struct cred *new);
+extern int security_uring_sqpoll(void);
+#else
+static inline int security_uring_override_creds(const struct cred *new)
+{
+	return 0;
+}
+static inline int security_uring_sqpoll(void)
+{
+	return 0;
+}
+#endif /* CONFIG_SECURITY */
+#endif /* CONFIG_IO_URING */
+
 #endif /* ! __LINUX_SECURITY_H */
diff --git a/security/security.c b/security/security.c
index b38155b2de83..3d6b3a2cacf5 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2624,3 +2624,15 @@  int security_perf_event_write(struct perf_event *event)
 	return call_int_hook(perf_event_write, 0, event);
 }
 #endif /* CONFIG_PERF_EVENTS */
+
+#ifdef CONFIG_IO_URING
+int security_uring_override_creds(const struct cred *new)
+{
+	return call_int_hook(uring_override_creds, 0, new);
+}
+
+int security_uring_sqpoll(void)
+{
+	return call_int_hook(uring_sqpoll, 0);
+}
+#endif /* CONFIG_IO_URING */