io_uring: enable audit and restrict cred override for IORING_OP_FIXED_FD_INSTALL

We need to correct some aspects of the IORING_OP_FIXED_FD_INSTALL
command to take into account the security implications of making an
io_uring-private file descriptor generally accessible to a userspace
task.

The first change in this patch is to enable auditing of the FD_INSTALL
operation as installing a file descriptor into a task's file descriptor
table is a security relevant operation and something that admins/users
may want to audit.

The second change is to disable the io_uring credential override
functionality, also known as io_uring "personalities", in the
FD_INSTALL command.  The credential override in FD_INSTALL is
particularly problematic as it affects the credentials used in the
security_file_receive() LSM hook.  If a task were to request a
credential override via REQ_F_CREDS on a FD_INSTALL operation, the LSM
would incorrectly check to see if the overridden credentials of the
io_uring were able to "receive" the file as opposed to the task's
credentials.  After discussions upstream, it's difficult to imagine a
use case where we would want to allow a credential override on a
FD_INSTALL operation so we are simply going to block REQ_F_CREDS on
IORING_OP_FIXED_FD_INSTALL operations.

Fixes: dc18b89 ("io_uring/openclose: add support for IORING_OP_FIXED_FD_INSTALL")
Signed-off-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20240123215501.289566-2-paul@paul-moore.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: pcmoore

io_uring/opdef.c

@@ -471,7 +471,6 @@ const struct io_issue_def io_issue_defs[] = {
 	},
 	[IORING_OP_FIXED_FD_INSTALL] = {
 		.needs_file		= 1,
-		.audit_skip		= 1,
 		.prep			= io_install_fixed_fd_prep,
 		.issue			= io_install_fixed_fd,
 	},

io_uring/openclose.c

@@ -277,6 +277,10 @@ int io_install_fixed_fd_prep(struct io_kiocb *req, const struct io_uring_sqe *sq
 	if (flags & ~IORING_FIXED_FD_NO_CLOEXEC)
 		return -EINVAL;
 
+	/* ensure the task's creds are used when installing/receiving fds */
+	if (req->flags & REQ_F_CREDS)
+		return -EPERM;
+
 	/* default to O_CLOEXEC, disable if IORING_FIXED_FD_NO_CLOEXEC is set */
 	ifi = io_kiocb_to_cmd(req, struct io_fixed_install);
 	ifi->o_flags = O_CLOEXEC;