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: dc18b89ab113 ("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>

diff --git a/io_uring/opdef.c b/io_uring/opdef.c
index 6705634e5f52aa625b797f3c7931903c55130359..b1ee3a9c38072933dd020848b7f3586af257d912 100644 (file)
--- a/io_uring/opdef.c
+++ b/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,
        },

diff --git a/io_uring/openclose.c b/io_uring/openclose.c
index 0fe0dd30554623edb87cd159b4ffe0c52288211a..e3357dfa14ca42dd5b25e6cf9ce4a4be8b7ee0f4 100644 (file)
--- a/io_uring/openclose.c
+++ b/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;