diff options
author | Rodrigo Campos <rodrigo@kinvolk.io> | 2021-05-17 12:39:07 -0700 |
---|---|---|
committer | Kees Cook <keescook@chromium.org> | 2021-06-28 12:49:52 -0700 |
commit | 0ae71c7720e3ae3aabd2e8a072d27f7bd173d25c (patch) | |
tree | 6508fb49112c155cd9113a62fb7968c586f1f4d0 /include/uapi | |
parent | ddc473916955f7710d1eb17c1273d91c8622a9fe (diff) |
seccomp: Support atomic "addfd + send reply"
Alban Crequy reported a race condition userspace faces when we want to
add some fds and make the syscall return them[1] using seccomp notify.
The problem is that currently two different ioctl() calls are needed by
the process handling the syscalls (agent) for another userspace process
(target): SECCOMP_IOCTL_NOTIF_ADDFD to allocate the fd and
SECCOMP_IOCTL_NOTIF_SEND to return that value. Therefore, it is possible
for the agent to do the first ioctl to add a file descriptor but the
target is interrupted (EINTR) before the agent does the second ioctl()
call.
This patch adds a flag to the ADDFD ioctl() so it adds the fd and
returns that value atomically to the target program, as suggested by
Kees Cook[2]. This is done by simply allowing
seccomp_do_user_notification() to add the fd and return it in this case.
Therefore, in this case the target wakes up from the wait in
seccomp_do_user_notification() either to interrupt the syscall or to add
the fd and return it.
This "allocate an fd and return" functionality is useful for syscalls
that return a file descriptor only, like connect(2). Other syscalls that
return a file descriptor but not as return value (or return more than
one fd), like socketpair(), pipe(), recvmsg with SCM_RIGHTs, will not
work with this flag.
This effectively combines SECCOMP_IOCTL_NOTIF_ADDFD and
SECCOMP_IOCTL_NOTIF_SEND into an atomic opteration. The notification's
return value, nor error can be set by the user. Upon successful invocation
of the SECCOMP_IOCTL_NOTIF_ADDFD ioctl with the SECCOMP_ADDFD_FLAG_SEND
flag, the notifying process's errno will be 0, and the return value will
be the file descriptor number that was installed.
[1]: https://lore.kernel.org/lkml/CADZs7q4sw71iNHmV8EOOXhUKJMORPzF7thraxZYddTZsxta-KQ@mail.gmail.com/
[2]: https://lore.kernel.org/lkml/202012011322.26DCBC64F2@keescook/
Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210517193908.3113-4-sargun@sargun.me
Diffstat (limited to 'include/uapi')
-rw-r--r-- | include/uapi/linux/seccomp.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h index 6ba18b82a02e..78074254ab98 100644 --- a/include/uapi/linux/seccomp.h +++ b/include/uapi/linux/seccomp.h @@ -115,6 +115,7 @@ struct seccomp_notif_resp { /* valid flags for seccomp_notif_addfd */ #define SECCOMP_ADDFD_FLAG_SETFD (1UL << 0) /* Specify remote fd */ +#define SECCOMP_ADDFD_FLAG_SEND (1UL << 1) /* Addfd and return it, atomically */ /** * struct seccomp_notif_addfd |