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Message-ID: <20200707114103.lkfbt3kdtturp42z@wittgenstein>
Date: Tue, 7 Jul 2020 13:41:03 +0200
From: Christian Brauner <christian.brauner@...ntu.com>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, Sargun Dhillon <sargun@...gun.me>,
Christian Brauner <christian@...uner.io>,
Tycho Andersen <tycho@...ho.ws>,
David Laight <David.Laight@...LAB.COM>,
Christoph Hellwig <hch@....de>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Aleksa Sarai <cyphar@...har.com>,
Matt Denton <mpdenton@...gle.com>,
Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
Robert Sesek <rsesek@...gle.com>,
Giuseppe Scrivano <gscrivan@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Andy Lutomirski <luto@...capital.net>,
Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
netdev@...r.kernel.org, containers@...ts.linux-foundation.org,
linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 1/7] net/scm: Regularize compat handling of
scm_detach_fds()
On Mon, Jul 06, 2020 at 01:17:14PM -0700, Kees Cook wrote:
> Duplicate the cleanups from commit 2618d530dd8b ("net/scm: cleanup
> scm_detach_fds") into the compat code.
>
> Move the check added in commit 1f466e1f15cf ("net: cleanly handle kernel
> vs user buffers for ->msg_control") to before the compat call, even
> though it should be impossible for an in-kernel call to also be compat.
>
> Correct the int "flags" argument to unsigned int to match fd_install()
> and similar APIs.
>
> Regularize any remaining differences, including a whitespace issue,
> a checkpatch warning, and add the check from commit 6900317f5eff ("net,
> scm: fix PaX detected msg_controllen overflow in scm_detach_fds") which
> fixed an overflow unique to 64-bit. To avoid confusion when comparing
> the compat handler to the native handler, just include the same check
> in the compat handler.
>
> Fixes: 48a87cc26c13 ("net: netprio: fd passed in SCM_RIGHTS datagram not set correctly")
> Fixes: d84295067fc7 ("net: net_cls: fd passed in SCM_RIGHTS datagram not set correctly")
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
Thanks. Just a comment below.
Acked-by: Christian Brauner <christian.brauner@...ntu.com>
> include/net/scm.h | 1 +
> net/compat.c | 55 +++++++++++++++++++++--------------------------
> net/core/scm.c | 18 ++++++++--------
> 3 files changed, 35 insertions(+), 39 deletions(-)
>
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 1ce365f4c256..581a94d6c613 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -37,6 +37,7 @@ struct scm_cookie {
> #endif
> };
>
> +int __scm_install_fd(struct file *file, int __user *ufd, unsigned int o_flags);
> void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
> void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
> int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);
> diff --git a/net/compat.c b/net/compat.c
> index 5e3041a2c37d..27d477fdcaa0 100644
> --- a/net/compat.c
> +++ b/net/compat.c
> @@ -281,39 +281,31 @@ int put_cmsg_compat(struct msghdr *kmsg, int level, int type, int len, void *dat
> return 0;
> }
>
> -void scm_detach_fds_compat(struct msghdr *kmsg, struct scm_cookie *scm)
> +static int scm_max_fds_compat(struct msghdr *msg)
> {
> - struct compat_cmsghdr __user *cm = (struct compat_cmsghdr __user *) kmsg->msg_control;
> - int fdmax = (kmsg->msg_controllen - sizeof(struct compat_cmsghdr)) / sizeof(int);
> - int fdnum = scm->fp->count;
> - struct file **fp = scm->fp->fp;
> - int __user *cmfptr;
> - int err = 0, i;
> + if (msg->msg_controllen <= sizeof(struct compat_cmsghdr))
> + return 0;
> + return (msg->msg_controllen - sizeof(struct compat_cmsghdr)) / sizeof(int);
> +}
>
> - if (fdnum < fdmax)
> - fdmax = fdnum;
> +void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
> +{
> + struct compat_cmsghdr __user *cm =
> + (struct compat_cmsghdr __user *)msg->msg_control;
> + unsigned int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
> + int fdmax = min_t(int, scm_max_fds_compat(msg), scm->fp->count);
Just a note that SCM_RIGHTS fd-sending is limited to 253 (SCM_MAX_FD)
fds so min_t should never ouput > SCM_MAX_FD here afaict.
> + int __user *cmsg_data = CMSG_USER_DATA(cm);
> + int err = 0, i;
>
> - for (i = 0, cmfptr = (int __user *) CMSG_COMPAT_DATA(cm); i < fdmax; i++, cmfptr++) {
> - int new_fd;
> - err = security_file_receive(fp[i]);
> + for (i = 0; i < fdmax; i++) {
> + err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
> if (err)
> break;
> - err = get_unused_fd_flags(MSG_CMSG_CLOEXEC & kmsg->msg_flags
> - ? O_CLOEXEC : 0);
> - if (err < 0)
> - break;
> - new_fd = err;
> - err = put_user(new_fd, cmfptr);
> - if (err) {
> - put_unused_fd(new_fd);
> - break;
> - }
> - /* Bump the usage count and install the file. */
> - fd_install(new_fd, get_file(fp[i]));
> }
>
> if (i > 0) {
> int cmlen = CMSG_COMPAT_LEN(i * sizeof(int));
> +
> err = put_user(SOL_SOCKET, &cm->cmsg_level);
> if (!err)
> err = put_user(SCM_RIGHTS, &cm->cmsg_type);
> @@ -321,16 +313,19 @@ void scm_detach_fds_compat(struct msghdr *kmsg, struct scm_cookie *scm)
> err = put_user(cmlen, &cm->cmsg_len);
> if (!err) {
> cmlen = CMSG_COMPAT_SPACE(i * sizeof(int));
> - kmsg->msg_control += cmlen;
> - kmsg->msg_controllen -= cmlen;
> + if (msg->msg_controllen < cmlen)
> + cmlen = msg->msg_controllen;
> + msg->msg_control += cmlen;
> + msg->msg_controllen -= cmlen;
> }
> }
> - if (i < fdnum)
> - kmsg->msg_flags |= MSG_CTRUNC;
> +
> + if (i < scm->fp->count || (scm->fp->count && fdmax <= 0))
I think fdmax can't be < 0 after your changes? scm_max_fds() guarantees
that fdmax is always >= 0 and min_t() guarantees that fdmax <= scm->fp->count.
So the check should technically be :)
if (i < scm->fp->count || scm->fp->count && fdmax == 0)
> + msg->msg_flags |= MSG_CTRUNC;
>
> /*
> - * All of the files that fit in the message have had their
> - * usage counts incremented, so we just free the list.
> + * All of the files that fit in the message have had their usage counts
> + * incremented, so we just free the list.
> */
> __scm_destroy(scm);
> }
> diff --git a/net/core/scm.c b/net/core/scm.c
> index 875df1c2989d..6151678c73ed 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -280,7 +280,7 @@ void put_cmsg_scm_timestamping(struct msghdr *msg, struct scm_timestamping_inter
> }
> EXPORT_SYMBOL(put_cmsg_scm_timestamping);
>
> -static int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
> +int __scm_install_fd(struct file *file, int __user *ufd, unsigned int o_flags)
> {
> struct socket *sock;
> int new_fd;
> @@ -319,29 +319,29 @@ static int scm_max_fds(struct msghdr *msg)
>
> void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
> {
> - struct cmsghdr __user *cm
> - = (__force struct cmsghdr __user*)msg->msg_control;
> - int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
> + struct cmsghdr __user *cm =
> + (__force struct cmsghdr __user *)msg->msg_control;
> + unsigned int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
> int fdmax = min_t(int, scm_max_fds(msg), scm->fp->count);
> int __user *cmsg_data = CMSG_USER_DATA(cm);
> int err = 0, i;
>
> + /* no use for FD passing from kernel space callers */
> + if (WARN_ON_ONCE(!msg->msg_control_is_user))
> + return;
> +
> if (msg->msg_flags & MSG_CMSG_COMPAT) {
> scm_detach_fds_compat(msg, scm);
> return;
> }
>
> - /* no use for FD passing from kernel space callers */
> - if (WARN_ON_ONCE(!msg->msg_control_is_user))
> - return;
> -
> for (i = 0; i < fdmax; i++) {
> err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
> if (err)
> break;
> }
>
> - if (i > 0) {
> + if (i > 0) {
> int cmlen = CMSG_LEN(i * sizeof(int));
>
> err = put_user(SOL_SOCKET, &cm->cmsg_level);
> --
> 2.25.1
>
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