| Message ID | 20241117091313.10251-1-stsp2@yandex.ru (mailing list archive) |
|---|---|
| State | Rejected |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [net,v2] scm: fix negative fds with SO_PASSPIDFD | expand |
Am So., 17. Nov. 2024 um 10:13 Uhr schrieb Stas Sergeev <stsp2@yandex.ru>: > > pidfd_prepare() can return negative values as an error codes. > But scm_pidfd_recv() didn't check for that condition. > As the result, it is possible to create the race that leads to > the negative fds. The race happens if the peer process sends > something to SO_PASSPIDFD-enabled recipient, and quickly exits. > pidfd_prepare() has this code: > > if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID)) > return -EINVAL; > > So if you exit quickly enough, you can hit that EINVAL. > Getting the fd=-22 is very weird, if not exploitable. > > This patch adds the missing check and sets MSG_CTRUNC on error. > Recipient can now detect an error by checking this flag. > > Changes in v2: add Fixes tag > > Signed-off-by: Stas Sergeev <stsp2@yandex.ru> Hi Stas, Actually, it's not a forgotten check. It's intended behavior to pass through errors from pidfd_prepare() to the userspace. In my first version [1] of the patch I tried to return ESRCH instead of EINVAL in your case, but then during discussions we decided to remove that. [1] https://lore.kernel.org/all/20230316131526.283569-2-aleksandr.mikhalitsyn@canonical.com/ Kind regards, Alex > > Fixes: 5e2ff6704a2 ("scm: add SO_PASSPIDFD and SCM_PIDFD") > > CC: Alexander Mikhalitsyn <alexander@mihalicyn.com> > CC: "David S. Miller" <davem@davemloft.net> > CC: Eric Dumazet <edumazet@google.com> > CC: Jakub Kicinski <kuba@kernel.org> > CC: Paolo Abeni <pabeni@redhat.com> > CC: Simon Horman <horms@kernel.org> > CC: Christian Brauner <brauner@kernel.org> > CC: Kees Cook <kees@kernel.org> > CC: Kuniyuki Iwashima <kuniyu@amazon.com> > CC: netdev@vger.kernel.org > CC: linux-kernel@vger.kernel.org > --- > include/net/scm.h | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/include/net/scm.h b/include/net/scm.h > index 0d35c7c77a74..3ccf8546c506 100644 > --- a/include/net/scm.h > +++ b/include/net/scm.h > @@ -155,6 +155,10 @@ static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm > return; > > pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file); > + if (pidfd < 0) { > + msg->msg_flags |= MSG_CTRUNC; > + return; > + } > > if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) { > if (pidfd_file) { > -- > 2.47.0 >
17.11.2024 12:40, Alexander Mikhalitsyn пишет: > Hi Stas, > > Actually, it's not a forgotten check. It's intended behavior to pass > through errors from pidfd_prepare() to > the userspace. In my first version [1] of the patch I tried to return > ESRCH instead of EINVAL in your case, but > then during discussions we decided to remove that. > > [1] https://lore.kernel.org/all/20230316131526.283569-2-aleksandr.mikhalitsyn@canonical.com/ Yes, the patch you referenced above, only calls put_cmsg() with an error code. But the code I can see now in git, does much more. Namely, if (pidfd_file) fd_install(pidfd, pidfd_file); Or: put_unused_fd(pidfd); And I really can't find any ">=0" check in those funcs. What am I missing? Is it safe to call fd_install(-22, pidfd_file)?
17.11.2024 13:04, stsp пишет: > 17.11.2024 12:40, Alexander Mikhalitsyn пишет: >> Hi Stas, >> >> Actually, it's not a forgotten check. It's intended behavior to pass >> through errors from pidfd_prepare() to >> the userspace. In my first version [1] of the patch I tried to return >> ESRCH instead of EINVAL in your case, but >> then during discussions we decided to remove that. >> >> [1] >> https://lore.kernel.org/all/20230316131526.283569-2-aleksandr.mikhalitsyn@canonical.com/ > Yes, the patch you referenced above, > only calls put_cmsg() with an error code. > > But the code I can see now in git, does > much more. Namely, > if (pidfd_file) > fd_install(pidfd, pidfd_file); Ah, I guess pidfd_file is a culprit. Thanks.
Am So., 17. Nov. 2024 um 11:11 Uhr schrieb stsp <stsp2@yandex.ru>: > > 17.11.2024 13:04, stsp пишет: > > 17.11.2024 12:40, Alexander Mikhalitsyn пишет: > >> Hi Stas, > >> > >> Actually, it's not a forgotten check. It's intended behavior to pass > >> through errors from pidfd_prepare() to > >> the userspace. In my first version [1] of the patch I tried to return > >> ESRCH instead of EINVAL in your case, but > >> then during discussions we decided to remove that. > >> > >> [1] > >> https://lore.kernel.org/all/20230316131526.283569-2-aleksandr.mikhalitsyn@canonical.com/ > > Yes, the patch you referenced above, > > only calls put_cmsg() with an error code. > > > > But the code I can see now in git, does > > much more. Namely, > > if (pidfd_file) > > fd_install(pidfd, pidfd_file); > Ah, I guess pidfd_file is a culprit. Hey, Precisely, when an error happens then pidfd_file is NULL. Kind regards, Alex > Thanks.
diff --git a/include/net/scm.h b/include/net/scm.h index 0d35c7c77a74..3ccf8546c506 100644 --- a/include/net/scm.h +++ b/include/net/scm.h @@ -155,6 +155,10 @@ static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm return; pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file); + if (pidfd < 0) { + msg->msg_flags |= MSG_CTRUNC; + return; + } if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) { if (pidfd_file) {
pidfd_prepare() can return negative values as an error codes. But scm_pidfd_recv() didn't check for that condition. As the result, it is possible to create the race that leads to the negative fds. The race happens if the peer process sends something to SO_PASSPIDFD-enabled recipient, and quickly exits. pidfd_prepare() has this code: if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID)) return -EINVAL; So if you exit quickly enough, you can hit that EINVAL. Getting the fd=-22 is very weird, if not exploitable. This patch adds the missing check and sets MSG_CTRUNC on error. Recipient can now detect an error by checking this flag. Changes in v2: add Fixes tag Signed-off-by: Stas Sergeev <stsp2@yandex.ru> Fixes: 5e2ff6704a2 ("scm: add SO_PASSPIDFD and SCM_PIDFD") CC: Alexander Mikhalitsyn <alexander@mihalicyn.com> CC: "David S. Miller" <davem@davemloft.net> CC: Eric Dumazet <edumazet@google.com> CC: Jakub Kicinski <kuba@kernel.org> CC: Paolo Abeni <pabeni@redhat.com> CC: Simon Horman <horms@kernel.org> CC: Christian Brauner <brauner@kernel.org> CC: Kees Cook <kees@kernel.org> CC: Kuniyuki Iwashima <kuniyu@amazon.com> CC: netdev@vger.kernel.org CC: linux-kernel@vger.kernel.org --- include/net/scm.h | 4 ++++ 1 file changed, 4 insertions(+)