From: Kees Cook Date: Sat, 13 May 2017 11:51:38 +0000 (-0700) Subject: doc: ReSTify no_new_privs.txt X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=40fde647ccb0ae8c11d256d271e24d385eed595b;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git doc: ReSTify no_new_privs.txt This updates no_new_privs documentation to ReST markup and adds it to the user-space API documentation. Signed-off-by: Kees Cook Signed-off-by: Jonathan Corbet --- diff --git a/Documentation/prctl/no_new_privs.txt b/Documentation/prctl/no_new_privs.txt deleted file mode 100644 index f7be84fba910..000000000000 --- a/Documentation/prctl/no_new_privs.txt +++ /dev/null @@ -1,57 +0,0 @@ -The execve system call can grant a newly-started program privileges that -its parent did not have. The most obvious examples are setuid/setgid -programs and file capabilities. To prevent the parent program from -gaining these privileges as well, the kernel and user code must be -careful to prevent the parent from doing anything that could subvert the -child. For example: - - - The dynamic loader handles LD_* environment variables differently if - a program is setuid. - - - chroot is disallowed to unprivileged processes, since it would allow - /etc/passwd to be replaced from the point of view of a process that - inherited chroot. - - - The exec code has special handling for ptrace. - -These are all ad-hoc fixes. The no_new_privs bit (since Linux 3.5) is a -new, generic mechanism to make it safe for a process to modify its -execution environment in a manner that persists across execve. Any task -can set no_new_privs. Once the bit is set, it is inherited across fork, -clone, and execve and cannot be unset. With no_new_privs set, execve -promises not to grant the privilege to do anything that could not have -been done without the execve call. For example, the setuid and setgid -bits will no longer change the uid or gid; file capabilities will not -add to the permitted set, and LSMs will not relax constraints after -execve. - -To set no_new_privs, use prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0). - -Be careful, though: LSMs might also not tighten constraints on exec -in no_new_privs mode. (This means that setting up a general-purpose -service launcher to set no_new_privs before execing daemons may -interfere with LSM-based sandboxing.) - -Note that no_new_privs does not prevent privilege changes that do not -involve execve. An appropriately privileged task can still call -setuid(2) and receive SCM_RIGHTS datagrams. - -There are two main use cases for no_new_privs so far: - - - Filters installed for the seccomp mode 2 sandbox persist across - execve and can change the behavior of newly-executed programs. - Unprivileged users are therefore only allowed to install such filters - if no_new_privs is set. - - - By itself, no_new_privs can be used to reduce the attack surface - available to an unprivileged user. If everything running with a - given uid has no_new_privs set, then that uid will be unable to - escalate its privileges by directly attacking setuid, setgid, and - fcap-using binaries; it will need to compromise something without the - no_new_privs bit set first. - -In the future, other potentially dangerous kernel features could become -available to unprivileged tasks if no_new_privs is set. In principle, -several options to unshare(2) and clone(2) would be safe when -no_new_privs is set, and no_new_privs + chroot is considerable less -dangerous than chroot by itself. diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst index 15ff12342db8..7b2eb1b7d4ca 100644 --- a/Documentation/userspace-api/index.rst +++ b/Documentation/userspace-api/index.rst @@ -16,6 +16,7 @@ place where this information is gathered. .. toctree:: :maxdepth: 2 + no_new_privs seccomp_filter unshare diff --git a/Documentation/userspace-api/no_new_privs.rst b/Documentation/userspace-api/no_new_privs.rst new file mode 100644 index 000000000000..d060ea217ea1 --- /dev/null +++ b/Documentation/userspace-api/no_new_privs.rst @@ -0,0 +1,63 @@ +====================== +No New Privileges Flag +====================== + +The execve system call can grant a newly-started program privileges that +its parent did not have. The most obvious examples are setuid/setgid +programs and file capabilities. To prevent the parent program from +gaining these privileges as well, the kernel and user code must be +careful to prevent the parent from doing anything that could subvert the +child. For example: + + - The dynamic loader handles ``LD_*`` environment variables differently if + a program is setuid. + + - chroot is disallowed to unprivileged processes, since it would allow + ``/etc/passwd`` to be replaced from the point of view of a process that + inherited chroot. + + - The exec code has special handling for ptrace. + +These are all ad-hoc fixes. The ``no_new_privs`` bit (since Linux 3.5) is a +new, generic mechanism to make it safe for a process to modify its +execution environment in a manner that persists across execve. Any task +can set ``no_new_privs``. Once the bit is set, it is inherited across fork, +clone, and execve and cannot be unset. With ``no_new_privs`` set, ``execve()`` +promises not to grant the privilege to do anything that could not have +been done without the execve call. For example, the setuid and setgid +bits will no longer change the uid or gid; file capabilities will not +add to the permitted set, and LSMs will not relax constraints after +execve. + +To set ``no_new_privs``, use:: + + prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); + +Be careful, though: LSMs might also not tighten constraints on exec +in ``no_new_privs`` mode. (This means that setting up a general-purpose +service launcher to set ``no_new_privs`` before execing daemons may +interfere with LSM-based sandboxing.) + +Note that ``no_new_privs`` does not prevent privilege changes that do not +involve ``execve()``. An appropriately privileged task can still call +``setuid(2)`` and receive SCM_RIGHTS datagrams. + +There are two main use cases for ``no_new_privs`` so far: + + - Filters installed for the seccomp mode 2 sandbox persist across + execve and can change the behavior of newly-executed programs. + Unprivileged users are therefore only allowed to install such filters + if ``no_new_privs`` is set. + + - By itself, ``no_new_privs`` can be used to reduce the attack surface + available to an unprivileged user. If everything running with a + given uid has ``no_new_privs`` set, then that uid will be unable to + escalate its privileges by directly attacking setuid, setgid, and + fcap-using binaries; it will need to compromise something without the + ``no_new_privs`` bit set first. + +In the future, other potentially dangerous kernel features could become +available to unprivileged tasks if ``no_new_privs`` is set. In principle, +several options to ``unshare(2)`` and ``clone(2)`` would be safe when +``no_new_privs`` is set, and ``no_new_privs`` + ``chroot`` is considerable less +dangerous than chroot by itself.