From 09b243577be319ef55310b45c65737008f3ebf12 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 2 Jul 2012 14:03:58 -0700 Subject: [PATCH 1/2] security: document no_new_privs Document no_new_privs. Signed-off-by: Andy Lutomirski Acked-by: Kees Cook Signed-off-by: James Morris --- Documentation/prctl/no_new_privs.txt | 50 ++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) create mode 100644 Documentation/prctl/no_new_privs.txt diff --git a/Documentation/prctl/no_new_privs.txt b/Documentation/prctl/no_new_privs.txt new file mode 100644 index 000000000000..cb705ec69abe --- /dev/null +++ b/Documentation/prctl/no_new_privs.txt @@ -0,0 +1,50 @@ +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. + +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. From 75331a597cf4cde51d9b0bb22cbd03b9837ef9e4 Mon Sep 17 00:00:00 2001 From: Paul Mundt Date: Mon, 2 Jul 2012 14:34:11 +0900 Subject: [PATCH 2/2] security: Fix nommu build. The security + nommu configuration presently blows up with an undefined reference to BDI_CAP_EXEC_MAP: security/security.c: In function 'mmap_prot': security/security.c:687:36: error: dereferencing pointer to incomplete type security/security.c:688:16: error: 'BDI_CAP_EXEC_MAP' undeclared (first use in this function) security/security.c:688:16: note: each undeclared identifier is reported only once for each function it appears in include backing-dev.h directly to fix it up. Signed-off-by: Paul Mundt Signed-off-by: James Morris --- security/security.c | 1 + 1 file changed, 1 insertion(+) diff --git a/security/security.c b/security/security.c index 3efc9b12aef4..860aeb349cb3 100644 --- a/security/security.c +++ b/security/security.c @@ -23,6 +23,7 @@ #include #include #include +#include #include #define MAX_LSM_EVM_XATTR 2