From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933805Ab2AKR0H (ORCPT ); Wed, 11 Jan 2012 12:26:07 -0500 Received: from mail-gy0-f174.google.com ([209.85.160.174]:38150 "EHLO mail-gy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933640Ab2AKR0A (ORCPT ); Wed, 11 Jan 2012 12:26:00 -0500 From: Will Drewry To: linux-kernel@vger.kernel.org Cc: keescook@chromium.org, john.johansen@canonical.com, serge.hallyn@canonical.com, coreyb@linux.vnet.ibm.com, pmoore@redhat.com, eparis@redhat.com, djm@mindrot.org, torvalds@linux-foundation.org, segoon@openwall.com, rostedt@goodmis.org, jmorris@namei.org, scarybeasts@gmail.com, avi@redhat.com, penberg@cs.helsinki.fi, viro@zeniv.linux.org.uk, wad@chromium.org, luto@mit.edu, mingo@elte.hu, akpm@linux-foundation.org, khilman@ti.com, borislav.petkov@amd.com, amwang@redhat.com, oleg@redhat.com, ak@linux.intel.com, eric.dumazet@gmail.com, gregkh@suse.de, dhowells@redhat.com, daniel.lezcano@free.fr, linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, olofj@chromium.org, mhalcrow@google.com, dlaor@redhat.com Subject: [RFC,PATCH 2/2] Documentation: prctl/seccomp_filter Date: Wed, 11 Jan 2012 11:25:10 -0600 Message-Id: <1326302710-9427-3-git-send-email-wad@chromium.org> X-Mailer: git-send-email 1.7.5.4 In-Reply-To: <1326302710-9427-1-git-send-email-wad@chromium.org> References: <1326302710-9427-1-git-send-email-wad@chromium.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Document how system call filtering with BPF works and can be used. Signed-off-by: Will Drewry --- Documentation/prctl/seccomp_filter.txt | 159 ++++++++++++++++++++++++++++++++ 1 files changed, 159 insertions(+), 0 deletions(-) create mode 100644 Documentation/prctl/seccomp_filter.txt diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt new file mode 100644 index 0000000..5fb3f44 --- /dev/null +++ b/Documentation/prctl/seccomp_filter.txt @@ -0,0 +1,159 @@ + Seccomp filtering + ================= + +Introduction +------------ + +A large number of system calls are exposed to every userland process +with many of them going unused for the entire lifetime of the process. +As system calls change and mature, bugs are found and eradicated. A +certain subset of userland applications benefit by having a reduced set +of available system calls. The resulting set reduces the total kernel +surface exposed to the application. System call filtering is meant for +use with those applications. + +Seccomp filtering provides a means for a process to specify a filter +for incoming system calls. The filter is expressed as a Berkeley Packet +Filter program, as with socket filters, except that the data operated on +is the current user_regs_struct. This allows for expressive filtering +of system calls using the pre-existing system call ABI and using a filter +program language with a long history of being exposed to userland. +Additionally, BPF makes it impossible for users of seccomp to fall prey to +time-of-check-time-of-use (TOCTOU) attacks that are common in system call +interposition frameworks because the evaluated data is solely register state +just after system call entry. + +What it isn't +------------- + +System call filtering isn't a sandbox. It provides a clearly defined +mechanism for minimizing the exposed kernel surface. Beyond that, +policy for logical behavior and information flow should be managed with +a combinations of other system hardening techniques and, potentially, a +LSM of your choosing. Expressive, dynamic filters provide further options down +this path (avoiding pathological sizes or selecting which of the multiplexed +system calls in socketcall() is allowed, for instance) which could be +construed, incorrectly, as a more complete sandboxing solution. + +Usage +----- + +An additional seccomp mode is added, but they are not directly set by the +consuming process. The new mode, '2', is only available if +CONFIG_SECCOMP_FILTER is set and enabled using prctl with the +PR_ATTACH_SECCOMP_FILTER argument. + +Interacting with seccomp filters is done using one prctl(2) call. + +PR_ATTACH_SECCOMP_FILTER: + Allows the specification of a new filter using a BPF program. + The BPF program will be executed over a user_regs_struct data + reflecting system call time except with the system call number + resident in orig_[register]. To allow a system call, the size + of the data must be returned. At present, all other return values + result in the system call being blocked, but it is recommended to + return 0 in those cases. This will allow for future custom return + values to be introduced, if ever desired. + + Usage: + prctl(PR_ATTACH_SECCOMP_FILTER, prog); + + The 'prog' argument is a pointer to a struct sock_fprog which will + contain the filter program. If the program is invalid, the call + will return -1 and set errno to -EINVAL. + + The struct user_regs_struct the @prog will see is based on the + personality of the task at the time of this prctl call. Additionally, + is_compat_task is also tracked for the @prog. This means that once set + the calling task will have all of its system calls blocked if it + switches its system call ABI (via personality or other means). + + If the @prog is installed while the task has CAP_SYS_ADMIN in its user + namespace, the @prog will be marked as inheritable across execve. Any + inherited filters are still subject to the system call ABI constraints + above and any ABI mismatched system calls will result in process death. + +All of the above calls return 0 on success and non-zero on error. + + +Example +------- + +Assume a process would like to cleanly read and write to stdin/out/err and exit +cleanly. Without using a BPF compiler, it may be done as follows on x86 32-bit: + +#include +#include +#include +#include +#include +#include + +#define regoffset(_reg) (offsetof(struct user_regs_struct, _reg)) +int install_filter(void) +{ + struct sock_filter filter[] = { + /* Grab the system call number */ + BPF_STMT(BPF_LD+BPF_W+BPF_IND, regoffset(orig_eax)), + /* Jump table for the allowed syscalls */ + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 10, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 9, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 8, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 7, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 2, 6), + + /* Check that read is only using stdin. */ + BPF_STMT(BPF_LD+BPF_W+BPF_IND, regoffset(ebx)), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 3, 4), + + /* Check that write is only using stdout/stderr */ + BPF_STMT(BPF_LD+BPF_W+BPF_IND, regoffset(ebx)), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0), + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 0, 1), + + /* Put the "accept" value in A */ + BPF_STMT(BPF_LD+BPF_W+BPF_LEN, 0), + + BPF_STMT(BPF_RET+BPF_A,0), + }; + struct sock_fprog prog = { + .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])), + .filter = filter, + }; + if (prctl(36, &prog)) { + perror("prctl"); + return 1; + } + return 0; +} + +#define payload(_c) _c, sizeof(_c) +int main(int argc, char **argv) { + char buf[4096]; + ssize_t bytes = 0; + if (install_filter()) + return 1; + syscall(__NR_write, STDOUT_FILENO, payload("OHAI! WHAT IS YOUR NAME? ")); + bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)); + syscall(__NR_write, STDOUT_FILENO, payload("HELLO, ")); + syscall(__NR_write, STDOUT_FILENO, buf, bytes); + return 0; +} + +Additionally, if prctl(2) is allowed by the installed filter, additional +filters may be layered on which will increase evaluation time, but allow for +further decreasing the attack surface during execution of a process. + + +Caveats +------- + +- execve will fail unless the most recently attached filter was installed by + a process with CAP_SYS_ADMIN (in its namespace). + +Adding architecture support +----------------------- + +Any platform with seccomp support will support seccomp filters +as long as CONFIG_SECCOMP_FILTER is enabled. -- 1.7.5.4