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    Subject[RFC,PATCH 2/2] Documentation: prctl/seccomp_filter
    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
    + =================
    +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.
    +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
    +Interacting with seccomp filters is done using one prctl(2) call.
    + 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.
    +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 <asm/unistd.h>
    +#include <linux/filter.h>
    +#include <stdio.h>
    +#include <stddef.h>
    +#include <sys/user.h>
    +#include <unistd.h>
    +#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)),
    + /* Check that write is only using stdout/stderr */
    + BPF_STMT(BPF_LD+BPF_W+BPF_IND, regoffset(ebx)),
    + /* Put the "accept" value in A */
    + };
    + 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.
    +- 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.

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