Re: [PATCH 1/2] seccomp.2: document seccomp syscall

[Michael Kerrisk (man-pages)]

Hi Kees,

On 09/26/2014 12:47 AM, Kees Cook wrote:
> Combines documentation from prctl, in-kernel seccomp_filter.txt and
> dropper.c, along with details specific to the new syscall.

I am working on integrating this page at the moment, and I'll have
an edited draft for you sometime soon, I hope.

In the meantime, I have a question. Could you show a sample run
of the example program given in the man page? In my attempts so far,
I always get EINVAL from seccomp(). Obviously, I am missing something.

Cheers,

Michael

 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  man2/seccomp.2 | 400 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 400 insertions(+)
>  create mode 100644 man2/seccomp.2
> 
> diff --git a/man2/seccomp.2 b/man2/seccomp.2
> new file mode 100644
> index 0000000..f64950f
> --- /dev/null
> +++ b/man2/seccomp.2
> @@ -0,0 +1,400 @@
> +.\" Copyright (C) 2014 Kees Cook <keescook@xxxxxxxxxxxx>
> +.\" and Copyright (C) 2012 Will Drewry <wad@xxxxxxxxxxxx>
> +.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of this
> +.\" manual under the conditions for verbatim copying, provided that the
> +.\" entire resulting derived work is distributed under the terms of a
> +.\" permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date.  The author(s) assume no
> +.\" responsibility for errors or omissions, or for damages resulting from
> +.\" the use of the information contained herein.  The author(s) may not
> +.\" have taken the same level of care in the production of this manual,
> +.\" which is licensed free of charge, as they might when working
> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.\"
> +.TH SECCOMP 2 2014-06-23 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +seccomp \-
> +operate on Secure Computing state of the process
> +.SH SYNOPSIS
> +.nf
> +.B #include <linux/seccomp.h>
> +.B #include <linux/filter.h>
> +.B #include <linux/audit.h>
> +.B #include <linux/signal.h>
> +.B #include <sys/ptrace.h>
> +
> +.BI "int seccomp(unsigned int " operation ", unsigned int " flags ,
> +.BI "            void *" args );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR seccomp ()
> +system call operates on the Secure Computing (seccomp) state of the
> +current process.
> +
> +Currently, Linux supports the following
> +.IR operation
> +values:
> +.TP
> +.BR SECCOMP_SET_MODE_STRICT
> +Only system calls that the thread is permitted to make are
> +.BR read (2),
> +.BR write (2),
> +.BR _exit (2),
> +and
> +.BR sigreturn (2).
> +Other system calls result in the delivery of a
> +.BR SIGKILL
> +signal. Strict secure computing mode is useful for number-crunching
> +applications that may need to execute untrusted byte code, perhaps
> +obtained by reading from a pipe or socket.
> +
> +This operation is available only if the kernel is configured with
> +.BR CONFIG_SECCOMP
> +enabled.
> +
> +The value of
> +.IR flags
> +must be 0, and
> +.IR args
> +must be NULL.
> +
> +This operation is functionally identical to calling
> +.IR "prctl(PR_SET_SECCOMP,\ SECCOMP_MODE_STRICT)" .
> +.TP
> +.BR SECCOMP_SET_MODE_FILTER
> +The system calls allowed are defined by a pointer to a Berkeley Packet
> +Filter (BPF) passed via
> +.IR args .
> +This argument is a pointer to
> +.IR "struct\ sock_fprog" ;
> +it can be designed to filter arbitrary system calls and system call
> +arguments. If the filter is invalid, the call will fail, returning
> +.BR EACCESS
> +in
> +.IR errno .
> +
> +If
> +.BR fork (2),
> +.BR clone (2),
> +or
> +.BR execve (2)
> +are allowed by the filter, any child processes will be constrained to
> +the same filters and system calls as the parent.
> +
> +Prior to using this operation, the process must call
> +.IR "prctl(PR_SET_NO_NEW_PRIVS,\ 1)"
> +or run with
> +.BR CAP_SYS_ADMIN
> +privileges in its namespace. If these are not true, the call will fail
> +and return
> +.BR EACCES
> +in
> +.IR errno .
> +This requirement ensures that filter programs cannot be applied to child
> +processes with greater privileges than the process that installed them.
> +
> +Additionally, if
> +.BR prctl (2)
> +or
> +.BR seccomp (2)
> +is allowed by the attached filter, additional filters may be layered on
> +which will increase evaluation time, but allow for further reduction of
> +the attack surface during execution of a process.
> +
> +This operation is available only if the kernel is configured with
> +.BR CONFIG_SECCOMP_FILTER
> +enabled.
> +
> +When
> +.IR flags
> +are 0, this operation is functionally identical to calling
> +.IR "prctl(PR_SET_SECCOMP,\ SECCOMP_MODE_FILTER,\ args)" .
> +
> +The recognized
> +.IR flags
> +are:
> +.RS
> +.TP
> +.BR SECCOMP_FILTER_FLAG_TSYNC
> +When adding a new filter, synchronize all other threads of the current
> +process to the same seccomp filter tree. If any thread cannot do this,
> +the call will not attach the new seccomp filter, and will fail returning
> +the first thread ID found that cannot synchronize.  Synchronization will
> +fail if another thread is in
> +.BR SECCOMP_MODE_STRICT
> +or if it has attached new seccomp filters to itself, diverging from the
> +calling thread's filter tree.
> +.RE
> +.SH FILTERS
> +When adding filters via
> +.BR SECCOMP_SET_MODE_FILTER ,
> +.IR args
> +points to a filter program:
> +
> +.in +4n
> +.nf
> +struct sock_fprog {
> +    unsigned short      len;    /* Number of BPF instructions */
> +    struct sock_filter *filter;
> +};
> +.fi
> +.in
> +
> +Each program must contain one or more BPF instructions:
> +
> +.in +4n
> +.nf
> +struct sock_filter {    /* Filter block */
> +    __u16   code;       /* Actual filter code */
> +    __u8    jt;         /* Jump true */
> +    __u8    jf;         /* Jump false */
> +    __u32   k;          /* Generic multiuse field */
> +};
> +.fi
> +.in
> +
> +When executing the instructions, the BPF program executes over the
> +syscall information made available via:
> +
> +.in +4n
> +.nf
> +struct seccomp_data {
> +    int nr;                     /* system call number */
> +    __u32 arch;                 /* AUDIT_ARCH_* value */
> +    __u64 instruction_pointer;  /* CPU instruction pointer */
> +    __u64 args[6];              /* up to 6 system call arguments */
> +};
> +.fi
> +.in
> +
> +A seccomp filter may return any of the following values. If multiple
> +filters exist, the return value for the evaluation of a given system
> +call will always use the highest precedent value. (For example,
> +.BR SECCOMP_RET_KILL
> +will always take precedence.)
> +
> +In precedence order, they are:
> +.TP
> +.BR SECCOMP_RET_KILL
> +Results in the task exiting immediately without executing the
> +system call.  The exit status of the task (status & 0x7f) will
> +be
> +.BR SIGSYS ,
> +not
> +.BR SIGKILL .
> +.TP
> +.BR SECCOMP_RET_TRAP
> +Results in the kernel sending a
> +.BR SIGSYS
> +signal to the triggering task without executing the system call.
> +.IR siginfo\->si_call_addr
> +will show the address of the system call instruction, and
> +.IR siginfo\->si_syscall
> +and
> +.IR siginfo\->si_arch
> +will indicate which syscall was attempted.  The program counter will be
> +as though the syscall happened (i.e. it will not point to the syscall
> +instruction).  The return value register will contain an arch\-dependent
> +value; if resuming execution, set it to something sensible.
> +(The architecture dependency is because replacing it with
> +.BR ENOSYS
> +could overwrite some useful information.)
> +
> +The
> +.BR SECCOMP_RET_DATA
> +portion of the return value will be passed as
> +.IR si_errno .
> +
> +.BR SIGSYS
> +triggered by seccomp will have a
> +.IR si_code
> +of
> +.BR SYS_SECCOMP .
> +.TP
> +.BR SECCOMP_RET_ERRNO
> +Results in the lower 16-bits of the return value being passed
> +to userland as the
> +.IR errno
> +without executing the system call.
> +.TP
> +.BR SECCOMP_RET_TRACE
> +When returned, this value will cause the kernel to attempt to
> +notify a ptrace()-based tracer prior to executing the system
> +call.  If there is no tracer present,
> +.BR ENOSYS
> +is returned to userland and the system call is not executed.
> +
> +A tracer will be notified if it requests
> +.BR PTRACE_O_TRACESECCOMP
> +using
> +.IR ptrace(PTRACE_SETOPTIONS) .
> +The tracer will be notified of a
> +.BR PTRACE_EVENT_SECCOMP
> +and the
> +.BR SECCOMP_RET_DATA
> +portion of the BPF program return value will be available to the tracer
> +via
> +.BR PTRACE_GETEVENTMSG .
> +
> +The tracer can skip the system call by changing the syscall number
> +to \-1.  Alternatively, the tracer can change the system call
> +requested by changing the system call to a valid syscall number.  If
> +the tracer asks to skip the system call, then the system call will
> +appear to return the value that the tracer puts in the return value
> +register.
> +
> +The seccomp check will not be run again after the tracer is
> +notified.  (This means that seccomp-based sandboxes MUST NOT
> +allow use of ptrace, even of other sandboxed processes, without
> +extreme care; ptracers can use this mechanism to escape.)
> +.TP
> +.BR SECCOMP_RET_ALLOW
> +Results in the system call being executed.
> +
> +If multiple filters exist, the return value for the evaluation of a
> +given system call will always use the highest precedent value.
> +
> +Precedence is only determined using the
> +.BR SECCOMP_RET_ACTION
> +mask.  When multiple filters return values of the same precedence,
> +only the
> +.BR SECCOMP_RET_DATA
> +from the most recently installed filter will be returned.
> +.SH RETURN VALUE
> +On success,
> +.BR seccomp ()
> +returns 0.
> +On error, if
> +.BR SECCOMP_FILTER_FLAG_TSYNC
> +was used, the return value is the thread ID that caused the
> +synchronization failure. On other errors, \-1 is returned, and
> +.IR errno
> +is set to indicate the cause of the error.
> +.SH ERRORS
> +.BR seccomp ()
> +can fail for the following reasons:
> +.TP
> +.BR EACCESS
> +the caller did not have the
> +.BR CAP_SYS_ADMIN
> +capability, or had not set
> +.IR no_new_privs
> +before using
> +.BR SECCOMP_SET_MODE_FILTER .
> +.TP
> +.BR EFAULT
> +.IR args
> +was required to be a valid address.
> +.TP
> +.BR EINVAL
> +.IR operation
> +is unknown; or
> +.IR flags
> +are invalid for the given
> +.IR operation
> +.TP
> +.BR ESRCH
> +Another thread caused a failure during thread sync, but its ID could not
> +be determined.
> +.SH VERSIONS
> +This system call first appeared in Linux 3.16.
> +.\" FIXME Add glibc version
> +.SH CONFORMING TO
> +This system call is a nonstandard Linux extension.
> +.SH NOTES
> +.BR seccomp ()
> +provides a superset of the functionality provided by
> +.IR PR_SET_SECCOMP
> +of
> +.BR prctl (2) .
> +(Which does not support
> +.IR flags .)
> +.SH EXAMPLE
> +.nf
> +#include <errno.h>
> +#include <stddef.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <linux/audit.h>
> +#include <linux/filter.h>
> +#include <linux/seccomp.h>
> +#include <sys/prctl.h>
> +
> +static int install_filter(int syscall, int arch, int error)
> +{
> +    struct sock_filter filter[] = {
> +        /* Load architecture. */
> +        BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
> +                 (offsetof(struct seccomp_data, arch))),
> +        /* Jump forward 4 instructions on architecture mismatch. */
> +        BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 4),
> +        /* Load syscall number. */
> +        BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
> +                 (offsetof(struct seccomp_data, nr))),
> +        /* Jump forward 1 instruction on syscall mismatch. */
> +        BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, syscall, 0, 1),
> +        /* Matching arch and syscall: return specific errno. */
> +        BPF_STMT(BPF_RET+BPF_K,
> +                 SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
> +        /* Destination of syscall mismatch: Allow other syscalls. */
> +        BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +        /* Destination of arch mismatch: Kill process. */
> +        BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
> +    };
> +    struct sock_fprog prog = {
> +        .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
> +        .filter = filter,
> +    };
> +    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
> +        perror("seccomp");
> +        return EXIT_FAILURE;
> +    }
> +    return EXIT_SUCCESS;
> +}
> +
> +int main(int argc, char **argv)
> +{
> +    if (argc < 5) {
> +        fprintf(stderr, "Usage:\\n"
> +                "refuse <syscall_nr> <arch> <errno> <prog> [<args>]\\n"
> +                "Hint:  AUDIT_ARCH_I386: 0x%X\\n"
> +                "       AUDIT_ARCH_X86_64: 0x%X\\n"
> +                "\\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
> +        return EXIT_FAILURE;
> +    }
> +    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
> +        perror("prctl");
> +        return EXIT_FAILURE;
> +    }
> +    if (install_filter(strtol(argv[1], NULL, 0),
> +                       strtol(argv[2], NULL, 0),
> +                       strtol(argv[3], NULL, 0)))
> +        return EXIT_FAILURE;
> +    execv(argv[4], &argv[4]);
> +    perror("execv");
> +    return EXIT_FAILURE;
> +}
> +.fi
> +.SH SEE ALSO
> +.ad l
> +.nh
> +.BR prctl (2),
> +.BR ptrace (2),
> +.BR signal (7),
> +.BR socket (7)
> +.ad
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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