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

[Michael Kerrisk (man-pages)]

On 11/02/2014 05:36 PM, Michael Kerrisk (man-pages) wrote:
> 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.

Don't worry -- I found the problem. Silly error on my part. (I got the 
syscall number wrong, when rolling my wrappers.)

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 +.\" manual page may be incorrect or out-of-date.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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