Re: Edited seccomp.2 man page for review

[Andy Lutomirski]

On Sat, Nov 8, 2014 at 4:22 AM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> Hi Kees, (and all),
>
> Thanks for the seccomp.2 draft man page that you provided a few
> weeks ago (https://lkml.org/lkml/2014/9/25/685), and my apologies
> for the slow follow-up.
>

Answers to some of your questions below.

> .BR execve (2)
> is allowed by the filter,
> the filters and constraints on permitted system calls are preserved across an
> .BR execve (2).
>
> .\" FIXME I (mtk) reworded the following paragraph substantially.
> .\" Please check it.
> In order to use the
> .BR SECCOMP_SET_MODE_FILTER
> operation, either the caller must have the
> .BR CAP_SYS_ADMIN
> capability or the call must be preceded by the call:
>
>     prctl(PR_SET_NO_NEW_PRIVS, 1);
>
> Otherwise, the
> .BR SECCOMP_SET_MODE_FILTER
> operation will fail and return
> .BR EACCES
> in
> .IR errno .
> This requirement ensures that filter programs cannot be applied to child
> .\" FIXME What does "installed" in the following line mean?
> processes with greater privileges than the process that installed them.
>

This requirement ensures that an unprivileged process cannot apply a
malicious filter and then invoke a setuid or other privileged program
using execve, thus potentially compromising that program.

> If
> .BR prctl (2)
> or
> .BR seccomp (2)
> is allowed by the attached filter, further filters may be added.
> This will increase evaluation time, but allows for further reduction of
> the attack surface during execution of a process.
>
> The
> .BR SECCOMP_SET_MODE_FILTER
> operation is available only if the kernel is configured with
> .BR CONFIG_SECCOMP_FILTER
> enabled.
>
> When
> .IR flags
> is 0, this operation is functionally identical to the call:
>
>     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 calling
> process to the same seccomp filter tree.
> .\" FIXME Nowhere in this page is the term "filter tree" defined.
> .\" There should be a definition somewhere.
> .\" Is it: "the set of filters attached to a thread"?

It's the ordered list of filters attached to a thread, where attaching
identical filters in separate syscalls results in different filters
from this perspective.

> 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
> system call 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
>
> .\" FIXME I find the next piece a little hard to understand, so,
> .\"       some questions:
> .\"       * If there are multiple filters, in what order are they executed?
> .\"         (The man page should probably detail the answer to this question.)

All of them are executed.  The precedence rules determine what happens
if the filters return different values.

> .\"       * If there are multiple filters, are they all always executed?
> .\"         I assume not, but the notion that
> .\"             "the return value for the evaluation of a given system call
> .\"              will always use the value with the highest precedence"
> .\"         implies that even that if one filter generates (say)
> .\"         SECCOMP_RET_ERRNO, then further filters may still be executed,
> .\"         including one that generates (say) the "higher priority"
> .\"         SECCOMP_RET_KILL condition.
> .\"       Can you clarify the above?
> A seccomp filter returns one of the values listed below.
> If multiple filters exist,
> the return value for the evaluation of a given system call
> will always use the value with the highest precedence.
> (For example,
> .BR SECCOMP_RET_KILL
> will always take precedence.)
>
> In decreasing order order of precedence,
> the values that may be returned by a seccomp filter are:
> .TP
> .BR SECCOMP_RET_KILL
> Results in the task exiting immediately without executing the system call.
> The task terminates as though killed by a
> .B SIGSYS
> signal
> .RI ( 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 system call was attempted.
> The program counter will be as though the system call happened
> (i.e., it will not point to the system call instruction).
> The return value register will contain an architecture\-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.)
>
> .\" FIXME The following sentence is the first time that SECCOMP_RET_DATA
> .\"       is mentioned. SECCOMP_RET_DATA needs to be described in this
> .\"       man page.
> The
> .BR SECCOMP_RET_DATA
> portion of the return value will be passed as
> .IR si_errno .
>
> .BR SIGSYS
> triggered by seccomp will have the value
> .BR SYS_SECCOMP
> in the
> .IR si_code
> field.
> .TP
> .BR SECCOMP_RET_ERRNO
> .\" FIXME What does "the return value" refer to in the next sentence?
> .\"       It is not obvious to me.

The return value is the value returned by the BPF program.

--Andy
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