Re: Edited seccomp.2 man page for review [v2]

[Kees Cook]

On Tue, Dec 30, 2014 at 4:14 AM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> Hi Kees, (and all),
>
> Thanks for your comments on the previous draft of the seccomp(2)
> man page and (once again) my apologies for the slow follow-up.
>
> I have done some further editing of the page. Could you check
> the revised version below. I have added a number of FIXMEs
> for points where I'd either like you to check new text that I
> added (in case it contains errors) or where I hope you can
> provide answers to questions relating to details that may need
> clarifying in the page.
>
> I've appended the revised page at the foot of this mail. You can also
> find the branch holding this page in Git at:
> http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_seccomp
>
> Notable changes from the previous draft:
> * Several new error cases added under ERRORS
> * New subsection on Seccomp-specific BPF details
> * Add some detail in discussion of 'siginfo_t' fields
> * Tweaked comments on BPF program in EXAMPLE section
> * Added various FIXMEs
>
> I also have one API quibble, regarding the name of the
> SYS_SECCOMP constant; see below.
>
> Feedback as inline comments to the below would be great!
>
> Cheers,
>
> Michael
>
> .\" Copyright (C) 2014 Kees Cook <keescook@xxxxxxxxxxxx>
> .\" and Copyright (C) 2012 Will Drewry <wad@xxxxxxxxxxxx>
> .\" and Copyright (C) 2008, 2014 Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> .\"
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> .\"
> .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>
> .\" Kees Cook noted: Anything that uses SECCOMP_RET_TRACE returns will
> .\"                  need <sys/ptrace.h>
>
> .BI "int seccomp(unsigned int " operation ", unsigned int " flags \
> ", void *" args );
> .fi
> .SH DESCRIPTION
> The
> .BR seccomp ()
> system call operates on the Secure Computing (seccomp) state of the
> calling process.
>
> Currently, Linux supports the following
> .IR operation
> values:
> .TP
> .BR SECCOMP_SET_MODE_STRICT
> The only system calls that the calling 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 the call:
>
>     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 a
> .IR "struct\ sock_fprog" ;
> it can be designed to filter arbitrary system calls and system call
> arguments.
> If the filter is invalid,
> .BR seccomp ()
> fails, returning
> .BR EINVAL
> in
> .IR errno .
>
> If
> .BR fork (2)
> or
> .BR clone (2)
> is allowed by the filter, any child processes will be constrained to
> the same system call filters as the parent.
> If
> .BR execve (2)
> is allowed,
> the existing filters will be preserved across a call to
> .BR execve (2).
>
> In order to use the
> .BR SECCOMP_SET_MODE_FILTER
> operation, either the caller must have the
> .BR CAP_SYS_ADMIN
> capability, or the thread must already have the
> .I no_new_privs
> bit set.
> If that bit was not already set by an ancestor of this thread,
> the thread must make the following 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 an unprivileged process cannot apply
> a malicious filter and then invoke a set-user-ID or
> other privileged program using
> .BR execve (2),
> thus potentially compromising that program.
> (Such a malicious filter might, for example, cause an attempt to use
> .BR setuid (2)
> to set the caller's user IDs to non-zero values to instead
> return 0 without actually making the system call.
> Thus, the program might be tricked into retaining superuser privileges
> in circumstances where it is possible to influence it to do
> dangerous things because it did not actually drop privileges.)
>
> 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 thread.
>
> 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.
> A "filter tree" is the ordered list of filters attached to a thread.
> (Attaching identical filters in separate
> .BR seccomp ()
> calls results in different filters from this perspective.)
>
> If any thread cannot synchronize to the same filter tree,
> 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 in the same process 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
> .SS 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; /* Pointer to array of
>                                    BPF instructions */
> };
> .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
>
> .\" FIXME I reworded/enhanced the following sentence. Is it okay?
> When executing the instructions, the BPF program operates on the
> system call information made available (i.e., use the
> .BR BPF_ABS
> addressing mode) as a buffer of the following form:

That looks correct to me, yes.

>
> .in +4n
> .nf
> struct seccomp_data {
>     int   nr;                   /* System call number */
>     __u32 arch;                 /* AUDIT_ARCH_* value
>                                    (see <linux/audit.h>) */
>     __u64 instruction_pointer;  /* CPU instruction pointer */
>     __u64 args[6];              /* Up to 6 system call arguments */
> };
> .fi
> .in
>
> A seccomp filter returns a 32-bit value consisting of two parts:
> the most significant 16 bits
> (corresponding to the mask defined by the constant
> .BR SECCOMP_RET_ACTION )
> contain one of the "action" values listed below;
> the least significant 16-bits (defined by the constant
> .BR SECCOMP_RET_DATA )
> are "data" to be associated with this return value.
>
> If multiple filters exist, they are all executed,
> in reverse order of their addition to the filter tree
> (i.e., the most recently installed filter is executed first).
> The return value for the evaluation of a given system call is the first-seen
> .BR SECCOMP_RET_ACTION
> value of highest precedence (along with its accompanying data)
> returned by execution of all of the filters.
>
> In decreasing order of precedence,
> the values that may be returned by a seccomp filter are:
> .TP
> .BR SECCOMP_RET_KILL
> This value results in the process exiting immediately
> without executing the system call.
> The process terminates as though killed by a
> .B SIGSYS
> signal
> .RI ( not
> .BR SIGKILL ).
> .TP
> .BR SECCOMP_RET_TRAP
> This value results in the kernel sending a
> .BR SIGSYS
> signal to the triggering process without executing the system call.
> Various fields will be set in the
> .I siginfo_t
> structure (see
> .BR sigaction (2))
> associated with signal:
> .RS
> .IP * 3
> .I si_signo
> will contain
> .BR SIGSYS .
> .IP *
> .IR si_call_addr
> will show the address of the system call instruction.
> .IP *
> .IR si_syscall
> and
> .IR si_arch
> will indicate which system call was attempted.
> .IP *
> .I si_code
> .\" FIXME Why is the constant thus named? All of the other 'si_code'
> .\"       constants are prefixed 'SI_'. Why the inconsistency?
> will contain
> .BR SYS_SECCOMP .

Only certain reserved values have the SI_ prefix. All the
signal-specific values have their signal name as the prefix. See ILL_*
FPE_* SEGV_* BUS_* TRAP_* CLD_* POLL_* and SYS_*. I see these in
/usr/include/asm-generic/siginfo.h

> .IP *
> .I si_errno
> will contain the
> .BR SECCOMP_RET_DATA
> portion of the filter return value.
> .RE
> .IP
> 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.
> .\" FIXME Regarding the preceding line, can you give an example(s)
> .\"       of "something sensible"? (Depending on the answer, maybe it
> .\"       might be useful to add some text on this point.)

This means sensible in the context of the syscall made, or the desired
behavior. For example, setting the return value to ELOOP for something
like a "bind" syscall isn't very sensible.

> .\"
> .\" FIXME Please check:
> .\"     In an attempt to make the text clearer, I changed
> .\"     "replacing it with" to "setting the return value register to"
> .\"     Okay?
> (The architecture dependency is because setting the return value register to
> .BR ENOSYS
> could overwrite some useful information.)

Well, the arch dependency is really because _how_ to change the
register, and the register itself, is different between architectures.
(i.e. which ptrace call is needed, and which register is being
changed.) The overwriting of useful information is certainly true too,
though.

> .TP
> .BR SECCOMP_RET_ERRNO
> This value results in the
> .B SECCOMP_RET_DATA
> portion of the filter's return value being passed to user space as the
> .IR errno
> value without executing the system call.
> .TP
> .BR SECCOMP_RET_TRACE
> When returned, this value will cause the kernel to attempt to notify a
> .BR ptrace (2)-based
> tracer prior to executing the system call.
> If there is no tracer present,
> the system call is not executed and returns a failure status with
> .I errno
> set to
> .BR ENOSYS .
>
> 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 filter's return value will be available to the tracer via
> .BR PTRACE_GETEVENTMSG .
>
> The tracer can skip the system call by changing the system call number
> to \-1.
> Alternatively, the tracer can change the system call
> requested by changing the system call to a valid system call 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
> .B "must not"
> allow use of
> .BR ptrace (2)\(emeven
> of other
> sandboxed processes\(emwithout extreme care;
> .\" FIXME Below, I think it would be helpful to add some words after
> .\"       "to escape", as in "to escape [what?]" I suppose the wording
> .\"       would be something like "to escape the seccomp sandbox mechanism"
> .\"       but perhaps you have a better wording.
> ptracers can use this mechanism to escape.)

Yeah, that could be further clarified to "... use this mechanism to
escape from the seccomp sandbox." How does that sound?

> .TP
> .BR SECCOMP_RET_ALLOW
> This value results in the system call being executed.
> .SH RETURN VALUE
> On success,
> .BR seccomp ()
> returns 0.
> On error, if
> .BR SECCOMP_FILTER_FLAG_TSYNC
> was used,
> the return value is the ID of the thread
> that caused the synchronization failure.
> (This ID is a kernel thread ID of the type returned by
> .BR clone (2)
> and
> .BR gettid (2).)
> 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 not a valid address.
> .TP
> .BR EINVAL
> .IR operation
> is unknown; or
> .IR flags
> are invalid for the given
> .IR operation .
> .\" FIXME Please review the following
> .TP
> .BR EINVAL
> .I operation
> included
> .BR BPF_ABS ,
> but the specified offset was not aligned to a 32-bit boundary or exceeded
> .IR "sizeof(struct\ seccomp_data)" .
> .\" FIXME Please review the following
> .TP
> .BR EINVAL
> .\" See kernel/seccomp.c::seccomp_may_assign_mode() in 3.18 sources
> A secure computing mode has already been set, and
> .I operation
> differs from the existing setting.
> .\" FIXME Please review the following
> .TP
> .BR EINVAL
> .\" See stub kernel/seccomp.c::seccomp_set_mode_filter() in 3.18 sources
> .I operation
> specified
> .BR SECCOMP_SET_MODE_FILTER ,
> but the kernel was not built with
> .B CONFIG_SECCOMP_FILTER
> enabled.
> .\" FIXME Please review the following
> .TP
> .BR EINVAL
> .I operation
> specified
> .BR SECCOMP_SET_MODE_FILTER ,
> but the filter program pointed to by
> .I args
> was not valid or the length of the filter program was zero or exceeded
> .B BPF_MAXINSNS
> (4096) instructions.
> .BR EINVAL
> .TP
> .BR ENOMEM
> Out of memory.
> .\" FIXME Please review the following
> .TP
> .BR ENOMEM
> .\" ENOMEM in kernel/seccomp.c::seccomp_attach_filter() in 3.18 sources
> The total length of all filter programs attached
> to the calling thread would exceed
> .B MAX_INSNS_PER_PATH
> (32768) instructions.
> Note that for the purposes of calculating this limit,
> each already existing filter program incurs an
> overhead penalty of 4 instructions.
> .TP
> .BR ESRCH
> Another thread caused a failure during thread sync, but its ID could not
> be determined.
> .SH VERSIONS
> The
> .BR seccomp()
> system call first appeared in Linux 3.17.
> .\" FIXME . Add glibc version
> .SH CONFORMING TO
> The
> .BR seccomp()
> system call is a nonstandard Linux extension.
> .SH NOTES
> .BR seccomp ()
> provides a superset of the functionality provided by the
> .BR prctl (2)
> .BR PR_SET_SECCOMP
> operation (which does not support
> .IR flags ).
> .\" FIXME Please review the following new subsection {{{
> .SS Seccomp-specific BPF details
> Note the following BPF details specific to seccomp filters:
> .IP * 3
> The
> .B BPF_H
> and
> .B BPF_B
> size modifiers are not supported: all operations must load and store
> (4-byte) words
> .RB ( BPF_W ).
> .IP *
> To access the contents of the
> .I seccomp_data
> buffer, use the
> .B BPF_ABS
> addressing mode modifier.
> .\" FIXME What is the significance of the line
> .\"           ftest->code = BPF_LDX | BPF_W | BPF_ABS;
> .\"       in kernel/seccomp.c::seccomp_check_filter()?

This is converting an accumulator load (BPF_LD) into a index load
(BPF_LDX). I think this is to avoid addressing modes 1 and 2, but Will
may remember more here. The LD|W|ABS structure is very common, so I
think this was a way to accept that in the filter, but change it into
a more limited command.

> .IP *
> The
> .B BPF_LEN
> addressing mode modifier yields an immediate mode operand
> whose value is the size of the
> .IR seccomp_data
> buffer.
> .\" FIXME Any other seccomp-specific BPF details that should be added here?
> .\"
> .\" FIXME End of new subsection for review }}}

All the rest of the FIXMEs above (excepting the standing glibc one)
looks correct to me.

> .SH EXAMPLE
> The program below accepts four or more arguments.
> The first three arguments are a system call number,
> a numeric architecture identifier, and an error number.
> The program uses these values to construct a BPF filter
> that is used at run time to perform the following checks:
> .IP [1] 4
> If the program is not running on the specified architecture,
> the BPF filter causes system calls to fail with the error
> .BR ENOSYS .
> .IP [2]
> If the program attempts to execute the system call with the specified number,
> the BPF filter causes the system call to fail, with
> .I errno
> being set to the specified error number.
> .PP
> The remaining command-line arguments specify
> the pathname and additional arguments of a program
> that the example program should attempt to execute using
> .BR execve (3)
> (a library function that employs the
> .BR execve (2)
> system call).
> Some example runs of the program are shown below.
>
> First, we display the architecture that we are running on (x86-64)
> and then construct a shell function that looks up system call
> numbers on this architecture:
>
> .nf
> .in +4n
> $ \fBuname -m\fP
> x86_64
> $ \fBsyscall_nr() {
>     cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \\
>     awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
> }\fP
> .in
> .fi
>
> When the BPF filter rejects a system call (case [2] above),
> it causes the system call to fail with the error number
> specified on the command line.
> In the experiments shown here, we'll use error number 99:
>
> .nf
> .in +4n
> $ \fBerrno 99\fP
> EADDRNOTAVAIL 99 Cannot assign requested address
> .in
> .fi
>
> In the following example, we attempt to run the command
> .BR whoami (1),
> but the BPF filter rejects the
> .BR execve (2)
> system call, so that the command is not even executed:
>
> .nf
> .in +4n
> $ \fBsyscall_nr execve\fP
> 59
> $ \fB./a.out\fP
> Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
> Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
>                  AUDIT_ARCH_X86_64: 0xC000003E
> $ \fB./a.out 59 0xC000003E 99 /bin/whoami\fP
> execv: Cannot assign requested address
> .in
> .fi
>
> In the next example, the BPF filter rejects the
> .BR write (2)
> system call, so that, although it is successfully started, the
> .BR whoami (1)
> command is not able to write output:
>
> .nf
> .in +4n
> $ \fBsyscall_nr write\fP
> 1
> $ \fB./a.out 1 0xC000003E 99 /bin/whoami\fP
> .in
> .fi
>
> In the final example,
> the BPF filter rejects a system call that is not used by the
> .BR whoami (1)
> command, so it is able to successfully execute and produce output:
>
> .nf
> .in +4n
> $ \fBsyscall_nr preadv\fP
> 295
> $ \fB./a.out 295 0xC000003E 99 /bin/whoami\fP
> cecilia
> .in
> .fi
> .SS Program source
> .fi
> .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_nr, int t_arch, int f_errno)
> {
> .\" FIXME In the BPF program below, you use '+' to build the instructions.
> .\"       However, most other BPF example code I see uses '|'. While I
> .\"       assume it's equivalent (i.e., the bit fields are nonoverlapping),
> .\"       was there a reason to use '+' rather than '|'? (To me, the
> .\"       latter is a little clearer in its intent.)

Ah, no, "|" should be used, good catch.

> .\"
> .\" FIXME I expanded comments [0], [1], [2], [3], [4] a little.
> .\"       Are they okay? */

Yup, these look good to me.

> .\"
>     struct sock_filter filter[] = {
>         /* [0] Load architecture from 'seccomp_data' buffer into
>                accumulator */
>         BPF_STMT(BPF_LD + BPF_W + BPF_ABS,
>                  (offsetof(struct seccomp_data, arch))),
>
>         /* [1] Jump forward 4 instructions if architecture does not
>                match 't_arch' */
>         BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, t_arch, 0, 4),
>
>         /* [2] Load system call number from 'seccomp_data' buffer into
>                accumulator */
>         BPF_STMT(BPF_LD + BPF_W + BPF_ABS,
>                  (offsetof(struct seccomp_data, nr))),
>
>         /* [3] Jump forward 1 instruction if system call number
>                does not match 'syscall_nr' */
>         BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, syscall_nr, 0, 1),
>
>         /* [4] Matching architecture and system call: don't execute
>                the system call, and return 'f_errno' in 'errno' */
>         BPF_STMT(BPF_RET + BPF_K,
>                  SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),
>
>         /* [5] Destination of system call number mismatch: allow other
>                system calls */
>         BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_ALLOW),
>
>         /* [6] Destination of architecture 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 1;
>     }
>
>     return 0;
> }
>
> int
> main(int argc, char **argv)
> {
>     if (argc < 5) {
>         fprintf(stderr, "Usage: "
>                 "%s <syscall_nr> <arch> <errno> <prog> [<args>]\\n"
>                 "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\\n"
>                 "                 AUDIT_ARCH_X86_64: 0x%X\\n"
>                 "\\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
>         exit(EXIT_FAILURE);
>     }
>
>     if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
>         perror("prctl");
>         exit(EXIT_FAILURE);
>     }
>
>     if (install_filter(strtol(argv[1], NULL, 0),
>                        strtol(argv[2], NULL, 0),
>                        strtol(argv[3], NULL, 0)))
>         exit(EXIT_FAILURE);
>
>     execv(argv[4], &argv[4]);
>     perror("execv");
>     exit(EXIT_FAILURE);
> }
> .fi
> .SH SEE ALSO
> .BR prctl (2),
> .BR ptrace (2),
> .BR signal (7),
> .BR socket (7)
> .sp
> The kernel source files
> .IR Documentation/networking/filter.txt
> and
> .IR Documentation/prctl/seccomp_filter.txt .
> .sp
> McCanne, S. and Jacobson, V. (1992)
> .IR "The BSD Packet Filter: A New Architecture for User-level Packet Capture" ,
> Proceedings of the USENIX Winter 1993 Conference
> .UR http://www.tcpdump.org/papers/bpf-usenix93.pdf
> .UE
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

Thanks for the additional details and clarifications!

-Kees

-- 
Kees Cook
Chrome OS Security
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