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Diffstat (limited to 'Documentation/networking')
| -rw-r--r-- | Documentation/networking/filter.txt | 608 |
1 files changed, 561 insertions, 47 deletions
diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt index cdb3e40..a06b48d 100644 --- a/Documentation/networking/filter.txt +++ b/Documentation/networking/filter.txt @@ -1,49 +1,563 @@ -filter.txt: Linux Socket Filtering -Written by: Jay Schulist <jschlst@samba.org> +Linux Socket Filtering aka Berkeley Packet Filter (BPF) +======================================================= Introduction -============ - - Linux Socket Filtering is derived from the Berkeley -Packet Filter. There are some distinct differences between -the BSD and Linux Kernel Filtering. - -Linux Socket Filtering (LSF) allows a user-space program to -attach a filter onto any socket and allow or disallow certain -types of data to come through the socket. LSF follows exactly -the same filter code structure as the BSD Berkeley Packet Filter -(BPF), so referring to the BSD bpf.4 manpage is very helpful in -creating filters. - -LSF is much simpler than BPF. One does not have to worry about -devices or anything like that. You simply create your filter -code, send it to the kernel via the SO_ATTACH_FILTER option and -if your filter code passes the kernel check on it, you then -immediately begin filtering data on that socket. - -You can also detach filters from your socket via the -SO_DETACH_FILTER option. This will probably not be used much -since when you close a socket that has a filter on it the -filter is automagically removed. The other less common case -may be adding a different filter on the same socket where you had another -filter that is still running: the kernel takes care of removing -the old one and placing your new one in its place, assuming your -filter has passed the checks, otherwise if it fails the old filter -will remain on that socket. - -SO_LOCK_FILTER option allows to lock the filter attached to a -socket. Once set, a filter cannot be removed or changed. This allows -one process to setup a socket, attach a filter, lock it then drop -privileges and be assured that the filter will be kept until the -socket is closed. - -Examples -======== - -Ioctls- -setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter)); -setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value)); -setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value)); - -See the BSD bpf.4 manpage and the BSD Packet Filter paper written by -Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory. +------------ + +Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter. +Though there are some distinct differences between the BSD and Linux +Kernel filtering, but when we speak of BPF or LSF in Linux context, we +mean the very same mechanism of filtering in the Linux kernel. + +BPF allows a user-space program to attach a filter onto any socket and +allow or disallow certain types of data to come through the socket. LSF +follows exactly the same filter code structure as BSD's BPF, so referring +to the BSD bpf.4 manpage is very helpful in creating filters. + +On Linux, BPF is much simpler than on BSD. One does not have to worry +about devices or anything like that. You simply create your filter code, +send it to the kernel via the SO_ATTACH_FILTER option and if your filter +code passes the kernel check on it, you then immediately begin filtering +data on that socket. + +You can also detach filters from your socket via the SO_DETACH_FILTER +option. This will probably not be used much since when you close a socket +that has a filter on it the filter is automagically removed. The other +less common case may be adding a different filter on the same socket where +you had another filter that is still running: the kernel takes care of +removing the old one and placing your new one in its place, assuming your +filter has passed the checks, otherwise if it fails the old filter will +remain on that socket. + +SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once +set, a filter cannot be removed or changed. This allows one process to +setup a socket, attach a filter, lock it then drop privileges and be +assured that the filter will be kept until the socket is closed. + +The biggest user of this construct might be libpcap. Issuing a high-level +filter command like `tcpdump -i em1 port 22` passes through the libpcap +internal compiler that generates a structure that can eventually be loaded +via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd` +displays what is being placed into this structure. + +Although we were only speaking about sockets here, BPF in Linux is used +in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel +qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places +such as team driver, PTP code, etc where BPF is being used. + + [1] Documentation/prctl/seccomp_filter.txt + +Original BPF paper: + +Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new +architecture for user-level packet capture. In Proceedings of the +USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993 +Conference Proceedings (USENIX'93). USENIX Association, Berkeley, +CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf] + +Structure +--------- + +User space applications include <linux/filter.h> which contains the +following relevant structures: + +struct sock_filter { /* Filter block */ + __u16 code; /* Actual filter code */ + __u8 jt; /* Jump true */ + __u8 jf; /* Jump false */ + __u32 k; /* Generic multiuse field */ +}; + +Such a structure is assembled as an array of 4-tuples, that contains +a code, jt, jf and k value. jt and jf are jump offsets and k a generic +value to be used for a provided code. + +struct sock_fprog { /* Required for SO_ATTACH_FILTER. */ + unsigned short len; /* Number of filter blocks */ + struct sock_filter __user *filter; +}; + +For socket filtering, a pointer to this structure (as shown in +follow-up example) is being passed to the kernel through setsockopt(2). + +Example +------- + +#include <sys/socket.h> +#include <sys/types.h> +#include <arpa/inet.h> +#include <linux/if_ether.h> +/* ... */ + +/* From the example above: tcpdump -i em1 port 22 -dd */ +struct sock_filter code[] = { + { 0x28, 0, 0, 0x0000000c }, + { 0x15, 0, 8, 0x000086dd }, + { 0x30, 0, 0, 0x00000014 }, + { 0x15, 2, 0, 0x00000084 }, + { 0x15, 1, 0, 0x00000006 }, + { 0x15, 0, 17, 0x00000011 }, + { 0x28, 0, 0, 0x00000036 }, + { 0x15, 14, 0, 0x00000016 }, + { 0x28, 0, 0, 0x00000038 }, + { 0x15, 12, 13, 0x00000016 }, + { 0x15, 0, 12, 0x00000800 }, + { 0x30, 0, 0, 0x00000017 }, + { 0x15, 2, 0, 0x00000084 }, + { 0x15, 1, 0, 0x00000006 }, + { 0x15, 0, 8, 0x00000011 }, + { 0x28, 0, 0, 0x00000014 }, + { 0x45, 6, 0, 0x00001fff }, + { 0xb1, 0, 0, 0x0000000e }, + { 0x48, 0, 0, 0x0000000e }, + { 0x15, 2, 0, 0x00000016 }, + { 0x48, 0, 0, 0x00000010 }, + { 0x15, 0, 1, 0x00000016 }, + { 0x06, 0, 0, 0x0000ffff }, + { 0x06, 0, 0, 0x00000000 }, +}; + +struct sock_fprog bpf = { + .len = ARRAY_SIZE(code), + .filter = code, +}; + +sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); +if (sock < 0) + /* ... bail out ... */ + +ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf)); +if (ret < 0) + /* ... bail out ... */ + +/* ... */ +close(sock); + +The above example code attaches a socket filter for a PF_PACKET socket +in order to let all IPv4/IPv6 packets with port 22 pass. The rest will +be dropped for this socket. + +The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments +and SO_LOCK_FILTER for preventing the filter to be detached, takes an +integer value with 0 or 1. + +Note that socket filters are not restricted to PF_PACKET sockets only, +but can also be used on other socket families. + +Summary of system calls: + + * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val)); + * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val)); + * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val)); + +Normally, most use cases for socket filtering on packet sockets will be +covered by libpcap in high-level syntax, so as an application developer +you should stick to that. libpcap wraps its own layer around all that. + +Unless i) using/linking to libpcap is not an option, ii) the required BPF +filters use Linux extensions that are not supported by libpcap's compiler, +iii) a filter might be more complex and not cleanly implementable with +libpcap's compiler, or iv) particular filter codes should be optimized +differently than libpcap's internal compiler does; then in such cases +writing such a filter "by hand" can be of an alternative. For example, +xt_bpf and cls_bpf users might have requirements that could result in +more complex filter code, or one that cannot be expressed with libpcap +(e.g. different return codes for various code paths). Moreover, BPF JIT +implementors may wish to manually write test cases and thus need low-level +access to BPF code as well. + +BPF engine and instruction set +------------------------------ + +Under tools/net/ there's a small helper tool called bpf_asm which can +be used to write low-level filters for example scenarios mentioned in the +previous section. Asm-like syntax mentioned here has been implemented in +bpf_asm and will be used for further explanations (instead of dealing with +less readable opcodes directly, principles are the same). The syntax is +closely modelled after Steven McCanne's and Van Jacobson's BPF paper. + +The BPF architecture consists of the following basic elements: + + Element Description + + A 32 bit wide accumulator + X 32 bit wide X register + M[] 16 x 32 bit wide misc registers aka "scratch memory + store", addressable from 0 to 15 + +A program, that is translated by bpf_asm into "opcodes" is an array that +consists of the following elements (as already mentioned): + + op:16, jt:8, jf:8, k:32 + +The element op is a 16 bit wide opcode that has a particular instruction +encoded. jt and jf are two 8 bit wide jump targets, one for condition +"jump if true", the other one "jump if false". Eventually, element k +contains a miscellaneous argument that can be interpreted in different +ways depending on the given instruction in op. + +The instruction set consists of load, store, branch, alu, miscellaneous +and return instructions that are also represented in bpf_asm syntax. This +table lists all bpf_asm instructions available resp. what their underlying +opcodes as defined in linux/filter.h stand for: + + Instruction Addressing mode Description + + ld 1, 2, 3, 4, 10 Load word into A + ldi 4 Load word into A + ldh 1, 2 Load half-word into A + ldb 1, 2 Load byte into A + ldx 3, 4, 5, 10 Load word into X + ldxi 4 Load word into X + ldxb 5 Load byte into X + + st 3 Store A into M[] + stx 3 Store X into M[] + + jmp 6 Jump to label + ja 6 Jump to label + jeq 7, 8 Jump on k == A + jneq 8 Jump on k != A + jne 8 Jump on k != A + jlt 8 Jump on k < A + jle 8 Jump on k <= A + jgt 7, 8 Jump on k > A + jge 7, 8 Jump on k >= A + jset 7, 8 Jump on k & A + + add 0, 4 A + <x> + sub 0, 4 A - <x> + mul 0, 4 A * <x> + div 0, 4 A / <x> + mod 0, 4 A % <x> + neg 0, 4 !A + and 0, 4 A & <x> + or 0, 4 A | <x> + xor 0, 4 A ^ <x> + lsh 0, 4 A << <x> + rsh 0, 4 A >> <x> + + tax Copy A into X + txa Copy X into A + + ret 4, 9 Return + +The next table shows addressing formats from the 2nd column: + + Addressing mode Syntax Description + + 0 x/%x Register X + 1 [k] BHW at byte offset k in the packet + 2 [x + k] BHW at the offset X + k in the packet + 3 M[k] Word at offset k in M[] + 4 #k Literal value stored in k + 5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet + 6 L Jump label L + 7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf + 8 #k,Lt Jump to Lt if predicate is true + 9 a/%a Accumulator A + 10 extension BPF extension + +The Linux kernel also has a couple of BPF extensions that are used along +with the class of load instructions by "overloading" the k argument with +a negative offset + a particular extension offset. The result of such BPF +extensions are loaded into A. + +Possible BPF extensions are shown in the following table: + + Extension Description + + len skb->len + proto skb->protocol + type skb->pkt_type + poff Payload start offset + ifidx skb->dev->ifindex + nla Netlink attribute of type X with offset A + nlan Nested Netlink attribute of type X with offset A + mark skb->mark + queue skb->queue_mapping + hatype skb->dev->type + rxhash skb->rxhash + cpu raw_smp_processor_id() + vlan_tci vlan_tx_tag_get(skb) + vlan_pr vlan_tx_tag_present(skb) + +These extensions can also be prefixed with '#'. +Examples for low-level BPF: + +** ARP packets: + + ldh [12] + jne #0x806, drop + ret #-1 + drop: ret #0 + +** IPv4 TCP packets: + + ldh [12] + jne #0x800, drop + ldb [23] + jneq #6, drop + ret #-1 + drop: ret #0 + +** (Accelerated) VLAN w/ id 10: + + ld vlan_tci + jneq #10, drop + ret #-1 + drop: ret #0 + +** SECCOMP filter example: + + ld [4] /* offsetof(struct seccomp_data, arch) */ + jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */ + ld [0] /* offsetof(struct seccomp_data, nr) */ + jeq #15, good /* __NR_rt_sigreturn */ + jeq #231, good /* __NR_exit_group */ + jeq #60, good /* __NR_exit */ + jeq #0, good /* __NR_read */ + jeq #1, good /* __NR_write */ + jeq #5, good /* __NR_fstat */ + jeq #9, good /* __NR_mmap */ + jeq #14, good /* __NR_rt_sigprocmask */ + jeq #13, good /* __NR_rt_sigaction */ + jeq #35, good /* __NR_nanosleep */ + bad: ret #0 /* SECCOMP_RET_KILL */ + good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */ + +The above example code can be placed into a file (here called "foo"), and +then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf +and cls_bpf understands and can directly be loaded with. Example with above +ARP code: + +$ ./bpf_asm foo +4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0, + +In copy and paste C-like output: + +$ ./bpf_asm -c foo +{ 0x28, 0, 0, 0x0000000c }, +{ 0x15, 0, 1, 0x00000806 }, +{ 0x06, 0, 0, 0xffffffff }, +{ 0x06, 0, 0, 0000000000 }, + +In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF +filters that might not be obvious at first, it's good to test filters before +attaching to a live system. For that purpose, there's a small tool called +bpf_dbg under tools/net/ in the kernel source directory. This debugger allows +for testing BPF filters against given pcap files, single stepping through the +BPF code on the pcap's packets and to do BPF machine register dumps. + +Starting bpf_dbg is trivial and just requires issuing: + +# ./bpf_dbg + +In case input and output do not equal stdin/stdout, bpf_dbg takes an +alternative stdin source as a first argument, and an alternative stdout +sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`. + +Other than that, a particular libreadline configuration can be set via +file "~/.bpf_dbg_init" and the command history is stored in the file +"~/.bpf_dbg_history". + +Interaction in bpf_dbg happens through a shell that also has auto-completion +support (follow-up example commands starting with '>' denote bpf_dbg shell). +The usual workflow would be to ... + +> load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0 + Loads a BPF filter from standard output of bpf_asm, or transformed via + e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT + debugging (next section), this command creates a temporary socket and + loads the BPF code into the kernel. Thus, this will also be useful for + JIT developers. + +> load pcap foo.pcap + Loads standard tcpdump pcap file. + +> run [<n>] +bpf passes:1 fails:9 + Runs through all packets from a pcap to account how many passes and fails + the filter will generate. A limit of packets to traverse can be given. + +> disassemble +l0: ldh [12] +l1: jeq #0x800, l2, l5 +l2: ldb [23] +l3: jeq #0x1, l4, l5 +l4: ret #0xffff +l5: ret #0 + Prints out BPF code disassembly. + +> dump +/* { op, jt, jf, k }, */ +{ 0x28, 0, 0, 0x0000000c }, +{ 0x15, 0, 3, 0x00000800 }, +{ 0x30, 0, 0, 0x00000017 }, +{ 0x15, 0, 1, 0x00000001 }, +{ 0x06, 0, 0, 0x0000ffff }, +{ 0x06, 0, 0, 0000000000 }, + Prints out C-style BPF code dump. + +> breakpoint 0 +breakpoint at: l0: ldh [12] +> breakpoint 1 +breakpoint at: l1: jeq #0x800, l2, l5 + ... + Sets breakpoints at particular BPF instructions. Issuing a `run` command + will walk through the pcap file continuing from the current packet and + break when a breakpoint is being hit (another `run` will continue from + the currently active breakpoint executing next instructions): + + > run + -- register dump -- + pc: [0] <-- program counter + code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction + curr: l0: ldh [12] <-- disassembly of current instruction + A: [00000000][0] <-- content of A (hex, decimal) + X: [00000000][0] <-- content of X (hex, decimal) + M[0,15]: [00000000][0] <-- folded content of M (hex, decimal) + -- packet dump -- <-- Current packet from pcap (hex) + len: 42 + 0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01 + 16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26 + 32: 00 00 00 00 00 00 0a 3b 01 01 + (breakpoint) + > + +> breakpoint +breakpoints: 0 1 + Prints currently set breakpoints. + +> step [-<n>, +<n>] + Performs single stepping through the BPF program from the current pc + offset. Thus, on each step invocation, above register dump is issued. + This can go forwards and backwards in time, a plain `step` will break + on the next BPF instruction, thus +1. (No `run` needs to be issued here.) + +> select <n> + Selects a given packet from the pcap file to continue from. Thus, on + the next `run` or `step`, the BPF program is being evaluated against + the user pre-selected packet. Numbering starts just as in Wireshark + with index 1. + +> quit +# + Exits bpf_dbg. + +JIT compiler +------------ + +The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC, +ARM and s390 and can be enabled through CONFIG_BPF_JIT. The JIT compiler is +transparently invoked for each attached filter from user space or for internal +kernel users if it has been previously enabled by root: + + echo 1 > /proc/sys/net/core/bpf_jit_enable + +For JIT developers, doing audits etc, each compile run can output the generated +opcode image into the kernel log via: + + echo 2 > /proc/sys/net/core/bpf_jit_enable + +Example output from dmesg: + +[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f +[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68 +[ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00 +[ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00 +[ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00 +[ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3 + +In the kernel source tree under tools/net/, there's bpf_jit_disasm for +generating disassembly out of the kernel log's hexdump: + +# ./bpf_jit_disasm +70 bytes emitted from JIT compiler (pass:3, flen:6) +ffffffffa0069c8f + <x>: + 0: push %rbp + 1: mov %rsp,%rbp + 4: sub $0x60,%rsp + 8: mov %rbx,-0x8(%rbp) + c: mov 0x68(%rdi),%r9d + 10: sub 0x6c(%rdi),%r9d + 14: mov 0xd8(%rdi),%r8 + 1b: mov $0xc,%esi + 20: callq 0xffffffffe0ff9442 + 25: cmp $0x800,%eax + 2a: jne 0x0000000000000042 + 2c: mov $0x17,%esi + 31: callq 0xffffffffe0ff945e + 36: cmp $0x1,%eax + 39: jne 0x0000000000000042 + 3b: mov $0xffff,%eax + 40: jmp 0x0000000000000044 + 42: xor %eax,%eax + 44: leaveq + 45: retq + +Issuing option `-o` will "annotate" opcodes to resulting assembler +instructions, which can be very useful for JIT developers: + +# ./bpf_jit_disasm -o +70 bytes emitted from JIT compiler (pass:3, flen:6) +ffffffffa0069c8f + <x>: + 0: push %rbp + 55 + 1: mov %rsp,%rbp + 48 89 e5 + 4: sub $0x60,%rsp + 48 83 ec 60 + 8: mov %rbx,-0x8(%rbp) + 48 89 5d f8 + c: mov 0x68(%rdi),%r9d + 44 8b 4f 68 + 10: sub 0x6c(%rdi),%r9d + 44 2b 4f 6c + 14: mov 0xd8(%rdi),%r8 + 4c 8b 87 d8 00 00 00 + 1b: mov $0xc,%esi + be 0c 00 00 00 + 20: callq 0xffffffffe0ff9442 + e8 1d 94 ff e0 + 25: cmp $0x800,%eax + 3d 00 08 00 00 + 2a: jne 0x0000000000000042 + 75 16 + 2c: mov $0x17,%esi + be 17 00 00 00 + 31: callq 0xffffffffe0ff945e + e8 28 94 ff e0 + 36: cmp $0x1,%eax + 83 f8 01 + 39: jne 0x0000000000000042 + 75 07 + 3b: mov $0xffff,%eax + b8 ff ff 00 00 + 40: jmp 0x0000000000000044 + eb 02 + 42: xor %eax,%eax + 31 c0 + 44: leaveq + c9 + 45: retq + c3 + +For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful +toolchain for developing and testing the kernel's JIT compiler. + +Misc +---- + +Also trinity, the Linux syscall fuzzer, has built-in support for BPF and +SECCOMP-BPF kernel fuzzing. + +Written by +---------- + +The document was written in the hope that it is found useful and in order +to give potential BPF hackers or security auditors a better overview of +the underlying architecture. + +Jay Schulist <jschlst@samba.org> +Daniel Borkmann <dborkman@redhat.com> |
