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priod: in-kernel packet filtering

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This commit is contained in:
Přemysl Eric Janouch 2017-07-05 14:22:57 +02:00
parent 5b334e4111
commit 7713850987
Signed by: p
GPG Key ID: B715679E3A361BE6
1 changed files with 53 additions and 5 deletions
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58
priod.c
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@ -30,6 +30,7 @@
#include <linux/cn_proc.h>
#include <linux/netlink.h>
#include <linux/connector.h>
#include <linux/filter.h>
#include <sys/resource.h>
#include <sys/syscall.h>
@ -173,8 +174,10 @@ on_exec (struct app_context *ctx, int pid)
static void
on_netlink_message (struct app_context *ctx, struct nlmsghdr *mh)
{
if (mh->nlmsg_type == NLMSG_ERROR
|| mh->nlmsg_type == NLMSG_NOOP)
// In practice the kernel connector never sends multipart messages
if (!soft_assert (mh->nlmsg_type != 0)
|| !soft_assert (mh->nlmsg_flags == 0)
|| mh->nlmsg_type != NLMSG_DONE)
return;
struct cn_msg *m = NLMSG_DATA (mh);
@ -182,6 +185,7 @@ on_netlink_message (struct app_context *ctx, struct nlmsghdr *mh)
|| m->id.val != CN_VAL_PROC)
return;
// XXX: potential alignment issues
struct proc_event *e = (struct proc_event *) m->data;
if (e->what == PROC_EVENT_EXEC)
on_exec (ctx, e->event_data.exit.process_tgid);
@ -190,7 +194,7 @@ on_netlink_message (struct app_context *ctx, struct nlmsghdr *mh)
static void
on_event (const struct pollfd *pfd, struct app_context *ctx)
{
char buf[4096];
char buf[sysconf (_SC_PAGESIZE)];
struct sockaddr_nl addr;
while (true)
{
@ -208,6 +212,7 @@ on_event (const struct pollfd *pfd, struct app_context *ctx)
if (addr.nl_pid)
continue;
// In practice the kernel connector always sends one per dgram
for (struct nlmsghdr *mh = (struct nlmsghdr *) buf;
NLMSG_OK (mh, len); mh = NLMSG_NEXT (mh, len))
on_netlink_message (ctx, mh);
@ -269,6 +274,48 @@ parse_program_arguments (int argc, char **argv)
return argv[0];
}
/// Sets up a filter so that we're only woken up by the kernel on exec() events
static void
setup_exec_filter (int fd)
{
struct incoming
{
union { struct nlmsghdr netlink; char align[NLMSG_HDRLEN]; };
struct cn_msg connector;
struct proc_event event;
}
__attribute__ ((packed));
// Byteswapping is needed because the netlink protocol is host-endian
struct sock_filter filter[] =
{
// Only continue filtering dgrams with one "proc_event" message in them
BPF_STMT (BPF_LD | BPF_W | BPF_LEN, 0),
BPF_JUMP (BPF_JMP | BPF_JEQ | BPF_K, sizeof (struct incoming), 0, 9),
BPF_STMT (BPF_LD | BPF_H | BPF_ABS,
offsetof (struct incoming, netlink.nlmsg_type)),
BPF_JUMP (BPF_JMP | BPF_JEQ | BPF_K, htons (NLMSG_DONE), 0, 7),
BPF_STMT (BPF_LD | BPF_W | BPF_ABS,
offsetof (struct incoming, connector.id.idx)),
BPF_JUMP (BPF_JMP | BPF_JEQ | BPF_K, htonl (CN_IDX_PROC), 0, 5),
BPF_STMT (BPF_LD | BPF_W | BPF_ABS,
offsetof (struct incoming, connector.id.val)),
BPF_JUMP (BPF_JMP | BPF_JEQ | BPF_K, htonl (CN_VAL_PROC), 0, 3),
BPF_STMT (BPF_LD | BPF_W | BPF_ABS,
offsetof (struct incoming, event.what)),
BPF_JUMP (BPF_JMP | BPF_JEQ | BPF_K, htonl (PROC_EVENT_EXEC), 1, 0),
BPF_STMT (BPF_RET | BPF_K, 0),
BPF_STMT (BPF_RET | BPF_K, 0xffffffff),
};
struct sock_fprog fprog = { .filter = filter, .len = N_ELEMENTS (filter) };
const int yes = 1;
if (setsockopt (fd, SOL_SOCKET, SO_ATTACH_FILTER, &fprog, sizeof fprog) < 0
|| setsockopt (fd, SOL_NETLINK, NETLINK_NO_ENOBUFS, &yes, sizeof yes) < 0)
print_error ("setsockopt: %s", strerror (errno));
}
int
main (int argc, char *argv[])
{
@ -294,6 +341,8 @@ main (int argc, char *argv[])
if (ctx.proc_fd < 0)
exit_fatal ("cannot make a proc connector: %s", strerror (errno));
setup_exec_filter (ctx.proc_fd);
struct sockaddr_nl addr = { .nl_family = AF_NETLINK, .nl_pid = getpid (),
.nl_groups = CN_IDX_PROC };
if (bind (ctx.proc_fd, (struct sockaddr *) &addr, sizeof addr) < 0)
@ -301,12 +350,11 @@ main (int argc, char *argv[])
struct
{
// Beware of padding between fields, shouldn't be any on x86-64
union { struct nlmsghdr netlink; char align[NLMSG_HDRLEN]; };
struct cn_msg connector;
enum proc_cn_mcast_op op;
}
subscription =
__attribute__ ((packed)) subscription =
{
.netlink.nlmsg_len = sizeof subscription,
.netlink.nlmsg_type = NLMSG_DONE,