strparser: initialize all callbacks

commit bbb03029 ("strparser: Generalize strparser") added more
function pointers to 'struct strp_callbacks'; however, kcm_attach() was
not updated to initialize them.  This could cause the ->lock() and/or
->unlock() function pointers to be set to garbage values, causing a
crash in strp_work().

Fix the bug by moving the callback structs into static memory, so
unspecified members are zeroed.  Also constify them while we're at it.

This bug was found by syzkaller, which encountered the following splat:

    IP: 0x55
    PGD 3b1ca067
    P4D 3b1ca067
    PUD 3b12f067
    PMD 0

    Oops: 0010 [#1] SMP KASAN
    Dumping ftrace buffer:
       (ftrace buffer empty)
    Modules linked in:
    CPU: 2 PID: 1194 Comm: kworker/u8:1 Not tainted 4.13.0-rc4-next-20170811 #2
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
    Workqueue: kstrp strp_work
    task: ffff88006bb0e480 task.stack: ffff88006bb10000
    RIP: 0010:0x55
    RSP: 0018:ffff88006bb17540 EFLAGS: 00010246
    RAX: dffffc0000000000 RBX: ffff88006ce4bd60 RCX: 0000000000000000
    RDX: 1ffff1000d9c97bd RSI: 0000000000000000 RDI: ffff88006ce4bc48
    RBP: ffff88006bb17558 R08: ffffffff81467ab2 R09: 0000000000000000
    R10: ffff88006bb17438 R11: ffff88006bb17940 R12: ffff88006ce4bc48
    R13: ffff88003c683018 R14: ffff88006bb17980 R15: ffff88003c683000
    FS:  0000000000000000(0000) GS:ffff88006de00000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 0000000000000055 CR3: 000000003c145000 CR4: 00000000000006e0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
    Call Trace:
     process_one_work+0xbf3/0x1bc0 kernel/workqueue.c:2098
     worker_thread+0x223/0x1860 kernel/workqueue.c:2233
     kthread+0x35e/0x430 kernel/kthread.c:231
     ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:431
    Code:  Bad RIP value.
    RIP: 0x55 RSP: ffff88006bb17540
    CR2: 0000000000000055
    ---[ end trace f0e4920047069cee ]---

Here is a C reproducer (requires CONFIG_BPF_SYSCALL=y and
CONFIG_AF_KCM=y):

    #include <linux/bpf.h>
    #include <linux/kcm.h>
    #include <linux/types.h>
    #include <stdint.h>
    #include <sys/ioctl.h>
    #include <sys/socket.h>
    #include <sys/syscall.h>
    #include <unistd.h>

    static const struct bpf_insn bpf_insns[3] = {
        { .code = 0xb7 }, /* BPF_MOV64_IMM(0, 0) */
        { .code = 0x95 }, /* BPF_EXIT_INSN() */
    };

    static const union bpf_attr bpf_attr = {
        .prog_type = 1,
        .insn_cnt = 2,
        .insns = (uintptr_t)&bpf_insns,
        .license = (uintptr_t)"",
    };

    int main(void)
    {
        int bpf_fd = syscall(__NR_bpf, BPF_PROG_LOAD,
                             &bpf_attr, sizeof(bpf_attr));
        int inet_fd = socket(AF_INET, SOCK_STREAM, 0);
        int kcm_fd = socket(AF_KCM, SOCK_DGRAM, 0);

        ioctl(kcm_fd, SIOCKCMATTACH,
              &(struct kcm_attach) { .fd = inet_fd, .bpf_fd = bpf_fd });
    }

Fixes: bbb03029 ("strparser: Generalize strparser")
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Tom Herbert <tom@quantonium.net>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/strparser.txt

@@ -35,7 +35,7 @@ Functions
 =========
 
 strp_init(struct strparser *strp, struct sock *sk,
-	  struct strp_callbacks *cb)
+	  const struct strp_callbacks *cb)
 
      Called to initialize a stream parser. strp is a struct of type
      strparser that is allocated by the upper layer. sk is the TCP

include/net/strparser.h

@@ -138,7 +138,7 @@ void strp_done(struct strparser *strp);
 void strp_stop(struct strparser *strp);
 void strp_check_rcv(struct strparser *strp);
 int strp_init(struct strparser *strp, struct sock *sk,
-	      struct strp_callbacks *cb);
+	      const struct strp_callbacks *cb);
 void strp_data_ready(struct strparser *strp);
 int strp_process(struct strparser *strp, struct sk_buff *orig_skb,
 		 unsigned int orig_offset, size_t orig_len,

kernel/bpf/sockmap.c

@@ -368,12 +368,12 @@ static int smap_read_sock_done(struct strparser *strp, int err)
 static int smap_init_sock(struct smap_psock *psock,
 			  struct sock *sk)
 {
-	struct strp_callbacks cb;
+	static const struct strp_callbacks cb = {
+		.rcv_msg = smap_read_sock_strparser,
+		.parse_msg = smap_parse_func_strparser,
+		.read_sock_done = smap_read_sock_done,
+	};
 
-	memset(&cb, 0, sizeof(cb));
-	cb.rcv_msg = smap_read_sock_strparser;
-	cb.parse_msg = smap_parse_func_strparser;
-	cb.read_sock_done = smap_read_sock_done;
 	return strp_init(&psock->strp, sk, &cb);
 }
 

net/kcm/kcmsock.c

@@ -1376,7 +1376,11 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 	struct kcm_psock *psock = NULL, *tpsock;
 	struct list_head *head;
 	int index = 0;
-	struct strp_callbacks cb;
+	static const struct strp_callbacks cb = {
+		.rcv_msg = kcm_rcv_strparser,
+		.parse_msg = kcm_parse_func_strparser,
+		.read_sock_done = kcm_read_sock_done,
+	};
 	int err;
 
 	csk = csock->sk;
@@ -1391,11 +1395,6 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 	psock->sk = csk;
 	psock->bpf_prog = prog;
 
-	cb.rcv_msg = kcm_rcv_strparser;
-	cb.abort_parser = NULL;
-	cb.parse_msg = kcm_parse_func_strparser;
-	cb.read_sock_done = kcm_read_sock_done;
-
 	err = strp_init(&psock->strp, csk, &cb);
 	if (err) {
 		kmem_cache_free(kcm_psockp, psock);

net/strparser/strparser.c

@@ -472,7 +472,7 @@ static void strp_sock_unlock(struct strparser *strp)
 }
 
 int strp_init(struct strparser *strp, struct sock *sk,
-	      struct strp_callbacks *cb)
+	      const struct strp_callbacks *cb)
 {
 
 	if (!cb || !cb->rcv_msg || !cb->parse_msg)