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    Subject[PATCH 5.10 16/43] tipc: fix NULL deref in tipc_link_xmit()
    Date
    From: Hoang Le <hoang.h.le@dektech.com.au>

    [ Upstream commit b77413446408fdd256599daf00d5be72b5f3e7c6 ]

    The buffer list can have zero skb as following path:
    tipc_named_node_up()->tipc_node_xmit()->tipc_link_xmit(), so
    we need to check the list before casting an &sk_buff.

    Fault report:
    [] tipc: Bulk publication failure
    [] general protection fault, probably for non-canonical [#1] PREEMPT [...]
    [] KASAN: null-ptr-deref in range [0x00000000000000c8-0x00000000000000cf]
    [] CPU: 0 PID: 0 Comm: swapper/0 Kdump: loaded Not tainted 5.10.0-rc4+ #2
    [] Hardware name: Bochs ..., BIOS Bochs 01/01/2011
    [] RIP: 0010:tipc_link_xmit+0xc1/0x2180
    [] Code: 24 b8 00 00 00 00 4d 39 ec 4c 0f 44 e8 e8 d7 0a 10 f9 48 [...]
    [] RSP: 0018:ffffc90000006ea0 EFLAGS: 00010202
    [] RAX: dffffc0000000000 RBX: ffff8880224da000 RCX: 1ffff11003d3cc0d
    [] RDX: 0000000000000019 RSI: ffffffff886007b9 RDI: 00000000000000c8
    [] RBP: ffffc90000007018 R08: 0000000000000001 R09: fffff52000000ded
    [] R10: 0000000000000003 R11: fffff52000000dec R12: ffffc90000007148
    [] R13: 0000000000000000 R14: 0000000000000000 R15: ffffc90000007018
    [] FS: 0000000000000000(0000) GS:ffff888037400000(0000) knlGS:000[...]
    [] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [] CR2: 00007fffd2db5000 CR3: 000000002b08f000 CR4: 00000000000006f0

    Fixes: af9b028e270fd ("tipc: make media xmit call outside node spinlock context")
    Acked-by: Jon Maloy <jmaloy@redhat.com>
    Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
    Link: https://lore.kernel.org/r/20210108071337.3598-1-hoang.h.le@dektech.com.au
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/tipc/link.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- a/net/tipc/link.c
    +++ b/net/tipc/link.c
    @@ -996,7 +996,6 @@ void tipc_link_reset(struct tipc_link *l
    int tipc_link_xmit(struct tipc_link *l, struct sk_buff_head *list,
    struct sk_buff_head *xmitq)
    {
    - struct tipc_msg *hdr = buf_msg(skb_peek(list));
    struct sk_buff_head *backlogq = &l->backlogq;
    struct sk_buff_head *transmq = &l->transmq;
    struct sk_buff *skb, *_skb;
    @@ -1004,13 +1003,18 @@ int tipc_link_xmit(struct tipc_link *l,
    u16 ack = l->rcv_nxt - 1;
    u16 seqno = l->snd_nxt;
    int pkt_cnt = skb_queue_len(list);
    - int imp = msg_importance(hdr);
    unsigned int mss = tipc_link_mss(l);
    unsigned int cwin = l->window;
    unsigned int mtu = l->mtu;
    + struct tipc_msg *hdr;
    bool new_bundle;
    int rc = 0;
    + int imp;

    + if (pkt_cnt <= 0)
    + return 0;
    +
    + hdr = buf_msg(skb_peek(list));
    if (unlikely(msg_size(hdr) > mtu)) {
    pr_warn("Too large msg, purging xmit list %d %d %d %d %d!\n",
    skb_queue_len(list), msg_user(hdr),
    @@ -1019,6 +1023,7 @@ int tipc_link_xmit(struct tipc_link *l,
    return -EMSGSIZE;
    }

    + imp = msg_importance(hdr);
    /* Allow oversubscription of one data msg per source at congestion */
    if (unlikely(l->backlog[imp].len >= l->backlog[imp].limit)) {
    if (imp == TIPC_SYSTEM_IMPORTANCE) {

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