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    Subject[PATCH AUTOSEL 6.6 26/52] net: skbuff: add overflow debug check to pull/push helpers
    Date
    From: Florian Westphal <fw@strlen.de>

    [ Upstream commit 219eee9c0d16f1b754a8b85275854ab17df0850a ]

    syzbot managed to trigger following splat:
    BUG: KASAN: use-after-free in __skb_flow_dissect+0x4a3b/0x5e50
    Read of size 1 at addr ffff888208a4000e by task a.out/2313
    [..]
    __skb_flow_dissect+0x4a3b/0x5e50
    __skb_get_hash+0xb4/0x400
    ip_tunnel_xmit+0x77e/0x26f0
    ipip_tunnel_xmit+0x298/0x410
    ..

    Analysis shows that the skb has a valid ->head, but bogus ->data
    pointer.

    skb->data gets its bogus value via the neigh layer, which does:

    1556 __skb_pull(skb, skb_network_offset(skb));

    .. and the skb was already dodgy at this point:

    skb_network_offset(skb) returns a negative value due to an
    earlier overflow of skb->network_header (u16). __skb_pull thus
    "adjusts" skb->data by a huge offset, pointing outside skb->head
    area.

    Allow debug builds to splat when we try to pull/push more than
    INT_MAX bytes.

    After this, the syzkaller reproducer yields a more precise splat
    before the flow dissector attempts to read off skb->data memory:

    WARNING: CPU: 5 PID: 2313 at include/linux/skbuff.h:2653 neigh_connected_output+0x28e/0x400
    ip_finish_output2+0xb25/0xed0
    iptunnel_xmit+0x4ff/0x870
    ipgre_xmit+0x78e/0xbb0

    Signed-off-by: Florian Westphal <fw@strlen.de>
    Reviewed-by: Simon Horman <horms@kernel.org>
    Link: https://lore.kernel.org/r/20240216113700.23013-1-fw@strlen.de
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    include/linux/skbuff.h | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
    index ddfe86deb4e7f..2063253b2a6bd 100644
    --- a/include/linux/skbuff.h
    +++ b/include/linux/skbuff.h
    @@ -2632,6 +2632,8 @@ static inline void skb_put_u8(struct sk_buff *skb, u8 val)
    void *skb_push(struct sk_buff *skb, unsigned int len);
    static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
    {
    + DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
    +
    skb->data -= len;
    skb->len += len;
    return skb->data;
    @@ -2640,6 +2642,8 @@ static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
    void *skb_pull(struct sk_buff *skb, unsigned int len);
    static inline void *__skb_pull(struct sk_buff *skb, unsigned int len)
    {
    + DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
    +
    skb->len -= len;
    if (unlikely(skb->len < skb->data_len)) {
    #if defined(CONFIG_DEBUG_NET)
    @@ -2664,6 +2668,8 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta);
    static inline enum skb_drop_reason
    pskb_may_pull_reason(struct sk_buff *skb, unsigned int len)
    {
    + DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
    +
    if (likely(len <= skb_headlen(skb)))
    return SKB_NOT_DROPPED_YET;

    --
    2.43.0

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