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    Subject[PATCH 3.16 082/148] inetpeer: fix data-race in inet_putpeer / inet_putpeer
    3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Dumazet <edumazet@google.com>

    commit 71685eb4ce80ae9c49eff82ca4dd15acab215de9 upstream.

    We need to explicitely forbid read/store tearing in inet_peer_gc()
    and inet_putpeer().

    The following syzbot report reminds us about inet_putpeer()
    running without a lock held.

    BUG: KCSAN: data-race in inet_putpeer / inet_putpeer

    write to 0xffff888121fb2ed0 of 4 bytes by interrupt on cpu 0:
    inet_putpeer+0x37/0xa0 net/ipv4/inetpeer.c:240
    ip4_frag_free+0x3d/0x50 net/ipv4/ip_fragment.c:102
    inet_frag_destroy_rcu+0x58/0x80 net/ipv4/inet_fragment.c:228
    __rcu_reclaim kernel/rcu/rcu.h:222 [inline]
    rcu_do_batch+0x256/0x5b0 kernel/rcu/tree.c:2157
    rcu_core+0x369/0x4d0 kernel/rcu/tree.c:2377
    rcu_core_si+0x12/0x20 kernel/rcu/tree.c:2386
    __do_softirq+0x115/0x33f kernel/softirq.c:292
    invoke_softirq kernel/softirq.c:373 [inline]
    irq_exit+0xbb/0xe0 kernel/softirq.c:413
    exiting_irq arch/x86/include/asm/apic.h:536 [inline]
    smp_apic_timer_interrupt+0xe6/0x280 arch/x86/kernel/apic/apic.c:1137
    apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
    native_safe_halt+0xe/0x10 arch/x86/kernel/paravirt.c:71
    arch_cpu_idle+0x1f/0x30 arch/x86/kernel/process.c:571
    default_idle_call+0x1e/0x40 kernel/sched/idle.c:94
    cpuidle_idle_call kernel/sched/idle.c:154 [inline]
    do_idle+0x1af/0x280 kernel/sched/idle.c:263

    write to 0xffff888121fb2ed0 of 4 bytes by interrupt on cpu 1:
    inet_putpeer+0x37/0xa0 net/ipv4/inetpeer.c:240
    ip4_frag_free+0x3d/0x50 net/ipv4/ip_fragment.c:102
    inet_frag_destroy_rcu+0x58/0x80 net/ipv4/inet_fragment.c:228
    __rcu_reclaim kernel/rcu/rcu.h:222 [inline]
    rcu_do_batch+0x256/0x5b0 kernel/rcu/tree.c:2157
    rcu_core+0x369/0x4d0 kernel/rcu/tree.c:2377
    rcu_core_si+0x12/0x20 kernel/rcu/tree.c:2386
    __do_softirq+0x115/0x33f kernel/softirq.c:292
    run_ksoftirqd+0x46/0x60 kernel/softirq.c:603
    smpboot_thread_fn+0x37d/0x4a0 kernel/smpboot.c:165
    kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
    ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

    Reported by Kernel Concurrency Sanitizer on:
    CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 5.4.0-rc3+ #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

    Fixes: 4b9d9be839fd ("inetpeer: remove unused list")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reported-by: syzbot <syzkaller@googlegroups.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    [bwh: Backported to 3.16:
    - Use ACCESS_ONCE() instead of {READ,WRITE}_ONCE()
    - Adjust context, indentation]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    net/ipv4/inetpeer.c | 12 ++++++++++--
    1 file changed, 10 insertions(+), 2 deletions(-)

    --- a/net/ipv4/inetpeer.c
    +++ b/net/ipv4/inetpeer.c
    @@ -419,7 +419,12 @@ static int inet_peer_gc(struct inet_peer
    p = rcu_deref_locked(**stackptr, base);
    if (atomic_read(&p->refcnt) == 0) {
    smp_rmb();
    - delta = (__u32)jiffies - p->dtime;
    +
    + /* The ACCESS_ONCE() pairs with the ACCESS_ONCE()
    + * in inet_putpeer()
    + */
    + delta = (__u32)jiffies - ACCESS_ONCE(p->dtime);
    +
    if (delta >= ttl &&
    atomic_cmpxchg(&p->refcnt, 0, -1) == 0) {
    p->gc_next = gchead;
    @@ -504,7 +509,10 @@ EXPORT_SYMBOL_GPL(inet_getpeer);

    void inet_putpeer(struct inet_peer *p)
    {
    - p->dtime = (__u32)jiffies;
    + /* The ACCESS_ONCE() pairs with itself (we run lockless)
    + * and the ACCESS_ONCE() in inet_peer_gc()
    + */
    + ACCESS_ONCE(p->dtime) = (__u32)jiffies;
    smp_mb__before_atomic();
    atomic_dec(&p->refcnt);
    }
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