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    Subject[PATCH 5.10 516/563] af_unix: annote lockless accesses to unix_tot_inflight & gc_in_progress
    Date
    From: Eric Dumazet <edumazet@google.com>

    commit 9d6d7f1cb67cdee15f1a0e85aacfb924e0e02435 upstream.

    wait_for_unix_gc() reads unix_tot_inflight & gc_in_progress
    without synchronization.

    Adds READ_ONCE()/WRITE_ONCE() and their associated comments
    to better document the intent.

    BUG: KCSAN: data-race in unix_inflight / wait_for_unix_gc

    write to 0xffffffff86e2b7c0 of 4 bytes by task 9380 on cpu 0:
    unix_inflight+0x1e8/0x260 net/unix/scm.c:63
    unix_attach_fds+0x10c/0x1e0 net/unix/scm.c:121
    unix_scm_to_skb net/unix/af_unix.c:1674 [inline]
    unix_dgram_sendmsg+0x679/0x16b0 net/unix/af_unix.c:1817
    unix_seqpacket_sendmsg+0xcc/0x110 net/unix/af_unix.c:2258
    sock_sendmsg_nosec net/socket.c:704 [inline]
    sock_sendmsg net/socket.c:724 [inline]
    ____sys_sendmsg+0x39a/0x510 net/socket.c:2409
    ___sys_sendmsg net/socket.c:2463 [inline]
    __sys_sendmmsg+0x267/0x4c0 net/socket.c:2549
    __do_sys_sendmmsg net/socket.c:2578 [inline]
    __se_sys_sendmmsg net/socket.c:2575 [inline]
    __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2575
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    read to 0xffffffff86e2b7c0 of 4 bytes by task 9375 on cpu 1:
    wait_for_unix_gc+0x24/0x160 net/unix/garbage.c:196
    unix_dgram_sendmsg+0x8e/0x16b0 net/unix/af_unix.c:1772
    unix_seqpacket_sendmsg+0xcc/0x110 net/unix/af_unix.c:2258
    sock_sendmsg_nosec net/socket.c:704 [inline]
    sock_sendmsg net/socket.c:724 [inline]
    ____sys_sendmsg+0x39a/0x510 net/socket.c:2409
    ___sys_sendmsg net/socket.c:2463 [inline]
    __sys_sendmmsg+0x267/0x4c0 net/socket.c:2549
    __do_sys_sendmmsg net/socket.c:2578 [inline]
    __se_sys_sendmmsg net/socket.c:2575 [inline]
    __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2575
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    value changed: 0x00000002 -> 0x00000004

    Reported by Kernel Concurrency Sanitizer on:
    CPU: 1 PID: 9375 Comm: syz-executor.1 Not tainted 5.16.0-rc7-syzkaller #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

    Fixes: 9915672d4127 ("af_unix: limit unix_tot_inflight")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reported-by: syzbot <syzkaller@googlegroups.com>
    Link: https://lore.kernel.org/r/20220114164328.2038499-1-eric.dumazet@gmail.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/unix/garbage.c | 14 +++++++++++---
    net/unix/scm.c | 6 ++++--
    2 files changed, 15 insertions(+), 5 deletions(-)

    --- a/net/unix/garbage.c
    +++ b/net/unix/garbage.c
    @@ -192,8 +192,11 @@ void wait_for_unix_gc(void)
    {
    /* If number of inflight sockets is insane,
    * force a garbage collect right now.
    + * Paired with the WRITE_ONCE() in unix_inflight(),
    + * unix_notinflight() and gc_in_progress().
    */
    - if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
    + if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
    + !READ_ONCE(gc_in_progress))
    unix_gc();
    wait_event(unix_gc_wait, gc_in_progress == false);
    }
    @@ -213,7 +216,9 @@ void unix_gc(void)
    if (gc_in_progress)
    goto out;

    - gc_in_progress = true;
    + /* Paired with READ_ONCE() in wait_for_unix_gc(). */
    + WRITE_ONCE(gc_in_progress, true);
    +
    /* First, select candidates for garbage collection. Only
    * in-flight sockets are considered, and from those only ones
    * which don't have any external reference.
    @@ -299,7 +304,10 @@ void unix_gc(void)

    /* All candidates should have been detached by now. */
    BUG_ON(!list_empty(&gc_candidates));
    - gc_in_progress = false;
    +
    + /* Paired with READ_ONCE() in wait_for_unix_gc(). */
    + WRITE_ONCE(gc_in_progress, false);
    +
    wake_up(&unix_gc_wait);

    out:
    --- a/net/unix/scm.c
    +++ b/net/unix/scm.c
    @@ -60,7 +60,8 @@ void unix_inflight(struct user_struct *u
    } else {
    BUG_ON(list_empty(&u->link));
    }
    - unix_tot_inflight++;
    + /* Paired with READ_ONCE() in wait_for_unix_gc() */
    + WRITE_ONCE(unix_tot_inflight, unix_tot_inflight + 1);
    }
    user->unix_inflight++;
    spin_unlock(&unix_gc_lock);
    @@ -80,7 +81,8 @@ void unix_notinflight(struct user_struct

    if (atomic_long_dec_and_test(&u->inflight))
    list_del_init(&u->link);
    - unix_tot_inflight--;
    + /* Paired with READ_ONCE() in wait_for_unix_gc() */
    + WRITE_ONCE(unix_tot_inflight, unix_tot_inflight - 1);
    }
    user->unix_inflight--;
    spin_unlock(&unix_gc_lock);

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