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    Subject[PATCH 3.16 223/305] uprobes: Fix handle_swbp() vs. unregister() + register() race once more
    3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Andrea Parri <andrea.parri@amarulasolutions.com>

    commit 09d3f015d1e1b4fee7e9bbdcf54201d239393391 upstream.

    Commit:

    142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")

    added the UPROBE_COPY_INSN flag, and corresponding smp_wmb() and smp_rmb()
    memory barriers, to ensure that handle_swbp() uses fully-initialized
    uprobes only.

    However, the smp_rmb() is mis-placed: this barrier should be placed
    after handle_swbp() has tested for the flag, thus guaranteeing that
    (program-order) subsequent loads from the uprobe can see the initial
    stores performed by prepare_uprobe().

    Move the smp_rmb() accordingly. Also amend the comments associated
    to the two memory barriers to indicate their actual locations.

    Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
    Acked-by: Oleg Nesterov <oleg@redhat.com>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Namhyung Kim <namhyung@kernel.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vince Weaver <vincent.weaver@maine.edu>
    Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
    Link: http://lkml.kernel.org/r/20181122161031.15179-1-andrea.parri@amarulasolutions.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    kernel/events/uprobes.c | 12 ++++++++++--
    1 file changed, 10 insertions(+), 2 deletions(-)

    --- a/kernel/events/uprobes.c
    +++ b/kernel/events/uprobes.c
    @@ -610,7 +610,7 @@ static int prepare_uprobe(struct uprobe
    BUG_ON((uprobe->offset & ~PAGE_MASK) +
    UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);

    - smp_wmb(); /* pairs with rmb() in find_active_uprobe() */
    + smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
    set_bit(UPROBE_COPY_INSN, &uprobe->flags);

    out:
    @@ -1858,10 +1858,18 @@ static void handle_swbp(struct pt_regs *
    * After we hit the bp, _unregister + _register can install the
    * new and not-yet-analyzed uprobe at the same address, restart.
    */
    - smp_rmb(); /* pairs with wmb() in install_breakpoint() */
    if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
    goto out;

    + /*
    + * Pairs with the smp_wmb() in prepare_uprobe().
    + *
    + * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
    + * we must also see the stores to &uprobe->arch performed by the
    + * prepare_uprobe() call.
    + */
    + smp_rmb();
    +
    /* Tracing handlers use ->utask to communicate with fetch methods */
    if (!get_utask())
    goto out;
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