uprobes: Fix handle_swbp() vs. unregister() + register() race once more

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>
Cc: stable@kernel.org
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>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 96d4bee83489b113a1f37452aeffb8e13461b230..322e97bbb4370a8ce4da1af39187dbc775fa83d4 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -829,7 +829,7 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
        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:
@@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *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;