[PATCH 3.18 23/54] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

From: Greg Kroah-Hartman
Date: Tue Dec 11 2018 - 10:43:09 EST

3.18-stable review patch. If anyone has any objections, please let me know.


From: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>

commit 09d3f015d1e1b4fee7e9bbdcf54201d239393391 upstream.


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@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Cc: stable@xxxxxxxxxx
Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
Link: http://lkml.kernel.org/r/20181122161031.15179-1-andrea.parri@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/events/uprobes.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -612,7 +612,7 @@ static int prepare_uprobe(struct uprobe
BUG_ON((uprobe->offset & ~PAGE_MASK) +

- 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);

@@ -1860,10 +1860,18 @@ 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;