linux-kernel - [tip:perf/urgent] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

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Message-ID: <tip-09d3f015d1e1b4fee7e9bbdcf54201d239393391@git.kernel.org>
Date:   Thu, 22 Nov 2018 23:33:51 -0800
From:   tip-bot for Andrea Parri <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     eranian@...gle.com, tglx@...utronix.de, mingo@...nel.org,
        akpm@...ux-foundation.org, hpa@...or.com,
        torvalds@...ux-foundation.org, namhyung@...nel.org,
        jolsa@...hat.com, andrea.parri@...rulasolutions.com,
        paulmck@...ux.vnet.ibm.com, acme@...hat.com, oleg@...hat.com,
        vincent.weaver@...ne.edu, alexander.shishkin@...ux.intel.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: [tip:perf/urgent] uprobes: Fix handle_swbp() vs. unregister() +
 register() race once more

Commit-ID:  09d3f015d1e1b4fee7e9bbdcf54201d239393391
Gitweb:     https://git.kernel.org/tip/09d3f015d1e1b4fee7e9bbdcf54201d239393391
Author:     Andrea Parri <andrea.parri@...rulasolutions.com>
AuthorDate: Thu, 22 Nov 2018 17:10:31 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 23 Nov 2018 08:31:19 +0100

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@...rulasolutions.com>
Acked-by: Oleg Nesterov <oleg@...hat.com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Cc: stable@...nel.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@...nel.org>
---
 kernel/events/uprobes.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 96d4bee83489..322e97bbb437 100644
--- 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;