linux-kernel - [PATCH] ring-buffer: fix uninitialized read

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <1339111642-869-1-git-send-email-dhsharp@google.com>
Date:	Thu,  7 Jun 2012 16:27:22 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	rostedt@...dmis.org, linux-kernel@...r.kernel.org
Cc:	vnagarnaik@...gle.com, David Sharp <dhsharp@...gle.com>
Subject: [PATCH] ring-buffer: fix uninitialized read_stamp

This fixes a scenario in which trace_pipe_raw will return events with invalid
timestamps when events are copied out one at a time (instead of swapping out
the reader page with a spare page). In this scenario, ring_buffer_read_page()
uses cpu_buffer->read_stamp to keep track of the time of the earliest event.
However, cpu_buffer->read_stamp was not always updated. The only function that
sets read_stamp to a new valid value is rb_reset_reader_page(), which is called
only by rb_get_reader_page(). rb_reset_reader_page() was not called when there
is data immediately available on the page to read (read < rb_page_size()). This
is the bug.

Setting the read_stamp on the first read from a page fixes the bug.

Tested: On certain lightly-loaded machines, repetitive reads from
trace_pipe_raw without using splice() would sometimes result in invalid
timestamps. Poisoning read_stamp in rb_init_page() with a negative value makes
the problem more visible. After this fix, the invalid timstamps disappear.

Google-Bug-Id: 6410455
Signed-off-by: David Sharp <dhsharp@...gle.com>
---
 kernel/trace/ring_buffer.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 1d0f6a8..ad0239b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3226,8 +3226,15 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 	reader = cpu_buffer->reader_page;

 	/* If there's more to read, return this page */
-	if (cpu_buffer->reader_page->read < rb_page_size(reader))
+	if (cpu_buffer->reader_page->read < rb_page_size(reader)) {
+		/*
+		 * If this is the first read from this page,
+		 * initialize the read timestamp.
+		 */
+		if (cpu_buffer->reader_page->read == 0)
+			cpu_buffer->read_stamp = reader->page->time_stamp;
 		goto out;
+	}

 	/* Never should we have an index greater than the size */
 	if (RB_WARN_ON(cpu_buffer,
-- 
1.7.7.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/