Patch "ring-buffer: Prevent overflow of size in ring_buffer_resize()" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

ring-buffer: Prevent overflow of size in ring_buffer_resize()

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ring-buffer-prevent-overflow-of-size-in-ring_buffer_resize.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Steven Rostedt (Red Hat)" <[email protected]>
Date: Fri, 13 May 2016 09:34:12 -0400
Subject: ring-buffer: Prevent overflow of size in ring_buffer_resize()

From: Steven Rostedt (Red Hat) <[email protected]>

commit 59643d1535eb220668692a5359de22545af579f6 upstream.

If the size passed to ring_buffer_resize() is greater than MAX_LONG - BUF_PAGE_SIZE then the DIV_ROUND_UP() will return zero.

Here's the details:

  # echo 18014398509481980 > /sys/kernel/debug/tracing/buffer_size_kb

tracing_entries_write() processes this and converts kb to bytes.

18014398509481980 << 10 = 18446744073709547520

and this is passed to ring_buffer_resize() as unsigned long size.

size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);

Where DIV_ROUND_UP(a, b) is (a + b - 1)/b

BUF_PAGE_SIZE is 4080 and here

18446744073709547520 + 4080 - 1 = 18446744073709551599

where 18446744073709551599 is still smaller than 2^64

2^64 - 18446744073709551599 = 17

But now 18446744073709551599 / 4080 = 4521260802379792

and size = size * 4080 = 18446744073709551360

This is checked to make sure its still greater than 2 * 4080, which it is.

Then we convert to the number of buffer pages needed.

nr_page = DIV_ROUND_UP(size, BUF_PAGE_SIZE)

but this time size is 18446744073709551360 and

2^64 - (18446744073709551360 + 4080 - 1) = -3823

Thus it overflows and the resulting number is less than 4080, which makes

3823 / 4080 = 0

an nr_pages is set to this. As we already checked against the minimum that nr_pages may be, this causes the logic to fail as well, and we crash the kernel.

There's no reason to have the two DIV_ROUND_UP() (that's just result of historical code changes), clean up the code and fix this bug.

---
 kernel/trace/ring_buffer.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

@@ -1645,14 +1645,13 @@ int ring_buffer_resize(struct ring_buffe
 	    !cpumask_test_cpu(cpu_id, buffer->cpumask))
 		return size;
 
-	size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
-	size *= BUF_PAGE_SIZE;
+	nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
 
 	/* we need a minimum of two pages */
-	if (size < BUF_PAGE_SIZE * 2)
-		size = BUF_PAGE_SIZE * 2;
+	if (nr_pages < 2)
+		nr_pages = 2;
 
-	nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
+	size = nr_pages * BUF_PAGE_SIZE;
 
 	/*
 	 * Don't succeed if resizing is disabled, as a reader might be

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/ring-buffer-prevent-overflow-of-size-in-ring_buffer_resize.patch queue-3.14/ring-buffer-use-long-for-nr_pages-to-avoid-overflow-failures.patch