CVE-2016-9754: Integer Overflow or Wraparound (CWE-190)
value: The ring_buffer_resize function in kernel/trace/ring_buffer.c in the profiling subsystem in the Linux kernel before 4.6.1 mishandles certain integer calculations, which allows local users to gain privileges by writing to the /sys/kernel/debug/tracing/buffer_size_kb file.
CWE description: The product performs a calculation that can
         produce an integer overflow or wraparound when the logic
         assumes that the resulting value will always be larger than
         the original value. This occurs when an integer value is
         incremented to a value that is too large to store in the
         associated representation. When this occurs, the value may
         become a very small or negative number.
CVSS v3 base score: 7.8
Published: 2017-01-05T11:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: kernel/trace/ring_buffer.c
Commit date: 2016-05-13 16:44:20 -0400
Commit message: ring-buffer: Prevent overflow of size in ring_buffer_resize()

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.

Cc: stable@vger.kernel.org # 3.5+
Fixes: 83f40318dab00 ("ring-buffer: Make removal of ring buffer pages atomic")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Diff: diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 99d64cd58c52d6..9c143739b8d73f 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1655,18 +1655,17 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
 	/* Make sure the requested buffer exists */
 	if (cpu_id != RING_BUFFER_ALL_CPUS &&
 	    !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
 	 * manipulating the ring buffer and is expecting a sane state while
 	 * this is true.

Vulnerable code: 	size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
	size *= BUF_PAGE_SIZE;
	if (size < BUF_PAGE_SIZE * 2)
		size = BUF_PAGE_SIZE * 2;
	nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
Fixed code: 	nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
	if (nr_pages < 2)
		nr_pages = 2;
	size = nr_pages * BUF_PAGE_SIZE;