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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.

Change-Id: I7744dfdd1c3be9676f767139002b5f57c41d87b2
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>
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rostedt authored and andi34 committed May 13, 2016
1 parent 1d6c887 commit cb5b1c7c3f3e40f9181db1fe3b589489fb9e2522
Showing with 5 additions and 5 deletions.
  1. +5 −5 kernel/trace/ring_buffer.c
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@@ -1324,13 +1324,14 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
if (!buffer)
return size;
size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
size *= BUF_PAGE_SIZE;
nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
buffer_size = buffer->pages * 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;
size = nr_pages * BUF_PAGE_SIZE;
if (size == buffer_size)
return size;
@@ -1343,7 +1344,6 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size)
mutex_lock(&buffer->mutex);
get_online_cpus();
nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
if (size < buffer_size) {

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