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 --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 99d64cd58c52d6ef8dc267423987736ce24d935b..9c143739b8d73f0c7982be06eb0e5cdb902bfc81 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1657,14 +1657,13 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
            !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