[PATCH 4.6 016/100] ring-buffer: Prevent overflow of size in ring_buffer_resize()

Greg Kroah-Hartman
2016-05-30T21:11:08Z

From: Steven Rostedt (Red Hat) <rostedt@goodmis.org>

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.

Fixes: 83f40318dab00 ("ring-buffer: Make removal of ring buffer pages atomic")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1657,14 +1657,13 @@ 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