[PATCH 3.2 45/79] dm bufio: fix integer overflow when limiting maximum cache size

Ben Hutchings
2018-02-11T04:33:41Z

From: Eric Biggers <ebiggers@google.com>

commit 74d4108d9e681dbbe4a2940ed8fdff1f6868184c upstream.

The default max_cache_size_bytes for dm-bufio is meant to be the lesser
of 25% of the size of the vmalloc area and 2% of the size of lowmem.
However, on 32-bit systems the intermediate result in the expression

(VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100

overflows, causing the wrong result to be computed. For example, on a
32-bit system where the vmalloc area is 520093696 bytes, the result is
1174405 rather than the expected 130023424, which makes the maximum
cache size much too small (far less than 2% of lowmem). This causes
severe performance problems for dm-verity users on affected systems.

Fix this by using mult_frac() to correctly multiply by a percentage. Do
this for all places in dm-bufio that multiply by a percentage. Also
replace (VMALLOC_END - VMALLOC_START) with VMALLOC_TOTAL, which contrary
to the comment is now defined in include/linux/vmalloc.h.

Depends-on: 9993bc635 ("sched/x86: Fix overflow in cyc2ns_offset")
Fixes: 95d402f057f2 ("dm: add bufio")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[bwh: Backported to 3.2: keep open-coded VMALLOC_TOTAL]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/md/dm-bufio.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)

--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -839,7 +839,8 @@ static void __get_memory_limit(struct dm
buffers = DM_BUFIO_MIN_BUFFERS;

*limit_buffers = buffers;
- *threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100;
+ *threshold_buffers = mult_frac(buffers,
+ DM_BUFIO_WRITEBACK_PERCENT, 100);
}

/*
@@ -1620,19 +1621,15 @@ static int __init dm_bufio_init(void)
memset(&dm_bufio_caches, 0, sizeof dm_bufio_caches);
memset(&dm_bufio_cache_names, 0, sizeof dm_bufio_cache_names);

- mem = (__u64)((totalram_pages - totalhigh_pages) *
- DM_BUFIO_MEMORY_PERCENT / 100) << PAGE_SHIFT;
+ mem = (__u64)mult_frac(totalram_pages - totalhigh_pages,
+ DM_BUFIO_MEMORY_PERCENT, 100) << PAGE_SHIFT;

if (mem > ULONG_MAX)
mem = ULONG_MAX;

#ifdef CONFIG_MMU
- /*
- * Get the size of vmalloc space the same way as VMALLOC_TOTAL
- * in fs/proc/internal.h
- */
- if (mem > (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100)
- mem = (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100;
+ if (mem > mult_frac(VMALLOC_END - VMALLOC_START, DM_BUFIO_VMALLOC_PERCENT, 100))
+ mem = mult_frac(VMALLOC_END - VMALLOC_START, DM_BUFIO_VMALLOC_PERCENT, 100);
#endif

dm_bufio_default_cache_size = mem;