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dm bufio: fix integer overflow when limiting maximum cache size

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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: 95d402f057 ("dm: add bufio")
Cc: <stable@vger.kernel.org> # v3.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Eric Biggers 2017-11-15 16:38:09 -08:00 committed by Mike Snitzer
parent 5d47c89f29
commit 74d4108d9e
1 changed files with 6 additions and 9 deletions
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15
drivers/md/dm-bufio.c
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@ -974,7 +974,8 @@ static void __get_memory_limit(struct dm_bufio_client *c,
buffers = c->minimum_buffers;
*limit_buffers = buffers;
*threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100;
*threshold_buffers = mult_frac(buffers,
DM_BUFIO_WRITEBACK_PERCENT, 100);
}
/*
@ -1910,19 +1911,15 @@ 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_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100))
mem = mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100);
#endif
dm_bufio_default_cache_size = mem;