From a06ad633a37c64a0cd4c229fc605cee8725d376e Mon Sep 17 00:00:00 2001 From: Tom Abraham Date: Tue, 10 Apr 2018 16:29:48 -0700 Subject: [PATCH] swap: divide-by-zero when zero length swap file on ssd Calling swapon() on a zero length swap file on SSD can lead to a divide-by-zero. Although creating such files isn't possible with mkswap and they woud be considered invalid, it would be better for the swapon code to be more robust and handle this condition gracefully (return -EINVAL). Especially since the fix is small and straightforward. To help with wear leveling on SSD, the swapon syscall calculates a random position in the swap file using modulo p->highest_bit, which is set to maxpages - 1 in read_swap_header. If the swap file is zero length, read_swap_header sets maxpages=1 and last_page=0, resulting in p->highest_bit=0 and we divide-by-zero when we modulo p->highest_bit in swapon syscall. This can be prevented by having read_swap_header return zero if last_page is zero. Link: http://lkml.kernel.org/r/5AC747C1020000A7001FA82C@prv-mh.provo.novell.com Signed-off-by: Thomas Abraham Reported-by: Reviewed-by: Andrew Morton Cc: Randy Dunlap Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swapfile.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/mm/swapfile.c b/mm/swapfile.c index c7a33717d079..a134d1e86795 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -2961,6 +2961,10 @@ static unsigned long read_swap_header(struct swap_info_struct *p, maxpages = swp_offset(pte_to_swp_entry( swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1; last_page = swap_header->info.last_page; + if (!last_page) { + pr_warn("Empty swap-file\n"); + return 0; + } if (last_page > maxpages) { pr_warn("Truncating oversized swap area, only using %luk out of %luk\n", maxpages << (PAGE_SHIFT - 10), -- 2.34.1