From 6d9e8c651dd979aa666bee15f086745f3ea9c4b3 Mon Sep 17 00:00:00 2001 From: Wen Yang Date: Mon, 13 Jan 2020 16:29:23 -0800 Subject: [PATCH] mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio() Patch series "use div64_ul() instead of div_u64() if the divisor is unsigned long". We were first inspired by commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom () calculation"), then refer to the recently analyzed mm code, we found this suspicious place. 201 if (min) { 202 min *= this_bw; 203 do_div(min, tot_bw); 204 } And we also disassembled and confirmed it: /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 201 0xffffffff811c37da <__wb_calc_thresh+234>: xor %r10d,%r10d 0xffffffff811c37dd <__wb_calc_thresh+237>: test %rax,%rax 0xffffffff811c37e0 <__wb_calc_thresh+240>: je 0xffffffff811c3800 <__wb_calc_thresh+272> /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 202 0xffffffff811c37e2 <__wb_calc_thresh+242>: imul %r8,%rax /usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 203 0xffffffff811c37e6 <__wb_calc_thresh+246>: mov %r9d,%r10d ---> truncates it to 32 bits here 0xffffffff811c37e9 <__wb_calc_thresh+249>: xor %edx,%edx 0xffffffff811c37eb <__wb_calc_thresh+251>: div %r10 0xffffffff811c37ee <__wb_calc_thresh+254>: imul %rbx,%rax 0xffffffff811c37f2 <__wb_calc_thresh+258>: shr $0x2,%rax 0xffffffff811c37f6 <__wb_calc_thresh+262>: mul %rcx 0xffffffff811c37f9 <__wb_calc_thresh+265>: shr $0x2,%rdx 0xffffffff811c37fd <__wb_calc_thresh+269>: mov %rdx,%r10 This series uses div64_ul() instead of div_u64() if the divisor is unsigned long, to avoid truncation to 32-bit on 64-bit platforms. This patch (of 3): The variables 'min' and 'max' are unsigned long and do_div truncates them to 32 bits, which means it can test non-zero and be truncated to zero for division. Fix this issue by using div64_ul() instead. Link: http://lkml.kernel.org/r/20200102081442.8273-2-wenyang@linux.alibaba.com Fixes: 693108a8a667 ("writeback: make bdi->min/max_ratio handling cgroup writeback aware") Signed-off-by: Wen Yang Reviewed-by: Andrew Morton Cc: Qian Cai Cc: Tejun Heo Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page-writeback.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 50055d2e4ea8..2d658b208319 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -201,11 +201,11 @@ static void wb_min_max_ratio(struct bdi_writeback *wb, if (this_bw < tot_bw) { if (min) { min *= this_bw; - do_div(min, tot_bw); + min = div64_ul(min, tot_bw); } if (max < 100) { max *= this_bw; - do_div(max, tot_bw); + max = div64_ul(max, tot_bw); } }