[PATCH 4.4 52/76] mm/page-writeback.c: avoid potential division by zero in wb_min_max_ratio()

Greg Kroah-Hartman
2020-01-22T09:32:17Z

From: Wen Yang <wenyang@linux.alibaba.com>

commit 6d9e8c651dd979aa666bee15f086745f3ea9c4b3 upstream.

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 <wenyang@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/page-writeback.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -200,11 +200,11 @@ static void wb_min_max_ratio(struct bdi_
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);
}
}