[PATCH -next 1/2] blk-iocost: fix divide by 0 error in calc_lcoefs()

[Li Nan]

echo max of u64 to cost.model can cause divide by 0 error.

  # echo 8:0 rbps=18446744073709551615 > /sys/fs/cgroup/io.cost.model

  divide error: 0000 [#1] PREEMPT SMP
  RIP: 0010:calc_lcoefs+0x4c/0xc0
  Call Trace:
   <TASK>
   ioc_refresh_params+0x2b3/0x4f0
   ioc_cost_model_write+0x3cb/0x4c0
   ? _copy_from_iter+0x6d/0x6c0
   ? kernfs_fop_write_iter+0xfc/0x270
   cgroup_file_write+0xa0/0x200
   kernfs_fop_write_iter+0x17d/0x270
   vfs_write+0x414/0x620
   ksys_write+0x73/0x160
   __x64_sys_write+0x1e/0x30
   do_syscall_64+0x35/0x80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

calc_lcoefs() uses the input value of cost.model in DIV_ROUND_UP_ULL,
overflow would happen if bps plus IOC_PAGE_SIZE is greater than
ULLONG_MAX, it can cause divide by 0 error.I_LCOEF_MAX is introduced to
prevent it.

Signed-off-by: Li Nan <linan122@xxxxxxxxxx>
---
 block/blk-iocost.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index f01359906c83..a38a5324bf10 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -306,6 +306,9 @@ enum {
 	IOC_PAGE_SIZE		= 1 << IOC_PAGE_SHIFT,
 	IOC_SECT_TO_PAGE_SHIFT	= IOC_PAGE_SHIFT - SECTOR_SHIFT,
 
+	/* avoid overflow */
+	I_LCOEF_MAX		= ULLONG_MAX - IOC_PAGE_SIZE,
+
 	/* if apart further than 16M, consider randio for linear model */
 	LCOEF_RANDIO_PAGES	= 4096,
 };
@@ -3406,6 +3409,8 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 			goto einval;
 		if (match_u64(&args[0], &v))
 			goto einval;
+		if (v > I_LCOEF_MAX)
+			goto einval;
 		u[tok] = v;
 		user = true;
 	}
-- 
2.31.1