The bug is here:
    bypass_pg(m, pg, bypassed);

The list iterator 'pg' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it will lead
to a invalid memory access.

To fix this bug, run bypass_pg(m, pg, bypassed); and return 0
when found, otherwise return -EINVAL.

Cc: [email protected]
Fixes: ^1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xiaomeng Tong <[email protected]>
---
 drivers/md/dm-mpath.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index f4719b65e5e3..6ba8f1133564 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1496,12 +1496,13 @@ static int bypass_pg_num(struct multipath *m, const 
char *pgstr, bool bypassed)
        }
 
        list_for_each_entry(pg, &m->priority_groups, list) {
-               if (!--pgnum)
-                       break;
+               if (!--pgnum) {
+                       bypass_pg(m, pg, bypassed);
+                       return 0;
+               }
        }
 
-       bypass_pg(m, pg, bypassed);
-       return 0;
+       return -EINVAL;
 }
 
 /*
-- 
2.17.1

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