linux-kernel - Possible race in loop.ko

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Message-ID: <e93eb913-f3e4-081e-c4f7-92bcb99b83b6@ispras.ru>
Date:   Fri, 28 Jul 2017 18:55:49 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     axboe@...com
Cc:     tom.leiming@...il.com, osandov@...com,
        linux-kernel@...r.kernel.org, ldv-project@...uxtesting.org,
        khoroshilov@...ras.ru
Subject: Possible race in loop.ko

Hello.
While searching for races in Linux kernel I've come across 
drivers/block/loop.ko module. Here is the question that I came up with 
while analyzing results. Lines are given using the info from Linux v4.12.

In loop_init function additional initialization happens after a 
successful call to misc_register() (loop.c: line 1961). Consider the 
following case:

Thread 1:                 Thread 2:
loop_init()
   misc_register()         loop_control_ioctl
   part_shift = 0          -> loop_add
   if (max_part > 0) {          alloc_disk(1 << part_shift)
     part_shift =
          <greater than 0>
     ...
   }

In this case alloc_disk() will be called with 1 as a parameter although 
part_shift should have been greater than 0. Maybe it would be better to 
move the call to a misc_register() function a bit further down (at least 
so it could be after the part_shift initialization)?

Thank you for your time.

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@...ras.ru