Messages in this thread | | | Date | Mon, 23 Feb 2015 18:09:16 +0100 | Subject | Fwd: w1/slaves/w1_therm: null-ptr access of sl->family_data | From | Thorsten Bschorr <> |
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Hi,
I have observed a null-pointer access in w1/slaves/w1_therm on my Raspberry Pi running 3.18.5 with several DS18S20 temperature sensors. I have already discussed the problem with Evgeniy.
@Evgeniy & David: sorry for re-sending my message, my email accidentally contained a HTML part.
w1_therm: w1_slave_show checks if the sensor uses external power. If this is the case, the mutex on dev->bus_mutex is unlocked while waiting 750 ms for the sensor to convert the temperature. Before reading the temperature, the mutex is again locked.
During this sleep-time, the sensor could get detached, for example by w1.c: w1_search_process_cb not finding the sensor (*): !test_bit(W1_SLAVE_ACTIVE, &sl->flags) ==true. This triggers w1_slave_detach, and hence w1_therm_remove_slave frees and nulls sl->family_data.
w1_slave_show does not check if familiy_data is null after re-acquiring the bus_mutex resulting in a null-ptr access when storing data.
After I've added checks for family_data!=0, I did not observe any more crashes; the other data of struct w1_slave seem to be valid as long as any thread executes w1_slave_show.
I have added debug-output to w1.c and w1_therm.c, here's a log of a potential crash:
[184199.510227] w1_master_driver w1_bus_master2: w1_search_process_cb, !W1_SLAVE_ACTIVE, calling w1_slave_detach [184199.510276] w1_slave_driver 10-000802d9c9e4: w1_slave_detach destroy_now 1 [184199.510297] w1_slave_driver 10-000802d9c9e4: w1_unref_slave refcnt 0 [184199.510321] w1_slave_driver 10-000802d9c9e4: w1_unref_slave: detaching 10-000802d9c9e4 [d5fb8800]. [184199.510347] w1_slave_driver 10-000802d9c9e4: w1_unref_slave -> w1_family_notify [184199.510365] w1_slave_driver 10-000802d9c9e4: w1_family_notify calling remove_slave [184199.510382] w1_slave_driver 10-000802d9c9e4: w1_therm_remove_slave [184199.510400] w1_slave_driver 10-000802d9c9e4: w1_therm_remove_slave refcnt -1 [184200.049745] w1_slave_driver 10-000802d9c9e4: w1_slave_show (after sleep), family_data==0 [184200.137133] w1_slave_driver 10-000802d9c9e4: w1_slave_show (before sleep), family_data==0 [184200.889551] w1_slave_driver 10-000802d9c9e4: w1_slave_show (after sleep), family_data==0 [184200.930866] w1_slave_driver 10-000802d9c9e4: w1_slave_show (after sleep), family_data==0 [184200.930907] w1_slave_driver 10-000802d9c9e4: Read failed CRC check [184200.931002] w1_slave_driver 10-000802d9c9e4: w1_unref_slave -> device_unregister [184200.931169] w1 w1_unref_slave -> kfree
Note: When this crash happened, multiple threads were reading the sensor.
I could trigger the problem several times, and each time device_unregister in w1_unref_slave was executed *after* w1_slave_show. In one case, the logged time-difference between the first family_data==0 message and device_unregister was about 8 seconds! I have not observed a w1_slave_show call after w1_unref_slave (as long as the device was not re-attached again).
From my observation, the w1_slave data seem to be valid as long as w1_slave_show is executed. My guess is that the call of sysfs_remove_groups in w1_family_notify hits a mutex (I did not add debug output here).
(*) On my tiny raspberry, this happens from time to time with high CPU and external disc load (timing and/or electrical issues); it seems that the sensor does not respond in time to the (periodic) search.
Please email me if you need further information.
Best regards, Thorsten Bschorr
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