Re: [PATCH v2] proc_sysctl: fix oops caused by incorrect command parameters.

[Xiaoming Ni <nixiaoming@huawei.com>]

On 2021/1/8 17:21, Michal Hocko wrote:
> On Fri 08-01-21 10:33:39, Xiaoming Ni wrote:
>> The process_sysctl_arg() does not check whether val is empty before
>>   invoking strlen(val). If the command line parameter () is incorrectly
>>   configured and val is empty, oops is triggered.
>>
>> For example, "hung_task_panic=1" is incorrectly written as "hung_task_panic".
>>
>> log:
>> 	Kernel command line: .... hung_task_panic
>> 	....
>> 	[000000000000000n] user address but active_mm is swapper
>> 	Internal error: Oops: 96000005 [#1] SMP
>> 	Modules linked in:
>> 	CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.1 #1
>> 	Hardware name: linux,dummy-virt (DT)
>> 	pstate: 40000005 (nZcv daif -PAN -UAO -TCO BTYPE=--)
>> 	pc : __pi_strlen+0x10/0x98
>> 	lr : process_sysctl_arg+0x1e4/0x2ac
>> 	sp : ffffffc01104bd40
>> 	x29: ffffffc01104bd40 x28: 0000000000000000
>> 	x27: ffffff80c0a4691e x26: ffffffc0102a7c8c
>> 	x25: 0000000000000000 x24: ffffffc01104be80
>> 	x23: ffffff80c22f0b00 x22: ffffff80c02e28c0
>> 	x21: ffffffc0109f9000 x20: 0000000000000000
>> 	x19: ffffffc0107c08de x18: 0000000000000003
>> 	x17: ffffffc01105d000 x16: 0000000000000054
>> 	x15: ffffffffffffffff x14: 3030253078413830
>> 	x13: 000000000000ffff x12: 0000000000000000
>> 	x11: 0101010101010101 x10: 0000000000000005
>> 	x9 : 0000000000000003 x8 : ffffff80c0980c08
>> 	x7 : 0000000000000000 x6 : 0000000000000002
>> 	x5 : ffffff80c0235000 x4 : ffffff810f7c7ee0
>> 	x3 : 000000000000043a x2 : 00bdcc4ebacf1a54
>> 	x1 : 0000000000000000 x0 : 0000000000000000
>> 	Call trace:
>> 	 __pi_strlen+0x10/0x98
>> 	 parse_args+0x278/0x344
>> 	 do_sysctl_args+0x8c/0xfc
>> 	 kernel_init+0x5c/0xf4
>> 	 ret_from_fork+0x10/0x30
>> 	Code: b200c3eb 927cec01 f2400c07 54000301 (a8c10c22)
>>
>> Fixes: 3db978d480e2843 ("kernel/sysctl: support setting sysctl parameters
>>   from kernel command line")
>> Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
> 
> Thanks for catching this!
> 
>> ---------
>> v2:
>>     Added log output of the failure branch based on the review comments of Kees Cook.
>> v1: https://lore.kernel.org/lkml/20201224074256.117413-1-nixiaoming@huawei.com/
>> ---------
>> ---
>>   fs/proc/proc_sysctl.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
>> index 317899222d7f..dc1a56515e86 100644
>> --- a/fs/proc/proc_sysctl.c
>> +++ b/fs/proc/proc_sysctl.c
>> @@ -1757,6 +1757,11 @@ static int process_sysctl_arg(char *param, char *val,
>>   	loff_t pos = 0;
>>   	ssize_t wret;
>>   
>> +	if (!val) {
>> +		pr_err("Missing param value! Expected '%s=...value...'\n", param);
>> +		return 0;
I may need to move the validation code for val to the end of the 
validation code for param to prevent non-sysctl arguments from 
triggering the current print.
Or delete the print and keep it silent for a little better performance.
Which is better?


>> +	}
> 
> Shouldn't you return an error here? Also my understanding is that
> parse_args is responsible for reporting the error.
> 
All exception branches in process_sysctl_arg record logs and return 0.
Do I need to keep the same processing in the new branch?


>> +
>>   	if (strncmp(param, "sysctl", sizeof("sysctl") - 1) == 0) {
>>   		param += sizeof("sysctl") - 1;
>>   
>> -- 
>> 2.27.0
> 

Thanks
Xiaoming Ni