linux-kernel - Re: [PATCH v3] proc_sysctl: fix oops caused by incorrect command parameters.

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Date:   Mon, 18 Jan 2021 11:50:25 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Xiaoming Ni <nixiaoming@...wei.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        rdunlap@...radead.org, hkallweit1@...il.com
Cc:     linux-kernel@...r.kernel.org, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com, adobriyan@...il.com,
        linux-fsdevel@...r.kernel.org, andy.shevchenko@...il.com,
        wangle6@...wei.com
Subject: Re: [PATCH v3] proc_sysctl: fix oops caused by incorrect command
 parameters.

On 1/17/21 3:59 AM, Xiaoming Ni wrote:
> On 2021/1/12 19:42, Vlastimil Babka wrote:
>> On 1/12/21 8:24 AM, Michal Hocko wrote:
>>>>>>
>>>>>> If we're going to do a separate "patch: make process_sysctl_arg()
>>>>>> return an errno instead of 0" then fine, we can discuss that.  But it's
>>>>>> conceptually a different work from fixing this situation.
>>>>>> .
>>>>>>
>>>>> However, are the logs generated by process_sysctl_arg() clearer and more
>>>>> accurate than parse_args()? Should the logs generated by
>>>>> process_sysctl_arg() be deleted?
>>>>
>>>> I think the individual logs are very useful and should be retained.
>>>
>>> Yes, other sysfs specific error messages are likely useful. I just fail
>>> to see why a missing value should be handled here when there is an
>>> existing handling in the caller. Not sure whether a complete shadow
>>> reporting in process_sysctl_arg is a deliberate decision or not.
>>> Vlastimil?
>>
>> Yes, it's a way to have more useful sysctl-specific reports than the generic
>> ones. And I think I was inspired by some other existing code, but don't remember
>> exactly. The options are:
>>
>> 1) the current sysctl-specific reports, return 0 as the values are only consumed
>> 2) be silent and return error, invent new error codes to have generic report be
>> more useful for sysctl, but inevitably lose some nuances anyway
>> 3) a mix where 2) is used for situations where generic report is sufficient
>> enough, 1) where not
>>
>> Patch v2 went with option 1), v3 with option 3). I think it's down to
>> preferences. I would personally go with v2 and message similar to the existing
>> ones, i.e.:
>>
>> "Failed to set sysctl parameter '%s': no value given\n"
>>
>> Also we seem to be silently doing nothing when strlen(val) == 0, i.e.
>> "hung_task_panic=" was passed. Worth reporting the same error.
>>
>> But v3 is fine with me as well. The generic error message works. We could just
>> add "if (!len) return -EINVAL" below the strlen() call.
>>
>> Also please Cc: stable.
>>
>>> Anyway one way or the other, all I care about is to have a reporting in
>>> place because this shouldn't be a silent failure.
>>>
> 
> 
> The current v2 is already in the linux-next branch and throws a new error:
> https://lore.kernel.org/lkml/cb54e349-7147-0a1f-a349-1e16ba603fce@infradead.org/
> 
> This bug has been mentioned in the previous discussion and has been fixed in the
> current v3 patch.
> https://lore.kernel.org/linux-fsdevel/202101111149.20A58E1@keescook/
> 
> What am I supposed to do now?
>     - Resend V3?

IMHO this. But also please handle also len == 0 like below. And add
Cc: <stable@...r.kernel.org>

>     - Rewrite a new fix patch based on the current code of linux-next.

AFAICS Andrew dropped the v2 already.

Thanks.

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 317899222d7f..f424010d1a60 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -1770,6 +1770,12 @@ static int process_sysctl_arg(char *param, char *val,
 			return 0;
 	}
 
+	if (!val)
+		return -EINVAL;
+	len = strlen(val);
+	if (!len)
+		return -EINVAL;
+
 	/*
 	 * To set sysctl options, we use a temporary mount of proc, look up the
 	 * respective sys/ file and write to it. To avoid mounting it when no
@@ -1811,7 +1817,6 @@ static int process_sysctl_arg(char *param, char *val,
 				file, param, val);
 		goto out;
 	}
-	len = strlen(val);
 	wret = kernel_write(file, val, len, &pos);
 	if (wret < 0) {
 		err = wret;