linux-kernel - Re: [PATCH] proc_sysctl: clamp sizes using table->maxlen

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Date:   Sat, 27 Feb 2021 09:41:36 -0500
From:   "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Luis Chamberlain <mcgrof@...nel.org>,
        Andrey Ignatov <rdna@...com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Iurii Zaikin <yzaikin@...gle.com>
Subject: Re: [PATCH] proc_sysctl: clamp sizes using table->maxlen

Excerpts from Christoph Hellwig's message of February 16, 2021 3:47 am:
> How do these maxlen = 0 entries even survive the sysctl_check_table
> check?

maxlen!=0 is only checked for "default" handlers, e.g. proc_dostring, 
proc_dointvec. it is not checked for non-default handlers, because some 
of them use fixed lengths.

my patch is not correct though because some drivers neither set proper 
maxlen nor use memcpy themselves; instead, they construct a ctl_table on 
the stack and call proc_*.

> Please split this into one patch each each subsystem that sets maxlen
> to 0 and the actual change to proc_sysctl.c.

I will do this with a new patch version once I figure out a way to 
comprehensively fix all the drivers setting bogus values for maxlen 
(sometimes maxlen=0 is valid if only blank writes are permitted, and 
some drivers set random values which have no relation to the actual read 
size).

Thank you for the review.