linux-kernel - Re: [PATCH v3] module: Fix display of wrong module .text address

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Date:   Wed, 2 May 2018 10:13:38 +0200
From:   Thomas-Mich Richter <tmricht@...ux.ibm.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jessica Yu <jeyu@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        brueckner@...ux.vnet.ibm.com,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        "Tobin C. Harding" <me@...in.cc>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] module: Fix display of wrong module .text address

On 05/02/2018 04:20 AM, Kees Cook wrote:
> On Wed, Apr 18, 2018 at 12:14 AM, Thomas Richter <tmricht@...ux.ibm.com> wrote:
>> Reading file /proc/modules shows the correct address:
>> [root@...lp76 ~]# cat /proc/modules | egrep '^qeth_l2'
>> qeth_l2 94208 1 - Live 0x000003ff80401000
>>
>> and reading file /sys/module/qeth_l2/sections/.text
>> [root@...lp76 ~]# cat /sys/module/qeth_l2/sections/.text
>> 0x0000000018ea8363
>> displays a random address.
>>
>> This breaks the perf tool which uses this address on s390
>> to calculate start of .text section in memory.
>>
>> Fix this by printing the correct (unhashed) address.
>>
>> Thanks to Jessica Yu for helping on this.
>>
>> Fixes: ef0010a30935 ("vsprintf: don't use 'restricted_pointer()' when not restricting")
>> Cc: <stable@...r.kernel.org> # v4.15+
>> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
>> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
>> Cc: Jessica Yu <jeyu@...nel.org>
>> ---
>>  kernel/module.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/module.c b/kernel/module.c
>> index a6e43a5806a1..40b42000bd80 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
>> @@ -1472,7 +1472,8 @@ static ssize_t module_sect_show(struct module_attribute *mattr,
>>  {
>>         struct module_sect_attr *sattr =
>>                 container_of(mattr, struct module_sect_attr, mattr);
>> -       return sprintf(buf, "0x%pK\n", (void *)sattr->address);
>> +       return sprintf(buf, "0x%px\n", kptr_restrict < 2 ?
>> +                      (void *)sattr->address : NULL);
> 
> Errr... this looks reversed to me.
> 
> I would expect: "kptr_restrict < 2 ? NULL : (void *)sattr->address"
> 
> -Kees
> 

I am confused:
In my patch, if kptr_restrict == 2 it prints NULL, which kptr_restrict 
being 0 or 1 it prints the address.

In your comment if kptr_restrict == 2 it prints the address, which 
kptr_restrict being 0 or 1 it prints NULL.

Looking into Documentation/sysctl/kernel.txt:
  When kptr_restrict is set to (2), kernel pointers printed using
  %pK will be replaced with 0's regardless of privileges.

With my patch, setting kptr_restrict to 0 or 1
prints the real kernel address (format %px, unmodified address
according to Documentation/printk-formats.txt).

I have tested this on s390 (which is the only arch using file
/sys/module/<XXX>/sections/.text) in the perf tool.

root@...60047 ~]# sysctl  kernel.kptr_restrict
kernel.kptr_restrict = 0
[root@...60047 ~]# cat /proc/modules | egrep '^qeth_l2'
qeth_l2 102400 1 - Live 0x000003ff8034d000
[root@...60047 ~]# cat /sys/module/qeth_l2/sections/.text 
0x000003ff8034da68
[root@...60047 ~]# sysctl  -w kernel.kptr_restrict=2
kernel.kptr_restrict = 2
[root@...60047 ~]# cat /proc/modules | egrep '^qeth_l2'
qeth_l2 102400 1 - Live 0x0000000000000000
[root@...60047 ~]# cat /sys/module/qeth_l2/sections/.text 
0x0000000000000000
[root@...60047 ~]# uname -a
Linux s8360047 4.17.0-rc3m-perf+ #6 SMP PREEMPT Wed May 2 10:02:38 CEST 2018 s390x s390x s390x GNU/Linux
[root@...60047 ~]# 

Hope this helps.
-- 
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
--
Vorsitzende des Aufsichtsrats: Martina Koederitz 
Geschäftsführung: Dirk Wittkopp
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