linux-kernel - Re: [PATCH] rpadlpar: fix potential drc

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Message-ID: <6b1728cf-c9b7-68fe-f338-3b79210357f2@linux.ibm.com>
Date:   Mon, 15 Mar 2021 09:58:55 -0700
From:   Tyrel Datwyler <tyreld@...ux.ibm.com>
To:     Michael Ellerman <michaele@....ibm.com>,
        Michal Suchánek <msuchanek@...e.de>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org, mmc@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rpadlpar: fix potential drc_name corruption in store
 functions

On 3/14/21 7:52 PM, Michael Ellerman wrote:
> Tyrel Datwyler <tyreld@...ux.ibm.com> writes:
>> On 3/13/21 1:17 AM, Michal Suchánek wrote:
>>> On Wed, Mar 10, 2021 at 04:30:21PM -0600, Tyrel Datwyler wrote:
>>>> Both add_slot_store() and remove_slot_store() try to fix up the drc_name
>>>> copied from the store buffer by placing a NULL terminator at nbyte + 1
>>>> or in place of a '\n' if present. However, the static buffer that we
>>>> copy the drc_name data into is not zeored and can contain anything past
>>>> the n-th byte. This is problematic if a '\n' byte appears in that buffer
>>>> after nbytes and the string copied into the store buffer was not NULL
>>>> terminated to start with as the strchr() search for a '\n' byte will mark
>>>> this incorrectly as the end of the drc_name string resulting in a drc_name
>>>> string that contains garbage data after the n-th byte. The following
>>>> debugging shows an example of the drmgr utility writing "PHB 4543" to
>>>> the add_slot sysfs attribute, but add_slot_store logging a corrupted
>>>> string value.
>>>>
>>>> [135823.702864] drmgr: drmgr: -c phb -a -s PHB 4543 -d 1
>>>> [135823.702879] add_slot_store: drc_name = PHB 4543°|<82>!, rc = -19
>>>>
>>>> Fix this by NULL terminating the string when we copy it into our static
>>>> buffer by coping nbytes + 1 of data from the store buffer. The code has
>>> Why is it OK to copy nbytes + 1 and why is it expected that the buffer
>>> contains a nul after the content?
>>
>> It is my understanding that the store function buffer is allocated as a
>> zeroed-page which the kernel copies up to at most (PAGE_SIZE - 1) of user data
>> into. Anything after nbytes would therefore be zeroed.
> 
> I think that's true, but it would be nice if we didn't have to rely on
> that obscure detail in order for this code to be correct & understandable.

I think its a security guarantee, but I guess barring a comment that explicitly
outlines the correctness it probably isn't obvious.

> 
>>> Isn't it much saner to just nul terminate the string after copying?
>>
>> At the cost of an extra line of code, sure.
> 
> Is there a reason we can't use strscpy()? That should deal with all the
> corner cases around the string copy, and then all you have to do is look
> for a newline and turn it into nul.

Fine with me. I'll spin v2 with strscpy().

-Tyrel

> 
> cheers
>