linux-hardening - Re: [PATCH] vt: Fix potential read overflow of kernel memory

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Date:   Thu, 31 Aug 2023 07:32:18 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Kees Cook <keescook@...omium.org>,
        Azeem Shaikh <azeemshaikh38@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] vt: Fix potential read overflow of kernel memory

On 30. 08. 23, 23:28, Kees Cook wrote:
> On Wed, Aug 30, 2023 at 03:25:54PM -0400, Azeem Shaikh wrote:
>> On Wed, Aug 30, 2023 at 1:57 PM Greg Kroah-Hartman
>> <gregkh@...uxfoundation.org> wrote:
>>>
>>> On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote:
>>>> strlcpy() reads the entire source buffer first.
>>>> This read may exceed the destination size limit if
>>>> a source string is not NUL-terminated [1].
>>>
>>> But that's not the case here, right?  So your "potential read overflow"
>>> isn't relevant here.
>>>
>>>> The copy_to_user() call uses @len returned from strlcpy() directly
>>>> without checking its value. This could potentially lead to read
>>>> overflow.
>>>
>>> But can it?  How?
>>>
>>
>> The case I was considering is when the null-terminated hardcoded
>> string @func_table[kb_func] has length @new_len > @len. In this case,
>> strlcpy() will assign @len = @new_len and copy_to_user() would read
>> @new_len from the kmalloc-ed memory of @len. This is the potential
>> read overflow I was referring to. Let me know if I'm mistaken.
> 
> First there is:
> 
> ssize_t len = sizeof(user_kdgkb->kb_string);
> 
> "struct user_kdgkb" is UAPI (therefore unlikely to change), and kb_string
> is 512:
> 
> struct kbsentry {
>          unsigned char kb_func;
>          unsigned char kb_string[512];
> };
> 
> Then we do:
> 
>                  len = strlcpy(kbs, func_table[kb_func] ? : "", len);
> 
> This is the anti-pattern (take the length of the _source_) we need to
> remove.

But len is the length of kbs, i.e. the destination. Or what am I missing?

                 kbs = kmalloc(len, GFP_KERNEL);
                 len = strlcpy(kbs, func_table[kb_func] ? : "", len);

> However, func_table[] is made up of either %NUL-terminated
> strings:
> 
> char func_buf[] = {
>          '\033', '[', '[', 'A', 0,
>          '\033', '[', '[', 'B', 0,
> ...
> char *func_table[MAX_NR_FUNC] = {
>          func_buf + 0,
>          func_buf + 5,
> ...
> 
> Or a NULL address itself. The ?: operator handles the NULL case, so
> "len" can only ever be 0 through the longest string in func_buf. So it's
> what I'd call "accidentally correct". i.e. it's using a fragile
> anti-pattern, but in this case everything is hard-coded and less than
> 512.
> 
> Regardless, we need to swap for a sane pattern, which you've done. But
> the commit log is misleading, so it needs some more detail. :)

I am still missing what is wrong in the above code with strlcpy(). The 
dest is deliberately made as long as the source, so the returned len is 
never less then the passed len. No checking needed IMO. Perhaps, we 
might switch to strcpy()?

FWIW I introduced this in commit 82e61c3909db5. So if it needs fixing, 
the patch deserves a Fixes: 82e61c3909db5 tag.

thanks,
-- 
js
suse labs