linux-kernel - Re: [PATCH] vsprintf: protect kernel from panic due to non-canonical pointer dereference

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Message-ID: <5d987403-a7bf-8996-d639-c99edeaabcdf@oracle.com>
Date:   Mon, 17 Oct 2022 19:31:53 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     "pmladek@...e.com" <pmladek@...e.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vsprintf: protect kernel from panic due to non-canonical
 pointer dereference

On 10/17/2022 12:25 PM, Andy Shevchenko wrote:
> On Mon, Oct 17, 2022 at 01:16:11PM -0600, Jane Chu wrote:
>> While debugging a separate issue, it was found that an invalid string
>> pointer could very well contain a non-canical address, such as
>> 0x7665645f63616465. In that case, this line of defense isn't enough
>> to protect the kernel from crashing due to general protection fault
>>
>> 	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
>>                  return "(efault)";
>>
>> So instead, use kern_addr_valid() to validate the string pointer.
> 
> How did you check that value of the (invalid string) pointer?
> 

In the bug scenario, the invalid string pointer was an out-of-bound 
string pointer. While the OOB referencing is fixed, the lingering issue 
is that the kernel ought to be able to protect itself, as the pointer 
contains a non-canonical address.  That said, I realized that not all 
architecture implement meaningful kern_addr_valid(), so this line
    if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
is still need.  I'll send v2.

thanks,
-jane

>