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

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Date:   Tue, 18 Oct 2022 14:45:20 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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 Mon 2022-10-17 19:31:53, Jane Chu wrote:
> 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,

Could you please provide more details about the fixed OOB?
What exact vsprintf()/printk() call was broken and eventually
how it was fixed, please?

> the lingering issue
> is that the kernel ought to be able to protect itself, as the pointer 
> contains a non-canonical address.

Was the pointer used only by the vsprintf()?
Or was it accessed also by another code, please?

I wonder if this patch would prevent the crash or if the broken
kernel would crash later anyway.

> 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.

Please, add linux-mm@...ck.org into CC. I wonder if kern_addr_valid()
is safe to use anywhere, especially during early boot. I wonder if
it would make sense to implement it on all architectures.

Best Regards,
Petr