linux-kernel - Re: [PATCH v3 1/1] vsprintf: protect kernel from panic due to non-canonical pointer dereference

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Message-ID: <Y1EUpXnGeUh9yslc@alley>
Date:   Thu, 20 Oct 2022 11:28:05 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     rostedt@...dmis.org, senozhatsky@...omium.org,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        wangkefeng.wang@...wei.com, konrad.wilk@...cle.com,
        haakon.bugge@...cle.com, john.haxby@...cle.com
Subject: Re: [PATCH v3 1/1] vsprintf: protect kernel from panic due to
 non-canonical pointer dereference

On Wed 2022-10-19 13:41:59, Jane Chu wrote:
> Having stepped on a local kernel bug where reading sysfs has led to
> out-of-bound pointer dereference by vsprintf() which led to GPF panic.
> And the reason for GPF is that the OOB pointer was turned to a
> non-canonical address such as 0x7665645f63616465.
> 
> vsprintf() already has this line of defense
> 	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
>                 return "(efault)";
> Since a non-canonical pointer can be detected by kern_addr_valid()
> on architectures that present VM holes as well as meaningful
> implementation of kern_addr_valid() that detects the non-canonical
> addresses, this patch adds a check on non-canonical string pointer by
> kern_addr_valid() and "(efault)" to alert user that something
> is wrong instead of unecessarily panic the server.
> 
> On the other hand, if the non-canonical string pointer is dereferenced
> else where in the kernel, by virtue of being non-canonical, a crash
> is expected to be immediate.

Just for record, this patch is going to be abandoned.

Some reasons are mentioned in this thread. Others are in the threads
for previous versions, see
https://lore.kernel.org/r/20221017194447.2579441-1-jane.chu@oracle.com
https://lore.kernel.org/r/20221017191611.2577466-1-jane.chu@oracle.com

Best Regards,
Petr