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

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Message-ID: <cc38380a-de69-d8f6-44b5-8ae4d073d916@rasmusvillemoes.dk>
Date:   Wed, 19 Oct 2022 23:00:09 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Jane Chu <jane.chu@...cle.com>, pmladek@...e.com,
        rostedt@...dmis.org, senozhatsky@...omium.org,
        andriy.shevchenko@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     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 19/10/2022 21.41, 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.

Just to be completely clear, the out-of-bounds dereference did not
happen in vsprintf if I understand your description right. Essentially
you have an array of char* pointers, and you accessed beyond that array,
where of course some random memory contents then turned out not to be a
real pointer, and that bogus pointer value was passed into vsprintf() as
a %s argument.

> And the reason for GPF is that the OOB pointer was turned to a
> non-canonical address such as 0x7665645f63616465.

That's ved_cade , or more properly edac_dev ...

> 
> 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.

I'm with Andy on this one, we don't add random checks like this in the
kernel, not in vsprintf or elsewhere.

check_pointer_msg is/was actually more about checking the various
%p<foo> extensions, where it is (more) expected that somebody does

  struct foo *f = get_a_foo();
  pr_debug("got %pfoo\n", f);
  if (IS_ERR(f)) { ... }

[possibly in a not so obvious path], and the PAGE_SIZE check is
similarly for cases where the "base" pointer is actually NULL but what
is passed is &f->member.

Rasmus