linux-kernel - Re: [PATCH] fs/proc/kcore.c: use probe_kernel

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Date:   Wed, 6 Dec 2017 16:43:50 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Al Viro <viro@...IV.linux.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/proc/kcore.c: use probe_kernel_read() instead of
 memcpy()

On Sat,  2 Dec 2017 14:27:39 +0100 Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> git commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext
> data") added a bounce buffer to avoid hardened usercopy
> checks. Copying to the bounce buffer was implemented with a simple
> memcpy() assuming that it is always valid to read from kernel memory
> iff the kern_addr_valid() check passed.
> 
> A simple, but pointless, test case like "dd if=/proc/kcore
> of=/dev/null" now can easily crash the kernel, since the former
> execption handling on invalid kernel addresses now doesn't work
> anymore.
> 
> Also adding a kern_addr_valid() implementation wouldn't help
> here. Most architectures simply return 1 here, while a couple
> implemented a page table walk to figure out if something is mapped at
> the address in question.
> With DEBUG_PAGEALLOC active mappings are established and removed all
> the time, so that relying on the result of kern_addr_valid() before
> executing the memcpy() also doesn't work.
> 
> Therefore simply use probe_kernel_read() to copy to the bounce
> buffer. This also allows to simplify read_kcore().
> 
> At least on s390 this fixes the observed crashes and doesn't introduce
> warnings that were removed with df04abfd181a ("fs/proc/kcore.c: Add
> bounce buffer for ktext data"), even though the generic
> probe_kernel_read() implementation uses uaccess functions.
> 
> While looking into this I'm also wondering if kern_addr_valid() could
> be completely removed...(?)
> 
> Fixes: df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
> Fixes: f5509cc18daa ("mm: Hardened usercopy")

It's a privileged operation, but oopsing root's kernel is still a bit
rude.  So I'll add cc:stable.  And let it bake until 4.16-rc1, since
the bug has been there for a year or more.  Sound OK?