linux-kernel - Re: [PATCH v5 0/6] compat: remove compat_alloc_user

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Date:   Fri, 30 Jul 2021 15:35:35 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christoph Hellwig <hch@...radead.org>,
        Feng Tang <feng.tang@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v5 0/6] compat: remove compat_alloc_user_space

On Fri, Jul 30, 2021 at 11:49 AM Heiko Carstens <hca@...ux.ibm.com> wrote:
> On Tue, Jul 27, 2021 at 04:48:53PM +0200, Arnd Bergmann wrote:
>
> Our CI reports this with linux-next and running strace selftest in
> compat mode:

Thanks a lot for the report! I managed track it down based on your
output, it turns out that I end up copying data from the stack according
to how much the user asked for, and in this case that was much more
than the 8 byte nodemask_t, copying all of the kernel stack all the
way into the guard page with CONFIG_VMAP_STACK, where it
crashed. Without CONFIG_VMAP_STACK, or with user space that
asks for less data, it would just be an information leak, so others
probably haven't noticed the problem.

The change below should fix that, I'll double-check the other callers
as well before sending a proper fixup patch to Andrew.

        Arnd

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4fabf2dddbc0..0d1f3be32723 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1438,6 +1438,7 @@ static int copy_nodes_to_user(unsigned long
__user *mask, unsigned long maxnode,
                if (clear_user((char __user *)mask + nbytes, copy - nbytes))
                        return -EFAULT;
                copy = nbytes;
+               maxnode = nr_node_ids;
        }

        if (compat)