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Date:   Mon, 16 Jan 2017 08:57:43 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Vladislav Yasevich <vyasevich@...il.com>,
        David Miller <davem@...emloft.net>, linux-sctp@...r.kernel.org,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: sctp: kernel memory overwrite attempt detected in
 sctp_getsockopt_assoc_stats

On Mon, Jan 16, 2017 at 08:11:40AM +0100, Dmitry Vyukov wrote:
> On Sun, Jan 15, 2017 at 9:35 PM, Neil Horman <nhorman@...driver.com> wrote:
> > On Sun, Jan 15, 2017 at 06:29:59PM +0100, Dmitry Vyukov wrote:
> >> Hello,
> >>
> >> I've enabled CONFIG_HARDENED_USERCOPY_PAGESPAN on syzkaller fuzzer and
> >> now I am seeing lots of:
> >>
> > If I'm not mistaken, its because thats specifically what that option does.  From
> > the Kconfig:
> > onfig HARDENED_USERCOPY_PAGESPAN
> >         bool "Refuse to copy allocations that span multiple pages"
> >         depends on HARDENED_USERCOPY
> >         depends on EXPERT
> >         help
> >           When a multi-page allocation is done without __GFP_COMP,
> >           hardened usercopy will reject attempts to copy it. There are,
> >           however, several cases of this in the kernel that have not all
> >           been removed. This config is intended to be used only while
> >           trying to find such users.
> >
> > So, if the fuzzer does a setsockopt and the data it passes crosses a page
> > boundary, it seems like this will get triggered.  Based on the fact that its
> > only used to find users that do this, it seems like not the sort of thing that
> > you want enabled while running a fuzzer that might do it arbitrarily.
> 
> 
> The code also takes into account compound pages. As far as I
> understand the intention of the check is to effectively find
> out-of-bounds copies (e.g. goes beyond the current heap allocation). I
> would expect that stacks are allocated as compound pages and don't
> trigger this check. I don't see it is firing in other similar places.
> 
Honestly, I'm not overly familiar with stack page allocation, at least not so
far as compound vs. single page allocation is concerned.  I suppose the question
your really asking here is: Have you found a case in which the syscall fuzzer
has forced the allocation of an insecure non-compound page on the stack, or is
this a false positive warning.  I can't provide the answer to that.

Neil

> 
> 
> >> usercopy: kernel memory overwrite attempt detected to ffff8801a74f6f10
> >> (<spans multiple pages>) (256 bytes)
> >>
> >> kernel BUG at mm/usercopy.c:75!
> >> invalid opcode: 0000 [#1] SMP KASAN
> >> Dumping ftrace buffer:
> >>    (ftrace buffer empty)
> >> Modules linked in:
> >> CPU: 1 PID: 15686 Comm: syz-executor3 Not tainted 4.9.0 #1
> >> Hardware name: Google Google Compute Engine/Google Compute Engine,
> >> BIOS Google 01/01/2011
> >> task: ffff8801c89b2500 task.stack: ffff8801a74f0000
> >> RIP: 0010:[<ffffffff81a1b041>]  [<ffffffff81a1b041>] report_usercopy
> >> mm/usercopy.c:67 [inline]
> >> RIP: 0010:[<ffffffff81a1b041>]  [<ffffffff81a1b041>]
> >> __check_object_size+0x2d1/0x9ec mm/usercopy.c:278
> >> RSP: 0018:ffff8801a74f6cd0  EFLAGS: 00010286
> >> RAX: 000000000000006b RBX: ffffffff84500120 RCX: 0000000000000000
> >> RDX: 000000000000006b RSI: ffffffff815a7791 RDI: ffffed0034e9ed8c
> >> RBP: ffff8801a74f6e48 R08: 0000000000000001 R09: 0000000000000000
> >> R10: 0000000000000000 R11: 0000000000000001 R12: ffff8801a74f6f10
> >> R13: 0000000000000100 R14: ffffffff845000e0 R15: ffff8801a74f700f
> >> FS:  00007f80918de700(0000) GS:ffff8801dc100000(0000) knlGS:0000000000000000
> >> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >> CR2: 0000000020058ffc CR3: 00000001cc1cc000 CR4: 00000000001406e0
> >> Stack:
> >>  ffffffff8598fcc8 0000000000000000 000077ff80000000 ffffea0005c99608
> >>  ffffffff844fff40 ffffffff844fff40 0000000041b58ab3 ffffffff84ae0fa0
> >>  ffffffff81a1ad70 ffff8801c89b2500 dead000000000100 ffffffff814d4425
> >> Call Trace:
> >>  [<ffffffff83e4ece9>] check_object_size include/linux/thread_info.h:129 [inline]
> >>  [<ffffffff83e4ece9>] copy_from_user arch/x86/include/asm/uaccess.h:692 [inline]
> >>  [<ffffffff83e4ece9>] sctp_getsockopt_assoc_stats+0x169/0xa10
> >> net/sctp/socket.c:6182
> >>  [<ffffffff83e5cc52>] sctp_getsockopt+0x1af2/0x66a0 net/sctp/socket.c:6556
> >>  [<ffffffff834f92c5>] sock_common_getsockopt+0x95/0xd0 net/core/sock.c:2649
> >>  [<ffffffff834f4910>] SYSC_getsockopt net/socket.c:1788 [inline]
> >>  [<ffffffff834f4910>] SyS_getsockopt+0x240/0x380 net/socket.c:1770
> >>  [<ffffffff81009798>] do_syscall_64+0x2e8/0x930 arch/x86/entry/common.c:280
> >>  [<ffffffff84370a49>] entry_SYSCALL64_slow_path+0x25/0x25
> >> Code: b0 fe ff ff e8 e1 25 ce ff 48 8b 85 b0 fe ff ff 4d 89 e9 4c 89
> >> e1 4c 89 f2 48 89 de 48 c7 c7 a0 01 50 84 49 89 c0 e8 51 d9 e0 ff <0f>
> >> 0b e8 b8 25 ce ff 4c 89 f2 4c 89 ee 4c 89 e7 e8 6a 1b fc ff
> >> RIP  [<ffffffff81a1b041>] report_usercopy mm/usercopy.c:67 [inline]
> >> RIP  [<ffffffff81a1b041>] __check_object_size+0x2d1/0x9ec mm/usercopy.c:278
> >>  RSP <ffff8801a74f6cd0>
> >> ---[ end trace 5e438996b2c0b35d ]---
> >>
> >>
> >> I am not sure why check_object_size flags this an a bug,
> >> copy_from_user copies into a stack object:
> >>
> >> static int sctp_getsockopt_assoc_stats(struct sock *sk, int len,
> >>                                        char __user *optval,
> >>                                        int __user *optlen)
> >> {
> >>         struct sctp_assoc_stats sas;
> >>         len = min_t(size_t, len, sizeof(sas));
> >>         if (copy_from_user(&sas, optval, len))
> >>                 return -EFAULT;
> >>
> >> Kees, can this be a false positive?
> >>
> >> On commit f4d3935e4f4884ba80561db5549394afb8eef8f7.
> >>
> 

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