Re: sctp: kernel memory overwrite attempt detected in sctp_getsockopt_assoc_stats

[Dmitry Vyukov]

On Sun, Jan 15, 2017 at 9:35 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> 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.



>> 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.
>>