netdev - Re: [PATCH net] net: ipv6: RTF_PCPU should not be settable from userspace

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Message-ID: <CAAeHK+zw3LX8rDJUrfaU5of8MrkUXsmwyLtUBMPaLtVykCUTQw@mail.gmail.com>
Date:   Thu, 20 Apr 2017 14:09:20 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     David Ahern <dsa@...ulusnetworks.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: ipv6: RTF_PCPU should not be settable from userspace

Thanks!

Tested-by: Andrey Konovalov <andreyknvl@...gle.com>

On Wed, Apr 19, 2017 at 11:52 PM, Martin KaFai Lau <kafai@...com> wrote:
> On Wed, Apr 19, 2017 at 02:19:43PM -0700, David Ahern wrote:
>> Andrey reported a fault in the IPv6 route code:
>>
>> kasan: GPF could be caused by NULL-ptr deref or user memory access
>> general protection fault: 0000 [#1] SMP KASAN
>> Modules linked in:
>> CPU: 1 PID: 4035 Comm: a.out Not tainted 4.11.0-rc7+ #250
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> task: ffff880069809600 task.stack: ffff880062dc8000
>> RIP: 0010:ip6_rt_cache_alloc+0xa6/0x560 net/ipv6/route.c:975
>> RSP: 0018:ffff880062dced30 EFLAGS: 00010206
>> RAX: dffffc0000000000 RBX: ffff8800670561c0 RCX: 0000000000000006
>> RDX: 0000000000000003 RSI: ffff880062dcfb28 RDI: 0000000000000018
>> RBP: ffff880062dced68 R08: 0000000000000001 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>> R13: ffff880062dcfb28 R14: dffffc0000000000 R15: 0000000000000000
>> FS:  00007feebe37e7c0(0000) GS:ffff88006cb00000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00000000205a0fe4 CR3: 000000006b5c9000 CR4: 00000000000006e0
>> Call Trace:
>>  ip6_pol_route+0x1512/0x1f20 net/ipv6/route.c:1128
>>  ip6_pol_route_output+0x4c/0x60 net/ipv6/route.c:1212
>> ...
>>
>> Andrey's syzkaller program passes rtmsg.rtmsg_flags with the RTF_PCPU bit
>> set. Flags passed to the kernel are blindly copied to the allocated
>> rt6_info by ip6_route_info_create making a newly inserted route appear
>> as though it is a per-cpu route. ip6_rt_cache_alloc sees the flag set
>> and expects rt->dst.from to be set - which it is not since it is not
>> really a per-cpu copy. The subsequent call to __ip6_dst_alloc then
>> generates the fault.
>>
>> Fix by checking for the flag and failing with EINVAL.
> Thanks for the fix.
>
> Acked-by: Martin KaFai Lau <kafai@...com>
>
>>
>> Fixes: d52d3997f843f ("ipv6: Create percpu rt6_info")
>> Reported-by: Andrey Konovalov <andreyknvl@...gle.com>
>> Signed-off-by: David Ahern <dsa@...ulusnetworks.com>
>> ---
>>  include/uapi/linux/ipv6_route.h | 2 +-
>>  net/ipv6/route.c                | 4 ++++
>>  2 files changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/ipv6_route.h b/include/uapi/linux/ipv6_route.h
>> index 85bbb1799df3..d496c02e14bc 100644
>> --- a/include/uapi/linux/ipv6_route.h
>> +++ b/include/uapi/linux/ipv6_route.h
>> @@ -35,7 +35,7 @@
>>  #define RTF_PREF(pref)       ((pref) << 27)
>>  #define RTF_PREF_MASK        0x18000000
>>
>> -#define RTF_PCPU     0x40000000
>> +#define RTF_PCPU     0x40000000      /* read-only: can not be set by user */
>>  #define RTF_LOCAL    0x80000000
>>
>>
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index 4ba7c49872ff..a1bf426c959b 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -1854,6 +1854,10 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
>>       int addr_type;
>>       int err = -EINVAL;
>>
>> +     /* RTF_PCPU is an internal flag; can not be set by userspace */
>> +     if (cfg->fc_flags & RTF_PCPU)
>> +             goto out;
>> +
>>       if (cfg->fc_dst_len > 128 || cfg->fc_src_len > 128)
>>               goto out;
>>  #ifndef CONFIG_IPV6_SUBTREES
>> --
>> 2.9.3
>>