Re: [PATCH] tun: bail out from tun_get_user() if the skb is empty

[Alexander Potapenko]

On Tue, Sep 26, 2017 at 5:02 PM, 'Eric Dumazet' via syzkaller
<syzkaller@xxxxxxxxxxxxxxxx> wrote:
> On Tue, Sep 26, 2017 at 7:53 AM, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>> KMSAN (https://github.com/google/kmsan) reported accessing uninitialized
>> skb->data[0] in the case the skb is empty (i.e. skb->len is 0):
>>
>> ================================================
>> BUG: KMSAN: use of uninitialized memory in tun_get_user+0x19ba/0x3770
>> CPU: 0 PID: 3051 Comm: probe Not tainted 4.13.0+ #3140
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>> ...
>>  __msan_warning_32+0x66/0xb0 mm/kmsan/kmsan_instr.c:477
>>  tun_get_user+0x19ba/0x3770 drivers/net/tun.c:1301
>>  tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
>>  call_write_iter ./include/linux/fs.h:1743
>>  new_sync_write fs/read_write.c:457
>>  __vfs_write+0x6c3/0x7f0 fs/read_write.c:470
>>  vfs_write+0x3e4/0x770 fs/read_write.c:518
>>  SYSC_write+0x12f/0x2b0 fs/read_write.c:565
>>  SyS_write+0x55/0x80 fs/read_write.c:557
>>  do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
>>  entry_SYSCALL64_slow_path+0x25/0x25 arch/x86/entry/entry_64.S:245
>> ...
>> origin:
>> ...
>>  kmsan_poison_shadow+0x6e/0xc0 mm/kmsan/kmsan.c:211
>>  slab_alloc_node mm/slub.c:2732
>>  __kmalloc_node_track_caller+0x351/0x370 mm/slub.c:4351
>>  __kmalloc_reserve net/core/skbuff.c:138
>>  __alloc_skb+0x26a/0x810 net/core/skbuff.c:231
>>  alloc_skb ./include/linux/skbuff.h:903
>>  alloc_skb_with_frags+0x1d7/0xc80 net/core/skbuff.c:4756
>>  sock_alloc_send_pskb+0xabf/0xfe0 net/core/sock.c:2037
>>  tun_alloc_skb drivers/net/tun.c:1144
>>  tun_get_user+0x9a8/0x3770 drivers/net/tun.c:1274
>>  tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
>>  call_write_iter ./include/linux/fs.h:1743
>>  new_sync_write fs/read_write.c:457
>>  __vfs_write+0x6c3/0x7f0 fs/read_write.c:470
>>  vfs_write+0x3e4/0x770 fs/read_write.c:518
>>  SYSC_write+0x12f/0x2b0 fs/read_write.c:565
>>  SyS_write+0x55/0x80 fs/read_write.c:557
>>  do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
>>  return_from_SYSCALL_64+0x0/0x6a arch/x86/entry/entry_64.S:245
>> ================================================
>>
>> Make sure tun_get_user() doesn't touch skb->data[0] unless there is
>> actual data.
>>
>> C reproducer below:
>> ==========================
>>     // autogenerated by syzkaller (http://github.com/google/syzkaller)
>>
>>     #define _GNU_SOURCE
>>
>>     #include <fcntl.h>
>>     #include <linux/if_tun.h>
>>     #include <netinet/ip.h>
>>     #include <net/if.h>
>>     #include <string.h>
>>     #include <sys/ioctl.h>
>>
>>     int main()
>>     {
>>       int sock = socket(PF_INET, SOCK_STREAM, IPPROTO_IP);
>>       int tun_fd = open("/dev/net/tun", O_RDWR);
>>       struct ifreq req;
>>       memset(&req, 0, sizeof(struct ifreq));
>>       strcpy((char*)&req.ifr_name, "gre0");
>>       req.ifr_flags = IFF_UP | IFF_MULTICAST;
>>       ioctl(tun_fd, TUNSETIFF, &req);
>>       ioctl(sock, SIOCSIFFLAGS, "gre0");
>>       write(tun_fd, "hi", 0);
>>       return 0;
>>     }
>> ==========================
>>
>> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
>> ---
>>  drivers/net/tun.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 3c9985f29950..25d96f54a729 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1496,6 +1496,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>         switch (tun->flags & TUN_TYPE_MASK) {
>>         case IFF_TUN:
>>                 if (tun->flags & IFF_NO_PI) {
>> +                       if (!skb->len)
>> +                               return -EINVAL;
>>                         switch (skb->data[0] & 0xf0) {
>>                         case 0x40:
>>                                 pi.proto = htons(ETH_P_IP);
>> --
>> 2.14.1.821.g8fa685d3b7-goog
>>
>
> Good catch, but...
>
> You are replacing an harmless read by a memory leak, which is far more
> problematic :/
Ah, you're right, I should have done kfree_skb() and incremented the
stats. That's also rx_dropped, correct?
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-- 
Alexander Potapenko
Software Engineer

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