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

[Eric Dumazet]

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 :/