Re: KCSAN: data-race in glue_cbc_decrypt_req_128bit / glue_cbc_decrypt_req_128bit

[Eric Biggers]

On Tue, Mar 31, 2020 at 12:35:13PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    b12d66a6 mm, kcsan: Instrument SLAB free with ASSERT_EXCLU..
> git tree:       https://github.com/google/ktsan.git kcsan
> console output: https://syzkaller.appspot.com/x/log.txt?x=111f0865e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=10bc0131c4924ba9
> dashboard link: https://syzkaller.appspot.com/bug?extid=6a6bca8169ffda8ce77b
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+6a6bca8169ffda8ce77b@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> BUG: KCSAN: data-race in glue_cbc_decrypt_req_128bit / glue_cbc_decrypt_req_128bit
> 
> write to 0xffff88809966e128 of 8 bytes by task 24119 on cpu 0:
>  u128_xor include/crypto/b128ops.h:67 [inline]
>  glue_cbc_decrypt_req_128bit+0x396/0x460 arch/x86/crypto/glue_helper.c:144
>  cbc_decrypt+0x26/0x40 arch/x86/crypto/serpent_avx2_glue.c:152
>  crypto_skcipher_decrypt+0x65/0x90 crypto/skcipher.c:652
>  _skcipher_recvmsg crypto/algif_skcipher.c:142 [inline]
>  skcipher_recvmsg+0x7fa/0x8c0 crypto/algif_skcipher.c:161
>  skcipher_recvmsg_nokey+0x5e/0x80 crypto/algif_skcipher.c:279
>  sock_recvmsg_nosec net/socket.c:886 [inline]
>  sock_recvmsg net/socket.c:904 [inline]
>  sock_recvmsg+0x92/0xb0 net/socket.c:900
>  ____sys_recvmsg+0x167/0x3a0 net/socket.c:2566
>  ___sys_recvmsg+0xb2/0x100 net/socket.c:2608
>  __sys_recvmsg+0x9d/0x160 net/socket.c:2642
>  __do_sys_recvmsg net/socket.c:2652 [inline]
>  __se_sys_recvmsg net/socket.c:2649 [inline]
>  __x64_sys_recvmsg+0x51/0x70 net/socket.c:2649
>  do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> read to 0xffff88809966e128 of 8 bytes by task 24118 on cpu 1:
>  u128_xor include/crypto/b128ops.h:67 [inline]
>  glue_cbc_decrypt_req_128bit+0x37c/0x460 arch/x86/crypto/glue_helper.c:144
>  cbc_decrypt+0x26/0x40 arch/x86/crypto/serpent_avx2_glue.c:152
>  crypto_skcipher_decrypt+0x65/0x90 crypto/skcipher.c:652
>  _skcipher_recvmsg crypto/algif_skcipher.c:142 [inline]
>  skcipher_recvmsg+0x7fa/0x8c0 crypto/algif_skcipher.c:161
>  skcipher_recvmsg_nokey+0x5e/0x80 crypto/algif_skcipher.c:279
>  sock_recvmsg_nosec net/socket.c:886 [inline]
>  sock_recvmsg net/socket.c:904 [inline]
>  sock_recvmsg+0x92/0xb0 net/socket.c:900
>  ____sys_recvmsg+0x167/0x3a0 net/socket.c:2566
>  ___sys_recvmsg+0xb2/0x100 net/socket.c:2608
>  __sys_recvmsg+0x9d/0x160 net/socket.c:2642
>  __do_sys_recvmsg net/socket.c:2652 [inline]
>  __se_sys_recvmsg net/socket.c:2649 [inline]
>  __x64_sys_recvmsg+0x51/0x70 net/socket.c:2649
>  do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 24118 Comm: syz-executor.1 Not tainted 5.6.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> ==================================================================
> 

I think this is a problem for almost all the crypto code.  Due to AF_ALG, both
the source and destination buffers can be userspace pages that were gotten with
get_user_pages().  Such pages can be concurrently modified, not just by the
kernel but also by userspace.

I'm not sure what can be done about this.

- Eric