Re: KCSAN: data-race in glue_cbc_decrypt_req_128bit / glue_cbc_decrypt_req_128bit

[Dmitry Vyukov]

On Tue, Mar 31, 2020 at 10:27 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> 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.

Oh, I thought it's something more serious like a shared crypto object.
Thanks for debugging.
I think I've seen this before in another context (b/149818448):

BUG: KCSAN: data-race in copyin / copyin

write to 0xffff888103c8b000 of 4096 bytes by task 20917 on cpu 0:
 instrument_copy_from_user include/linux/instrumented.h:106 [inline]
 copyin+0xab/0xc0 lib/iov_iter.c:151
 copy_page_from_iter_iovec lib/iov_iter.c:296 [inline]
 copy_page_from_iter+0x23f/0x5f0 lib/iov_iter.c:942
 process_vm_rw_pages mm/process_vm_access.c:46 [inline]
 process_vm_rw_single_vec mm/process_vm_access.c:120 [inline]
 process_vm_rw_core.isra.0+0x448/0x820 mm/process_vm_access.c:218
 process_vm_rw+0x1c4/0x1e0 mm/process_vm_access.c:286
 __do_sys_process_vm_writev mm/process_vm_access.c:308 [inline]
 __se_sys_process_vm_writev mm/process_vm_access.c:303 [inline]
 __x64_sys_process_vm_writev+0x8b/0xb0 mm/process_vm_access.c:303
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888103c8b000 of 4096 bytes by task 20918 on cpu 1:
 instrument_copy_from_user include/linux/instrumented.h:106 [inline]
 copyin+0xab/0xc0 lib/iov_iter.c:151
 copy_page_from_iter_iovec lib/iov_iter.c:296 [inline]
 copy_page_from_iter+0x23f/0x5f0 lib/iov_iter.c:942
 process_vm_rw_pages mm/process_vm_access.c:46 [inline]
 process_vm_rw_single_vec mm/process_vm_access.c:120 [inline]
 process_vm_rw_core.isra.0+0x448/0x820 mm/process_vm_access.c:218
 process_vm_rw+0x1c4/0x1e0 mm/process_vm_access.c:286
 __do_sys_process_vm_writev mm/process_vm_access.c:308 [inline]
 __se_sys_process_vm_writev mm/process_vm_access.c:303 [inline]
 __x64_sys_process_vm_writev+0x8b/0xb0 mm/process_vm_access.c:303
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9


Marco, I think we need to ignore all memory that comes from
get_user_pages() somehow. Either not set watchpoints at all, or
perhaps filter them out later if the check is not totally free.