1. 08 Feb, 2020 1 commit
    • Michael Ellerman's avatar
      powerpc/futex: Fix incorrect user access blocking · 9dc086f1
      Michael Ellerman authored
      The early versions of our kernel user access prevention (KUAP) were
      written by Russell and Christophe, and didn't have separate
      read/write access.
      
      At some point I picked up the series and added the read/write access,
      but I failed to update the usages in futex.h to correctly allow read
      and write.
      
      However we didn't notice because of another bug which was causing the
      low-level code to always enable read and write. That bug was fixed
      recently in commit 1d8f739b ("powerpc/kuap: Fix set direction in
      allow/prevent_user_access()").
      
      futex_atomic_cmpxchg_inatomic() is passed the user address as %3 and
      does:
      
        1:     lwarx   %1,  0, %3
               cmpw    0,  %1, %4
               bne-    3f
        2:     stwcx.  %5,  0, %3
      
      Which clearly loads and stores from/to %3. The logic in
      arch_futex_atomic_op_inuser() is similar, so fix both of them to use
      allow_read_write_user().
      
      Without this fix, and with PPC_KUAP_DEBUG=y, we see eg:
      
        Bug: Read fault blocked by AMR!
        WARNING: CPU: 94 PID: 149215 at arch/powerpc/include/asm/book3s/64/kup-radix.h:126 __do_page_fault+0x600/0xf30
        CPU: 94 PID: 149215 Comm: futex_requeue_p Tainted: G        W         5.5.0-rc7-gcc9x-g4c25df56 #1
        ...
        NIP [c000000000070680] __do_page_fault+0x600/0xf30
        LR [c00000000007067c] __do_page_fault+0x5fc/0xf30
        Call Trace:
        [c00020138e5637e0] [c00000000007067c] __do_page_fault+0x5fc/0xf30 (unreliable)
        [c00020138e5638c0] [c00000000000ada8] handle_page_fault+0x10/0x30
        --- interrupt: 301 at cmpxchg_futex_value_locked+0x68/0xd0
            LR = futex_lock_pi_atomic+0xe0/0x1f0
        [c00020138e563bc0] [c000000000217b50] futex_lock_pi_atomic+0x80/0x1f0 (unreliable)
        [c00020138e563c30] [c00000000021b668] futex_requeue+0x438/0xb60
        [c00020138e563d60] [c00000000021c6cc] do_futex+0x1ec/0x2b0
        [c00020138e563d90] [c00000000021c8b8] sys_futex+0x128/0x200
        [c00020138e563e20] [c00000000000b7ac] system_call+0x5c/0x68
      
      Fixes: de78a9c4 ("powerpc: Add a framework for Kernel Userspace Access Protection")
      Cc: stable@vger.kernel.org # v5.2+
      Reported-by: syzbot+e808452bad7c375cbee6@syzkaller-ppc64.appspotmail.com
      Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
      Reviewed-by: default avatarChristophe Leroy <christophe.leroy@c-s.fr>
      Link: https://lore.kernel.org/r/20200207122145.11928-1-mpe@ellerman.id.au
      9dc086f1
  2. 04 Feb, 2020 39 commits