On Fri, 2020-02-07 at 12:21:45 UTC, Michael Ellerman wrote:
> 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 1d8f739b07bd ("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-g4c25df5640ae #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: de78a9c42a79 ("powerpc: Add a framework for Kernel Userspace Access
> Protection")
> Cc: sta...@vger.kernel.org # v5.2+
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
Applied to powerpc fixes.
https://git.kernel.org/powerpc/c/9dc086f1e9ef39dd823bd27954b884b2062f9e70
cheers
