[PATCH] powerpc/futex: Fix incorrect user access blocking

[Christophe Leroy]

Le 07/02/2020 à 13:21, Michael Ellerman a écrit :
> 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: stable at vger.kernel.org # v5.2+
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Reviewed-by: Christophe Leroy <christophe.leroy at c-s.fr>

> ---
>   arch/powerpc/include/asm/futex.h | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/futex.h b/arch/powerpc/include/asm/futex.h
> index eea28ca679db..bc7d9d06a6d9 100644
> --- a/arch/powerpc/include/asm/futex.h
> +++ b/arch/powerpc/include/asm/futex.h
> @@ -35,7 +35,7 @@ static inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
>   {
>   	int oldval = 0, ret;
>   
> -	allow_write_to_user(uaddr, sizeof(*uaddr));
> +	allow_read_write_user(uaddr, uaddr, sizeof(*uaddr));
>   	pagefault_disable();
>   
>   	switch (op) {
> @@ -62,7 +62,7 @@ static inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
>   
>   	*oval = oldval;
>   
> -	prevent_write_to_user(uaddr, sizeof(*uaddr));
> +	prevent_read_write_user(uaddr, uaddr, sizeof(*uaddr));
>   	return ret;
>   }
>   
> @@ -76,7 +76,8 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
>   	if (!access_ok(uaddr, sizeof(u32)))
>   		return -EFAULT;
>   
> -	allow_write_to_user(uaddr, sizeof(*uaddr));
> +	allow_read_write_user(uaddr, uaddr, sizeof(*uaddr));
> +
>           __asm__ __volatile__ (
>           PPC_ATOMIC_ENTRY_BARRIER
>   "1:     lwarx   %1,0,%3         # futex_atomic_cmpxchg_inatomic\n\
> @@ -97,7 +98,8 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
>           : "cc", "memory");
>   
>   	*uval = prev;
> -	prevent_write_to_user(uaddr, sizeof(*uaddr));
> +	prevent_read_write_user(uaddr, uaddr, sizeof(*uaddr));
> +
>           return ret;
>   }
>   
>