From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Michael Ellerman <mpe@ellerman.id.au>, linuxppc-dev@ozlabs.org
Cc: dja@axtens.net
Subject: Re: [PATCH] powerpc/futex: Fix incorrect user access blocking
Date: Fri, 7 Feb 2020 17:03:39 +0100 [thread overview]
Message-ID: <e5d2e152-21af-cc25-c3d2-057661d3e329@c-s.fr> (raw)
In-Reply-To: <20200207122145.11928-1-mpe@ellerman.id.au>
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@vger.kernel.org # v5.2+
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christophe Leroy <christophe.leroy@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;
> }
>
>
next prev parent reply other threads:[~2020-02-07 16:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-07 12:21 [PATCH] powerpc/futex: Fix incorrect user access blocking Michael Ellerman
2020-02-07 12:30 ` Daniel Axtens
2020-02-07 16:03 ` Christophe Leroy [this message]
2020-02-08 12:50 ` Michael Ellerman
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