Re: [PATCH] powerpc/futex: Fix incorrect user access blocking
From: Christophe Leroy <hidden>
Date: 2020-02-07 16:05:20
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 <redacted>
quoted hunk ↗ jump to hunk
--- 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; }