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Subject[PATCH 4.4 099/190] arm64: futex: Fix undefined behaviour with FUTEX_OP_OPARG_SHIFT usage
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@arm.com>


[ Upstream commit 5f16a046f8e144c294ef98cd29d9458b5f8273e5 ]

FUTEX_OP_OPARG_SHIFT instructs the futex code to treat the 12-bit oparg
field as a shift value, potentially leading to a left shift value that
is negative or with an absolute value that is significantly larger then
the size of the type. UBSAN chokes with:

================================================================================
UBSAN: Undefined behaviour in ./arch/arm64/include/asm/futex.h:60:13
shift exponent -1 is negative
CPU: 1 PID: 1449 Comm: syz-executor0 Not tainted 4.11.0-rc4-00005-g977eb52-dirty #11
Hardware name: linux,dummy-virt (DT)
Call trace:
[<ffff200008094778>] dump_backtrace+0x0/0x538 arch/arm64/kernel/traps.c:73
[<ffff200008094cd0>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:228
[<ffff200008c194a8>] __dump_stack lib/dump_stack.c:16 [inline]
[<ffff200008c194a8>] dump_stack+0x120/0x188 lib/dump_stack.c:52
[<ffff200008cc24b8>] ubsan_epilogue+0x18/0x98 lib/ubsan.c:164
[<ffff200008cc3098>] __ubsan_handle_shift_out_of_bounds+0x250/0x294 lib/ubsan.c:421
[<ffff20000832002c>] futex_atomic_op_inuser arch/arm64/include/asm/futex.h:60 [inline]
[<ffff20000832002c>] futex_wake_op kernel/futex.c:1489 [inline]
[<ffff20000832002c>] do_futex+0x137c/0x1740 kernel/futex.c:3231
[<ffff200008320504>] SYSC_futex kernel/futex.c:3281 [inline]
[<ffff200008320504>] SyS_futex+0x114/0x268 kernel/futex.c:3249
[<ffff200008084770>] el0_svc_naked+0x24/0x28
================================================================================
syz-executor1 uses obsolete (PF_INET,SOCK_PACKET)
sock: process `syz-executor0' is using obsolete setsockopt SO_BSDCOMPAT

This patch attempts to fix some of this by:

* Making encoded_op an unsigned type, so we can shift it left even if
the top bit is set.

* Casting to signed prior to shifting right when extracting oparg
and cmparg

* Consider only the bottom 5 bits of oparg when using it as a left-shift
value.

Whilst I think this catches all of the issues, I'd much prefer to remove
this stuff, as I think it's unused and the bugs are copy-pasted between
a bunch of architectures.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/include/asm/futex.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -53,16 +53,16 @@
: "memory")

static inline int
-futex_atomic_op_inuser (int encoded_op, u32 __user *uaddr)
+futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
{
int op = (encoded_op >> 28) & 7;
int cmp = (encoded_op >> 24) & 15;
- int oparg = (encoded_op << 8) >> 20;
- int cmparg = (encoded_op << 20) >> 20;
+ int oparg = (int)(encoded_op << 8) >> 20;
+ int cmparg = (int)(encoded_op << 20) >> 20;
int oldval = 0, ret, tmp;

if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
- oparg = 1 << oparg;
+ oparg = 1U << (oparg & 0x1f);

if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
return -EFAULT;

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