Commit 5f16a046 authored by Will Deacon's avatar Will Deacon

arm64: futex: Fix undefined behaviour with FUTEX_OP_OPARG_SHIFT usage

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

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: default avatarRobin Murphy <>
Signed-off-by: default avatarWill Deacon <>
parent 690e95dd
......@@ -48,16 +48,16 @@ do { \
} while (0)
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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