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 --- arch/arm64/include/asm/futex.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h index f2585cdd32c2..20dcb196b240 100644 --- a/arch/arm64/include/asm/futex.h +++ b/arch/arm64/include/asm/futex.h @@ -51,16 +51,16 @@ : "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;