linux-kernel - bpf: undefined shift in __bpf_prog

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Message-ID: <CACT4Y+ZxoR3UjLgcNdUm4fECLMx2VdtfrENMtRRCdgHB2n0bJA@mail.gmail.com>
Date:	Fri, 4 Dec 2015 12:17:08 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Alexei Starovoitov <ast@...nel.org>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: bpf: undefined shift in __bpf_prog_run

Hello,

UBSAN reports the following undefined behavior:

UBSAN: Undefined behaviour in kernel/bpf/core.c:336:2
shift exponent 2835 is to large for 32-bit type 'unsigned int'
CPU: 1 PID: 14227 Comm: syzkaller_execu Not tainted 4.4.0-rc3+ #142
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 0000000000000001 ffff88003892f898 ffffffff82c747b8 0000000041b58ab3
 ffffffff878cbc05 ffffffff82c74706 ffff88003892f860 ffff88003892f9a0
 0000000000000000 0000000000000b13 ffffffff88178de2 0000000000000020
Call Trace:
 [<ffffffff82d684f0>] __ubsan_handle_shift_out_of_bounds+0x294/0x2e5
lib/ubsan.c:417
 [<ffffffff8160a408>] __bpf_prog_run+0x8f48/0x9ac0 kernel/bpf/core.c:336
 [<     inline     >] seccomp_run_filters kernel/seccomp.c:198
 [<     inline     >] __seccomp_phase1_filter kernel/seccomp.c:588
 [<ffffffff8156ddfb>] seccomp_phase1+0x1cb/0x990 kernel/seccomp.c:667
 [<ffffffff8100651f>] syscall_trace_enter_phase1+0x28f/0x4e0
arch/x86/entry/common.c:132
 [<ffffffff8691b939>] tracesys+0xd/0x44 arch/x86/entry/entry_64.S:240

On commit 31ade3b83e1821da5fbb2f11b5b3d4ab2ec39db8.

Such shifts have undefined behavior according to C standard and behave
differently on different archs. I guess we don't want to rely on any
kind of undefined behavior in bpf/seccomp. And generally want to
completely define results of all operations in bpf.
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