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Daniel Borkmann 2e66c36f13 bpf: Fix up register-based shifts in interpreter to silence KUBSAN [ Upstream commit `28131e9d93` ] syzbot reported a shift-out-of-bounds that KUBSAN observed in the interpreter: [...] UBSAN: shift-out-of-bounds in kernel/bpf/core.c:1420:2 shift exponent 255 is too large for 64-bit type 'long long unsigned int' CPU: 1 PID: 11097 Comm: syz-executor.4 Not tainted 5.12.0-rc2-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:79 [inline] dump_stack+0x141/0x1d7 lib/dump_stack.c:120 ubsan_epilogue+0xb/0x5a lib/ubsan.c:148 __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:327 ___bpf_prog_run.cold+0x19/0x56c kernel/bpf/core.c:1420 __bpf_prog_run32+0x8f/0xd0 kernel/bpf/core.c:1735 bpf_dispatcher_nop_func include/linux/bpf.h:644 [inline] bpf_prog_run_pin_on_cpu include/linux/filter.h:624 [inline] bpf_prog_run_clear_cb include/linux/filter.h:755 [inline] run_filter+0x1a1/0x470 net/packet/af_packet.c:2031 packet_rcv+0x313/0x13e0 net/packet/af_packet.c:2104 dev_queue_xmit_nit+0x7c2/0xa90 net/core/dev.c:2387 xmit_one net/core/dev.c:3588 [inline] dev_hard_start_xmit+0xad/0x920 net/core/dev.c:3609 __dev_queue_xmit+0x2121/0x2e00 net/core/dev.c:4182 __bpf_tx_skb net/core/filter.c:2116 [inline] __bpf_redirect_no_mac net/core/filter.c:2141 [inline] __bpf_redirect+0x548/0xc80 net/core/filter.c:2164 ____bpf_clone_redirect net/core/filter.c:2448 [inline] bpf_clone_redirect+0x2ae/0x420 net/core/filter.c:2420 ___bpf_prog_run+0x34e1/0x77d0 kernel/bpf/core.c:1523 __bpf_prog_run512+0x99/0xe0 kernel/bpf/core.c:1737 bpf_dispatcher_nop_func include/linux/bpf.h:644 [inline] bpf_test_run+0x3ed/0xc50 net/bpf/test_run.c:50 bpf_prog_test_run_skb+0xabc/0x1c50 net/bpf/test_run.c:582 bpf_prog_test_run kernel/bpf/syscall.c:3127 [inline] __do_sys_bpf+0x1ea9/0x4f00 kernel/bpf/syscall.c:4406 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46 entry_SYSCALL_64_after_hwframe+0x44/0xae [...] Generally speaking, KUBSAN reports from the kernel should be fixed. However, in case of BPF, this particular report caused concerns since the large shift is not wrong from BPF point of view, just undefined. In the verifier, K-based shifts that are >= {64,32} (depending on the bitwidth of the instruction) are already rejected. The register-based cases were not given their content might not be known at verification time. Ideas such as verifier instruction rewrite with an additional AND instruction for the source register were brought up, but regularly rejected due to the additional runtime overhead they incur. As Edward Cree rightly put it: Shifts by more than insn bitness are legal in the BPF ISA; they are implementation-defined behaviour [of the underlying architecture], rather than UB, and have been made legal for performance reasons. Each of the JIT backends compiles the BPF shift operations to machine instructions which produce implementation-defined results in such a case; the resulting contents of the register may be arbitrary but program behaviour as a whole remains defined. Guard checks in the fast path (i.e. affecting JITted code) will thus not be accepted. The case of division by zero is not truly analogous here, as division instructions on many of the JIT-targeted architectures will raise a machine exception / fault on division by zero, whereas (to the best of my knowledge) none will do so on an out-of-bounds shift. Given the KUBSAN report only affects the BPF interpreter, but not JITs, one solution is to add the ANDs with 63 or 31 into ___bpf_prog_run(). That would make the shifts defined, and thus shuts up KUBSAN, and the compiler would optimize out the AND on any CPU that interprets the shift amounts modulo the width anyway (e.g., confirmed from disassembly that on x86-64 and arm64 the generated interpreter code is the same before and after this fix). The BPF interpreter is slow path, and most likely compiled out anyway as distros select BPF_JIT_ALWAYS_ON to avoid speculative execution of BPF instructions by the interpreter. Given the main argument was to avoid sacrificing performance, the fact that the AND is optimized away from compiler for mainstream archs helps as well as a solution moving forward. Also add a comment on LSH/RSH/ARSH translation for JIT authors to provide guidance when they see the ___bpf_prog_run() interpreter code and use it as a model for a new JIT backend. Reported-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com Reported-by: Kurt Manucredo <fuzzybritches0@gmail.com> Signed-off-by: Eric Biggers <ebiggers@kernel.org> Co-developed-by: Eric Biggers <ebiggers@kernel.org> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Andrii Nakryiko <andrii@kernel.org> Tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com Cc: Edward Cree <ecree.xilinx@gmail.com> Link: https://lore.kernel.org/bpf/0000000000008f912605bd30d5d7@google.com Link: https://lore.kernel.org/bpf/bac16d8d-c174-bdc4-91bd-bfa62b410190@gmail.com Signed-off-by: Sasha Levin <sashal@kernel.org>		2021-07-19 09:44:50 +02:00
arch	MIPS: ingenic: Select CPU_SUPPORTS_CPUFREQ && MIPS_EXTERNAL_TIMER	2021-07-19 09:44:44 +02:00
block	blk-mq: update hctx->dispatch_busy in case of real scheduler	2021-07-14 16:56:13 +02:00
certs	certs: Add ability to preload revocation certs	2021-06-30 08:47:30 -04:00
crypto	crypto: sm2 - fix a memory leak in sm2	2021-07-14 16:56:06 +02:00
Documentation	powerpc/papr_scm: Make 'perf_stats' invisible if perf-stats unavailable	2021-07-14 16:56:49 +02:00
drivers	drm/amdkfd: Fix circular lock in nocpsch path	2021-07-19 09:44:50 +02:00
fs	udf: Fix NULL pointer dereference in udf_symlink function	2021-07-19 09:44:40 +02:00
include	net: mdio: provide shim implementation of devm_of_mdiobus_register	2021-07-19 09:44:39 +02:00
init	sched/core: Initialize the idle task with preemption disabled	2021-07-14 16:55:50 +02:00
ipc	ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry	2021-05-26 12:06:54 +02:00
kernel	bpf: Fix up register-based shifts in interpreter to silence KUBSAN	2021-07-19 09:44:50 +02:00
lib	lib/math/rational.c: fix divide by zero	2021-07-14 16:56:51 +02:00
LICENSES	LICENSES/deprecated: add Zlib license text	2020-09-16 14:33:49 +02:00
mm	mm/z3fold: use release_z3fold_page_locked() to release locked z3fold page	2021-07-14 16:56:51 +02:00
net	xfrm: Fix error reporting in xfrm_state_construct.	2021-07-19 09:44:49 +02:00
samples	samples/bpf: Fix the error return code of xdp_redirect's main()	2021-07-14 16:56:23 +02:00
scripts	kbuild: Fix objtool dependency for 'OBJECT_FILES_NON_STANDARD_<obj> := n'	2021-07-14 16:56:04 +02:00
security	selinux: use __GFP_NOWARN with GFP_NOWAIT in the AVC	2021-07-19 09:44:49 +02:00
sound	ALSA: firewire-lib: Fix 'amdtp_domain_start()' when no AMDTP_OUT_STREAM stream is found	2021-07-14 16:56:49 +02:00
tools	selftests: Clean forgotten resources as part of cleanup()	2021-07-19 09:44:49 +02:00
usr	Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs	2020-08-07 13:29:39 -07:00
virt	KVM: do not allow mapping valid but non-reference-counted pages	2021-06-30 08:47:25 -04:00
.clang-format	RDMA 5.10 pull request	2020-10-17 11:18:18 -07:00
.cocciconfig
.get_maintainer.ignore	Opt out of scripts/get_maintainer.pl	2019-05-16 10:53:40 -07:00
.gitattributes	.gitattributes: use 'dts' diff driver for dts files	2019-12-04 19:44:11 -08:00
.gitignore	kbuild: generate Module.symvers only when vmlinux exists	2021-05-19 10:12:59 +02:00
.mailmap	mailmap: add two more addresses of Uwe Kleine-König	2020-12-06 10:19:07 -08:00
COPYING	COPYING: state that all contributions really are covered by this file	2020-02-10 13:32:20 -08:00
CREDITS	MAINTAINERS: Move Jason Cooper to CREDITS	2020-11-30 10:20:34 +01:00
Kbuild	kbuild: rename hostprogs-y/always to hostprogs/always-y	2020-02-04 01:53:07 +09:00
Kconfig	kbuild: ensure full rebuild when the compiler is updated	2020-05-12 13:28:33 +09:00
MAINTAINERS	f2fs: move ioctl interface definitions to separated file	2021-05-19 10:13:00 +02:00
Makefile	Linux 5.10.50	2021-07-14 16:56:55 +02:00
README	Drop all 00-INDEX files from Documentation/	2018-09-09 15:08:58 -06:00

README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.