From: "Kurt Manucredo" <fuzzybritches0@gmail.com>
To: ebiggers@kernel.org, daniel@iogearbox.net
Cc: ecree.xilinx@gmail.com,
syzbot+bed360704c521841c85d@syzkaller.appspotmail.com,
keescook@chromium.org, yhs@fb.com, dvyukov@google.com,
andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org,
davem@davemloft.net, hawk@kernel.org, john.fastabend@gmail.com,
kafai@fb.com, kpsingh@kernel.org, kuba@kernel.org,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
songliubraving@fb.com, syzkaller-bugs@googlegroups.com,
nathan@kernel.org, ndesaulniers@google.com,
clang-built-linux@googlegroups.com,
kernel-hardening@lists.openwall.com, kasan-dev@googlegroups.com
Subject: Re: [PATCH v5] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
Date: Wed, 16 Jun 2021 00:31:49 +0200 [thread overview]
Message-ID: <31138-26823-curtm@phaethon> (raw)
In-Reply-To: <YMkkr5G6E8lcFymG@gmail.com>
On Tue, 15 Jun 2021 15:07:43 -0700, Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Tue, Jun 15, 2021 at 11:54:41PM +0200, Daniel Borkmann wrote:
> > On 6/15/21 11:38 PM, Eric Biggers wrote:
> > > On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote:
> > > > On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote:
> > > > > On 6/15/21 9:33 PM, Eric Biggers wrote:
> > > > > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote:
> > > > > > >
> > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter,
> > > > > > > which is the *slow path* and indeed on many production systems is
> > > > > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON).
> > > > > > > Perhaps a better approach to the fix would be to change the interpreter
> > > > > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and
> > > > > > > bitnesses), thus matching the behaviour of most chips' shift opcodes.
> > > > > > > This would shut up UBSAN, without affecting JIT code generation.
> > > > > >
> > > > > > Yes, I suggested that last week
> > > > > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even
> > > > > > get optimized out when compiling for most CPUs.
> > > > >
> > > > > Did you check if the generated interpreter code for e.g. x86 is the same
> > > > > before/after with that?
> > > >
> > > > Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same
> > > > both before and after (with UBSAN disabled). Here is the patch I used:
> > > >
> > > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > > index 5e31ee9f7512..996db8a1bbfb 100644
> > > > --- a/kernel/bpf/core.c
> > > > +++ b/kernel/bpf/core.c
> > > > @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
> > > > DST = (u32) DST OP (u32) IMM; > > > CONT;
> > > > + /*
> > > > + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined
> > > > + * behavior. Normally this won't affect the generated code.
> >
> > The last one should probably be more specific in terms of 'normally', e.g. that
> > it is expected that the compiler is optimizing this away for archs like x86. Is
> > arm64 also covered by this ... do you happen to know on which archs this won't
> > be the case?
> >
> > Additionally, I think such comment should probably be more clear in that it also
> > needs to give proper guidance to JIT authors that look at the interpreter code to
> > see what they need to implement, in other words, that they don't end up copying
> > an explicit AND instruction emission if not needed there.
>
> Same result on arm64 with gcc 10.2.0.
>
> On arm32 it is different, probably because the 64-bit shifts aren't native in
> that case. I don't know about other architectures. But there aren't many ways
> to implement shifts, and using just the low bits of the shift amount is the most
> logical way.
>
> Please feel free to send out a patch with whatever comment you want. The diff I
> gave was just an example and I am not an expert in BPF.
>
> >
> > > > + */
> > > > +#define ALU_SHIFT(OPCODE, OP) > > > + ALU64_##OPCODE##_X: > > > + DST = DST OP (SRC & 63);> > > + CONT; > > > + ALU_##OPCODE##_X: > > > + DST = (u32) DST OP ((u32)SRC & 31); > > > + CONT; > > > + ALU64_##OPCODE##_K: > > > + DST = DST OP (IMM & 63); > > > + CONT; > > > + ALU_##OPCODE##_K: > > > + DST = (u32) DST OP ((u32)IMM & 31); > > > + CONT;
> >
> > For the *_K cases these are explicitly rejected by the verifier already. Is this
> > required here nevertheless to suppress UBSAN false positive?
> >
>
> No, I just didn't know that these constants are never out of range. Please feel
> free to send out a patch that does this properly.
>
The shift-out-of-bounds on syzbot happens in ALU_##OPCODE##_X only. To
pass the syzbot test, only ALU_##OPCODE##_X needs to be guarded.
This old patch I tested on syzbot puts a check in all four.
https://syzkaller.appspot.com/text?tag=Patch&x=11f8cacbd00000
https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
thanks,
kind regards
Kurt Manucredo
next prev parent reply other threads:[~2021-06-15 22:44 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-10 16:05 [syzbot] UBSAN: shift-out-of-bounds in ___bpf_prog_run syzbot
2021-03-28 3:38 ` syzbot
2021-06-02 21:27 ` [PATCH v3] bpf: core: fix " Kurt Manucredo
2021-06-03 4:43 ` Greg KH
2021-06-05 15:01 ` [PATCH v4] " Kurt Manucredo
2021-06-05 17:55 ` Yonghong Song
2021-06-05 19:10 ` Alexei Starovoitov
2021-06-05 21:39 ` Yonghong Song
2021-06-06 19:44 ` Kurt Manucredo
2021-06-07 7:38 ` Dmitry Vyukov
2021-06-09 18:20 ` Kees Cook
2021-06-09 23:40 ` Yonghong Song
2021-06-10 5:32 ` Dmitry Vyukov
2021-06-10 6:06 ` Yonghong Song
2021-06-10 17:06 ` Kees Cook
2021-06-10 17:52 ` Alexei Starovoitov
2021-06-10 20:00 ` Eric Biggers
2021-06-15 16:42 ` [PATCH v5] " Kurt Manucredo
2021-06-15 18:51 ` Edward Cree
2021-06-15 19:33 ` Eric Biggers
2021-06-15 21:08 ` Daniel Borkmann
2021-06-15 21:32 ` Eric Biggers
2021-06-15 21:38 ` Eric Biggers
2021-06-15 21:54 ` Daniel Borkmann
2021-06-15 22:07 ` Eric Biggers
2021-06-15 22:31 ` Kurt Manucredo [this message]
2021-06-17 10:09 ` Daniel Borkmann
2021-06-06 19:15 ` [PATCH v4] " Kurt Manucredo
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