From: Eric Biggers <ebiggers@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Edward Cree <ecree.xilinx@gmail.com>,
Kurt Manucredo <fuzzybritches0@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: Tue, 15 Jun 2021 14:38:15 -0700 [thread overview]
Message-ID: <YMkdx1VB0i+fhjAY@gmail.com> (raw)
In-Reply-To: <YMkcYn4dyZBY/ze+@gmail.com>
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.
> + */
> +#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;
> +
> ALU(ADD, +)
> ALU(SUB, -)
> ALU(AND, &)
> ALU(OR, |)
> - ALU(LSH, <<)
> - ALU(RSH, >>)
> + ALU_SHIFT(LSH, <<)
> + ALU_SHIFT(RSH, >>)
> ALU(XOR, ^)
> ALU(MUL, *)
> #undef ALU
>
Note, I missed the arithmetic right shifts later on in the function. Same
result there, though.
- Eric
next prev parent reply other threads:[~2021-06-15 21:38 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <000000000000c2987605be907e41@google.com>
[not found] ` <20210602212726.7-1-fuzzybritches0@gmail.com>
[not found] ` <YLhd8BL3HGItbXmx@kroah.com>
[not found] ` <87609-531187-curtm@phaethon>
[not found] ` <6a392b66-6f26-4532-d25f-6b09770ce366@fb.com>
[not found] ` <CAADnVQKexxZQw0yK_7rmFOdaYabaFpi2EmF6RGs5bXvFHtUQaA@mail.gmail.com>
2021-06-07 7:38 ` [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run 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 [this message]
2021-06-15 21:54 ` Daniel Borkmann
2021-06-15 22:07 ` Eric Biggers
2021-06-15 22:31 ` Kurt Manucredo
2021-06-17 10:09 ` Daniel Borkmann
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