RE: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Reshetova, Elena
2018-01-04T09:16:33Z

> On Thu, Jan 04, 2018 at 02:15:53AM +0000, Alan Cox wrote:
> >
> > > > Elena has done the work of auditing static analysis reports to a dozen
> > > > or so locations that need some 'nospec' handling.
> > >
> > > How exactly is that related (especially in longer-term support terms) to
> > > BPF anyway?
> >
> > If you read the papers you need a very specific construct in order to not
> > only cause a speculative load of an address you choose but also to then
> > manage to cause a second operation that in some way reveals bits of data
> > or allows you to ask questions.
> >
> > BPF allows you to construct those sequences relatively easily and it's
> > the one case where a user space application can fairly easily place code
> > it wants to execute in the kernel. Without BPF you have to find the right
> > construct in the kernel, prime all the right predictions and measure the
> > result without getting killed off. There are places you can do that but
> > they are not so easy and we don't (at this point) think there are that
> > many.
>
> for BPF in particular we're thinking to do a different fix.
> Instead of killing speculation we can let cpu speculate.
> The fix will include rounding up bpf maps to nearest power of 2 and
> inserting bpf_and operation on the index after bounds check,
> so cpu can continue speculate beyond bounds check, but it will
> load from zero-ed memory.
> So this nospec arch dependent hack won't be necessary for bpf side,
> but may still be needed in other parts of the kernel.

I think this is a much better approach than what we have been doing internally so
far. My initial fix back in August for this was to insert a minimal set of lfence
barriers (osb() barrier below) that prevent speculation just based on how attack was using constants
to index eBPF maps:

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
#define BPF_R0 regs[BPF_REG_0]
@@ -939,6 +940,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
DST = IMM;
CONT;
LD_IMM_DW:
+ osb();
DST = (u64) (u32) insn[0].imm | ((u64) (u32) insn[1].imm) << 32;
insn++;
CONT;
@@ -1200,6 +1202,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
*(SIZE *)(unsigned long) (DST + insn->off) = IMM; \
CONT; \
LDX_MEM_##SIZEOP: \
+ osb(); \
DST = *(SIZE *)(unsigned long) (SRC + insn->off); \
CONT;

And similar stuff also for x86 JIT (blinding dependent):

@@ -400,7 +413,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
case BPF_ADD: b2 = 0x01; break;
case BPF_SUB: b2 = 0x29; break;
case BPF_AND: b2 = 0x21; break;
- case BPF_OR: b2 = 0x09; break;
+ case BPF_OR: b2 = 0x09; emit_memory_barrier(&prog); break;
case BPF_XOR: b2 = 0x31; break;
}
if (BPF_CLASS(insn->code) == BPF_ALU64)
@@ -647,6 +660,16 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
case BPF_ALU64 | BPF_RSH | BPF_X:
case BPF_ALU64 | BPF_ARSH | BPF_X:

+ /* If blinding is enabled, each
+ * BPF_LD | BPF_IMM | BPF_DW instruction
+ * is converted to 4 eBPF instructions with
+ * BPF_ALU64_IMM(BPF_LSH, BPF_REG_AX, 32)
+ * always present(number 3). Detect such cases
+ * and insert memory barriers. */
+ if ((BPF_CLASS(insn->code) == BPF_ALU64)
+ && (BPF_OP(insn->code) == BPF_LSH)
+ && (src_reg == BPF_REG_AX))
+ emit_memory_barrier(&prog);
/* check for bad case when dst_reg == rcx */
if (dst_reg == BPF_REG_4) {
/* mov r11, dst_reg */

But this is really ugly fix IMO to prevent speculation from happen, so with your
approach I think it is really much better.

If you need help in testing the fixes, I can do it unless you already have the
attack code yourself.

>
> Also note that variant 1 is talking about exploiting prog_array
> bpf feature which had 64-bit access prior to
> commit 90caccdd8cc0 ("bpf: fix bpf_tail_call() x64 JIT")
> That was a fix for JIT and not related to this cpu issue, but
> I believe it breaks the existing exploit.

Actually I could not get existing exploit working on anything past 4.11
but at that time I could not see any fundamental change in eBPF that
would prevent it.

Best Regards,
Elena.