Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2 bpf] bpf: introduce BPF_JIT_ALWAYS_ON config | From | Daniel Borkmann <> | Date | Tue, 9 Jan 2018 17:29:13 +0100 |
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On 01/09/2018 05:52 AM, Alexei Starovoitov wrote: > The BPF interpreter has been used as part of the spectre 2 attack CVE-2017-5715. > > A quote from goolge project zero blog: > "At this point, it would normally be necessary to locate gadgets in > the host kernel code that can be used to actually leak data by reading > from an attacker-controlled location, shifting and masking the result > appropriately and then using the result of that as offset to an > attacker-controlled address for a load. But piecing gadgets together > and figuring out which ones work in a speculation context seems annoying. > So instead, we decided to use the eBPF interpreter, which is built into > the host kernel - while there is no legitimate way to invoke it from inside > a VM, the presence of the code in the host kernel's text section is sufficient > to make it usable for the attack, just like with ordinary ROP gadgets." > > To make attacker job harder introduce BPF_JIT_ALWAYS_ON config > option that removes interpreter from the kernel in favor of JIT-only mode. > So far eBPF JIT is supported by: > x64, arm64, arm32, sparc64, s390, powerpc64, mips64 > > The start of JITed program is randomized and code page is marked as read-only. > In addition "constant blinding" can be turned on with net.core.bpf_jit_harden > > v1->v2: > - fix init order, test_bpf and cBPF (Daniel's feedback) > - fix offloaded bpf (Jakub's feedback) > - add 'return 0' dummy in case something can invoke prog->bpf_func > - retarget bpf tree. For bpf-next the patch would need one extra hunk. > It will be sent when the trees are merged back to net-next > > Considered doing: > int bpf_jit_enable __read_mostly = BPF_EBPF_JIT_DEFAULT; > but it seems better to land the patch as-is and in bpf-next remove > bpf_jit_enable global variable from all JITs, consolidate in one place > and remove this jit_init() function.
Ok, makes sense.
[...]
Still one minor thing left:
> @@ -1354,6 +1357,12 @@ static int bpf_check_tail_call(const struct bpf_prog *fp) > return 0; > } > > +static unsigned int __bpf_prog_ret0(const void *ctx, > + const struct bpf_insn *insn) > +{ > + return 0; > +}
When CONFIG_BPF_JIT_ALWAYS_ON is disabled, this will throw the following warning:
[...] CC kernel/bpf/core.o kernel/bpf/core.c:1360:21: warning: ‘__bpf_prog_ret0’ defined but not used [-Wunused-function] static unsigned int __bpf_prog_ret0(const void *ctx, ^~~~~~~~~~~~~~~
Probably just best to wrap it under ifdef CONFIG_BPF_JIT_ALWAYS_ON.
> /** > * bpf_prog_select_runtime - select exec runtime for BPF program > * @fp: bpf_prog populated with internal BPF program > @@ -1364,9 +1373,13 @@ static int bpf_check_tail_call(const struct bpf_prog *fp) > */ > struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) > { > +#ifndef CONFIG_BPF_JIT_ALWAYS_ON > u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1); > > fp->bpf_func = interpreters[(round_up(stack_depth, 32) / 32) - 1]; > +#else > + fp->bpf_func = __bpf_prog_ret0; > +#endif > > /* eBPF JITs can rewrite the program in case constant > * blinding is active. However, in case of error during > @@ -1376,6 +1389,12 @@ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) > */ > if (!bpf_prog_is_dev_bound(fp->aux)) { > fp = bpf_int_jit_compile(fp); > +#ifdef CONFIG_BPF_JIT_ALWAYS_ON > + if (!fp->jited) { > + *err = -ENOTSUPP; > + return fp; > + } > +#endif > } else { > *err = bpf_prog_offload_compile(fp); > if (*err)
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