With CONFIG_BPF_JIT, the kernel makes indirect calls to dynamically
generated code. This change adds basic sanity checking to ensure
we are jumping to a valid location, which narrows down the attack
surface on the stored pointer. This also prepares the code for future
Control-Flow Integrity (CFI) checking, which adds indirect call
validation to call targets that can be determined at compile-time, but
cannot validate calls to jited functions.
In addition, this change adds a weak arch_bpf_jit_check_func function,
which architectures that implement BPF JIT can override to perform
additional validation, such as verifying that the pointer points to
the correct memory region.
Signed-off-by: Sami Tolvanen <[email protected]>
---
include/linux/filter.h | 26 ++++++++++++++++++++++++--
kernel/bpf/core.c | 25 +++++++++++++++++++++++++
2 files changed, 49 insertions(+), 2 deletions(-)
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 92c6e31fb008..abfb0e1b21a8 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -511,7 +511,10 @@ struct sock_fprog_kern {
struct sock_filter *filter;
};
+#define BPF_BINARY_HEADER_MAGIC 0x05de0e82
+
struct bpf_binary_header {
+ u32 magic;
u32 pages;
/* Some arches need word alignment for their instructions */
u8 image[] __aligned(4);
@@ -553,20 +556,39 @@ struct sk_filter {
DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
+#ifdef CONFIG_BPF_JIT
+/*
+ * With JIT, the kernel makes an indirect call to dynamically generated
+ * code. Use bpf_call_func to perform additional validation of the call
+ * target to narrow down attack surface. Architectures implementing BPF
+ * JIT can override arch_bpf_jit_check_func for arch-specific checking.
+ */
+extern unsigned int bpf_call_func(const struct bpf_prog *prog,
+ const void *ctx);
+
+extern bool arch_bpf_jit_check_func(const struct bpf_prog *prog);
+#else
+static inline unsigned int bpf_call_func(const struct bpf_prog *prog,
+ const void *ctx)
+{
+ return prog->bpf_func(ctx, prog->insnsi);
+}
+#endif
+
#define BPF_PROG_RUN(prog, ctx) ({ \
u32 ret; \
cant_sleep(); \
if (static_branch_unlikely(&bpf_stats_enabled_key)) { \
struct bpf_prog_stats *stats; \
u64 start = sched_clock(); \
- ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
+ ret = bpf_call_func(prog, ctx); \
stats = this_cpu_ptr(prog->aux->stats); \
u64_stats_update_begin(&stats->syncp); \
stats->cnt++; \
stats->nsecs += sched_clock() - start; \
u64_stats_update_end(&stats->syncp); \
} else { \
- ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
+ ret = bpf_call_func(prog, ctx); \
} \
ret; })
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 66088a9e9b9e..7aad58f67105 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -792,6 +792,30 @@ void __weak bpf_jit_free_exec(void *addr)
module_memfree(addr);
}
+#ifdef CONFIG_BPF_JIT
+bool __weak arch_bpf_jit_check_func(const struct bpf_prog *prog)
+{
+ return true;
+}
+
+unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
+{
+ const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
+
+ if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
+ return prog->bpf_func(ctx, prog->insnsi);
+
+ if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
+ !arch_bpf_jit_check_func(prog))) {
+ WARN(1, "attempt to jump to an invalid address");
+ return 0;
+ }
+
+ return prog->bpf_func(ctx, prog->insnsi);
+}
+EXPORT_SYMBOL_GPL(bpf_call_func);
+#endif
+
struct bpf_binary_header *
bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
unsigned int alignment,
@@ -818,6 +842,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
/* Fill space with illegal/arch-dep instructions. */
bpf_fill_ill_insns(hdr, size);
+ hdr->magic = BPF_BINARY_HEADER_MAGIC;
hdr->pages = pages;
hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
PAGE_SIZE - sizeof(*hdr));
--
2.23.0.162.g0b9fbb3734-goog
On 9/9/19 11:32 PM, Sami Tolvanen wrote:
> With CONFIG_BPF_JIT, the kernel makes indirect calls to dynamically
> generated code. This change adds basic sanity checking to ensure
> we are jumping to a valid location, which narrows down the attack
> surface on the stored pointer. This also prepares the code for future
> Control-Flow Integrity (CFI) checking, which adds indirect call
> validation to call targets that can be determined at compile-time, but
> cannot validate calls to jited functions.
>
> In addition, this change adds a weak arch_bpf_jit_check_func function,
> which architectures that implement BPF JIT can override to perform
> additional validation, such as verifying that the pointer points to
> the correct memory region.
You did not mention BPF_BINARY_HEADER_MAGIC and added member
of `magic` in bpf_binary_header. Could you add some details
on what is the purpose for this `magic` member?
>
> Signed-off-by: Sami Tolvanen <[email protected]>
> ---
> include/linux/filter.h | 26 ++++++++++++++++++++++++--
> kernel/bpf/core.c | 25 +++++++++++++++++++++++++
> 2 files changed, 49 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 92c6e31fb008..abfb0e1b21a8 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -511,7 +511,10 @@ struct sock_fprog_kern {
> struct sock_filter *filter;
> };
>
> +#define BPF_BINARY_HEADER_MAGIC 0x05de0e82
> +
> struct bpf_binary_header {
> + u32 magic;
> u32 pages;
> /* Some arches need word alignment for their instructions */
> u8 image[] __aligned(4);
> @@ -553,20 +556,39 @@ struct sk_filter {
>
> DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
>
> +#ifdef CONFIG_BPF_JIT
> +/*
> + * With JIT, the kernel makes an indirect call to dynamically generated
> + * code. Use bpf_call_func to perform additional validation of the call
> + * target to narrow down attack surface. Architectures implementing BPF
> + * JIT can override arch_bpf_jit_check_func for arch-specific checking.
> + */
> +extern unsigned int bpf_call_func(const struct bpf_prog *prog,
> + const void *ctx);
> +
> +extern bool arch_bpf_jit_check_func(const struct bpf_prog *prog);
> +#else
> +static inline unsigned int bpf_call_func(const struct bpf_prog *prog,
> + const void *ctx)
> +{
> + return prog->bpf_func(ctx, prog->insnsi);
> +}
> +#endif
> +
> #define BPF_PROG_RUN(prog, ctx) ({ \
> u32 ret; \
> cant_sleep(); \
> if (static_branch_unlikely(&bpf_stats_enabled_key)) { \
> struct bpf_prog_stats *stats; \
> u64 start = sched_clock(); \
> - ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
> + ret = bpf_call_func(prog, ctx); \
> stats = this_cpu_ptr(prog->aux->stats); \
> u64_stats_update_begin(&stats->syncp); \
> stats->cnt++; \
> stats->nsecs += sched_clock() - start; \
> u64_stats_update_end(&stats->syncp); \
> } else { \
> - ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi); \
> + ret = bpf_call_func(prog, ctx); \
> } \
> ret; })
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 66088a9e9b9e..7aad58f67105 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -792,6 +792,30 @@ void __weak bpf_jit_free_exec(void *addr)
> module_memfree(addr);
> }
>
> +#ifdef CONFIG_BPF_JIT
> +bool __weak arch_bpf_jit_check_func(const struct bpf_prog *prog)
> +{
> + return true;
> +}
> +
> +unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
> +{
> + const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
> +
> + if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
> + return prog->bpf_func(ctx, prog->insnsi);
> +
> + if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> + !arch_bpf_jit_check_func(prog))) {
> + WARN(1, "attempt to jump to an invalid address");
> + return 0;
> + }
> +
> + return prog->bpf_func(ctx, prog->insnsi);
> +}
The above can be rewritten as
if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || prog->jited ||
hdr->magic != BPF_BINARY_HEADER_MAGIC ||
!arch_bpf_jit_check_func(prog))) {
WARN(1, "attempt to jump to an invalid address");
return 0;
}
return prog->bpf_func(ctx, prog->insnsi);
BPF_PROG_RUN() will be called during xdp fast path.
Have you measured how much slowdown the above change could
cost for the performance?
> +EXPORT_SYMBOL_GPL(bpf_call_func);
> +#endif
> +
> struct bpf_binary_header *
> bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> unsigned int alignment,
> @@ -818,6 +842,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> /* Fill space with illegal/arch-dep instructions. */
> bpf_fill_ill_insns(hdr, size);
>
> + hdr->magic = BPF_BINARY_HEADER_MAGIC;
> hdr->pages = pages;
> hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
> PAGE_SIZE - sizeof(*hdr));
>
On Tue, Sep 10, 2019 at 08:37:19AM +0000, Yonghong Song wrote:
> You did not mention BPF_BINARY_HEADER_MAGIC and added member
> of `magic` in bpf_binary_header. Could you add some details
> on what is the purpose for this `magic` member?
Sure, I'll add a description to the next version.
The magic is a random number used to identify bpf_binary_header in
memory. The purpose of this patch is to limit the possible call
targets for the function pointer and checking for the magic helps
ensure we are jumping to a page that contains a jited function,
instead of allowing calls to arbitrary targets.
This is particularly useful when combined with the compiler-based
Control-Flow Integrity (CFI) mitigation, which Google started shipping
in Pixel kernels last year. The compiler injects checks to all
indirect calls, but cannot obviously validate jumps to dynamically
generated code.
> > +unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
> > +{
> > + const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
> > +
> > + if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
> > + return prog->bpf_func(ctx, prog->insnsi);
> > +
> > + if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> > + !arch_bpf_jit_check_func(prog))) {
> > + WARN(1, "attempt to jump to an invalid address");
> > + return 0;
> > + }
> > +
> > + return prog->bpf_func(ctx, prog->insnsi);
> > +}
> The above can be rewritten as
> if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || prog->jited ||
> hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> !arch_bpf_jit_check_func(prog))) {
> WARN(1, "attempt to jump to an invalid address");
> return 0;
> }
That doesn't look quite equivalent, but yes, this can be rewritten as a
single if statement like this:
if ((IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
prog->jited) &&
(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
!arch_bpf_jit_check_func(prog)))
I think splitting the interpreter and JIT paths would be more readable,
but I can certainly change this if you prefer.
> BPF_PROG_RUN() will be called during xdp fast path.
> Have you measured how much slowdown the above change could
> cost for the performance?
I have not measured the overhead, but it shouldn't be significant. Is
there a particular benchmark you'd like me to run?
Sami
On 9/10/19 6:22 PM, Sami Tolvanen wrote:
> On Tue, Sep 10, 2019 at 08:37:19AM +0000, Yonghong Song wrote:
>> You did not mention BPF_BINARY_HEADER_MAGIC and added member
>> of `magic` in bpf_binary_header. Could you add some details
>> on what is the purpose for this `magic` member?
>
> Sure, I'll add a description to the next version.
>
> The magic is a random number used to identify bpf_binary_header in
> memory. The purpose of this patch is to limit the possible call
> targets for the function pointer and checking for the magic helps
> ensure we are jumping to a page that contains a jited function,
> instead of allowing calls to arbitrary targets.
>
> This is particularly useful when combined with the compiler-based
> Control-Flow Integrity (CFI) mitigation, which Google started shipping
> in Pixel kernels last year. The compiler injects checks to all
> indirect calls, but cannot obviously validate jumps to dynamically
> generated code.
>
>>> +unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
>>> +{
>>> + const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
>>> +
>>> + if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
>>> + return prog->bpf_func(ctx, prog->insnsi);
>>> +
>>> + if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
>>> + !arch_bpf_jit_check_func(prog))) {
>>> + WARN(1, "attempt to jump to an invalid address");
>>> + return 0;
>>> + }
>>> +
>>> + return prog->bpf_func(ctx, prog->insnsi);
>>> +}
>
>> The above can be rewritten as
>> if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || prog->jited ||
>> hdr->magic != BPF_BINARY_HEADER_MAGIC ||
>> !arch_bpf_jit_check_func(prog))) {
>> WARN(1, "attempt to jump to an invalid address");
>> return 0;
>> }
>
> That doesn't look quite equivalent, but yes, this can be rewritten as a
Indeed, I made a mistake. Your below change is correct.
> single if statement like this:
>
> if ((IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
> prog->jited) &&
> (hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> !arch_bpf_jit_check_func(prog)))
>
> I think splitting the interpreter and JIT paths would be more readable,
> but I can certainly change this if you prefer.
How about this:
if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
goto out;
if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
!arch_bpf_jit_check_func(prog))) {
WARN(1, "attempt to jump to an invalid address");
return 0;
}
out:
return prog->bpf_func(ctx, prog->insnsi);
>
>> BPF_PROG_RUN() will be called during xdp fast path.
>> Have you measured how much slowdown the above change could
>> cost for the performance?
>
> I have not measured the overhead, but it shouldn't be significant. Is
> there a particular benchmark you'd like me to run?
I am not an expert in XDP testing. Toke, Björn, could you give some
suggestions what to test for XDP performance here?
>
> Sami
>
On 2019-09-11 09:42, Yonghong Song wrote:
> I am not an expert in XDP testing. Toke, Björn, could you give some
> suggestions what to test for XDP performance here?
I ran the "xdp_rxq_info" sample with and without Sami's patch:
$ sudo ./xdp_rxq_info --dev enp134s0f0 --action XDP_DROP
Before:
Running XDP on dev:enp134s0f0 (ifindex:6) action:XDP_DROP options:no_touch
XDP stats CPU pps issue-pps
XDP-RX CPU 20 23923874 0
XDP-RX CPU total 23923874
RXQ stats RXQ:CPU pps issue-pps
rx_queue_index 20:20 23923878 0
rx_queue_index 20:sum 23923878
After Sami's patch:
Running XDP on dev:enp134s0f0 (ifindex:6) action:XDP_DROP options:no_touch
XDP stats CPU pps issue-pps
XDP-RX CPU 20 22998700 0
XDP-RX CPU total 22998700
RXQ stats RXQ:CPU pps issue-pps
rx_queue_index 20:20 22998705 0
rx_queue_index 20:sum 22998705
So, roughly ~4% for this somewhat naive scenario.
As for XDP performance tests; I guess some of the XDP selftests could be
used as well!
Björn
Björn Töpel <[email protected]> writes:
> On 2019-09-11 09:42, Yonghong Song wrote:
>> I am not an expert in XDP testing. Toke, Björn, could you give some
>> suggestions what to test for XDP performance here?
>
> I ran the "xdp_rxq_info" sample with and without Sami's patch:
Thanks for doing this!
> $ sudo ./xdp_rxq_info --dev enp134s0f0 --action XDP_DROP
>
> Before:
>
> Running XDP on dev:enp134s0f0 (ifindex:6) action:XDP_DROP options:no_touch
> XDP stats CPU pps issue-pps
> XDP-RX CPU 20 23923874 0
> XDP-RX CPU total 23923874
>
> RXQ stats RXQ:CPU pps issue-pps
> rx_queue_index 20:20 23923878 0
> rx_queue_index 20:sum 23923878
>
> After Sami's patch:
>
> Running XDP on dev:enp134s0f0 (ifindex:6) action:XDP_DROP options:no_touch
> XDP stats CPU pps issue-pps
> XDP-RX CPU 20 22998700 0
> XDP-RX CPU total 22998700
>
> RXQ stats RXQ:CPU pps issue-pps
> rx_queue_index 20:20 22998705 0
> rx_queue_index 20:sum 22998705
>
>
> So, roughly ~4% for this somewhat naive scenario.
Or (1/22998700 - 1/23923874) * 10**9 == 1.7 nanoseconds of overhead.
I guess that is not *too* bad; but it's still chipping away at
performance; anything we could do to lower the overhead?
-Toke
On Wed, Sep 11, 2019 at 5:09 AM Toke Høiland-Jørgensen <[email protected]> wrote:
>
> Björn Töpel <[email protected]> writes:
> > I ran the "xdp_rxq_info" sample with and without Sami's patch:
>
> Thanks for doing this!
Yes, thanks for testing this Björn!
> Or (1/22998700 - 1/23923874) * 10**9 == 1.7 nanoseconds of overhead.
>
> I guess that is not *too* bad; but it's still chipping away at
> performance; anything we could do to lower the overhead?
The check is already rather minimal, but I could move this to a static
inline function to help ensure the compiler doesn't generate an
additional function call for this. I'm also fine with gating this
behind a separate config option, but I'm not sure if that's worth it.
Any thoughts?
Sami
On Wed, Sep 11, 2019 at 12:43 AM Yonghong Song <[email protected]> wrote:
> How about this:
>
> if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
> goto out;
>
> if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> !arch_bpf_jit_check_func(prog))) {
> WARN(1, "attempt to jump to an invalid address");
> return 0;
> }
> out:
> return prog->bpf_func(ctx, prog->insnsi);
Sure, that does look cleaner. I'll use this in the next version. Thanks.
Sami
Sami Tolvanen <[email protected]> writes:
> On Wed, Sep 11, 2019 at 5:09 AM Toke Høiland-Jørgensen <[email protected]> wrote:
>>
>> Björn Töpel <[email protected]> writes:
>> > I ran the "xdp_rxq_info" sample with and without Sami's patch:
>>
>> Thanks for doing this!
>
> Yes, thanks for testing this Björn!
>
>> Or (1/22998700 - 1/23923874) * 10**9 == 1.7 nanoseconds of overhead.
>>
>> I guess that is not *too* bad; but it's still chipping away at
>> performance; anything we could do to lower the overhead?
>
> The check is already rather minimal, but I could move this to a static
> inline function to help ensure the compiler doesn't generate an
> additional function call for this. I'm also fine with gating this
> behind a separate config option, but I'm not sure if that's worth it.
> Any thoughts?
I think it would be good if you do both. I'm a bit worried that XDP
performance will end up in a "death by a thousand paper cuts" situation,
so I'd rather push back on even relatively small overheads like this; so
being able to turn it off in the config would be good.
Can you share more details about what the "future CFI checking" is
likely to look like?
-Toke
On Thu, Sep 12, 2019 at 3:52 AM Toke Høiland-Jørgensen <[email protected]> wrote:
> I think it would be good if you do both. I'm a bit worried that XDP
> performance will end up in a "death by a thousand paper cuts" situation,
> so I'd rather push back on even relatively small overheads like this; so
> being able to turn it off in the config would be good.
OK, thanks for the feedback. In that case, I think it's probably
better to wait until we have CFI ready for upstreaming and use the
same config for this one.
> Can you share more details about what the "future CFI checking" is
> likely to look like?
Sure, I posted an overview of CFI and what we're doing in Pixel devices here:
https://android-developers.googleblog.com/2018/10/control-flow-integrity-in-android-kernel.html
Sami
Sami Tolvanen <[email protected]> writes:
> On Thu, Sep 12, 2019 at 3:52 AM Toke Høiland-Jørgensen <[email protected]> wrote:
>> I think it would be good if you do both. I'm a bit worried that XDP
>> performance will end up in a "death by a thousand paper cuts" situation,
>> so I'd rather push back on even relatively small overheads like this; so
>> being able to turn it off in the config would be good.
>
> OK, thanks for the feedback. In that case, I think it's probably
> better to wait until we have CFI ready for upstreaming and use the
> same config for this one.
SGTM, thanks!
>> Can you share more details about what the "future CFI checking" is
>> likely to look like?
>
> Sure, I posted an overview of CFI and what we're doing in Pixel devices here:
>
> https://android-developers.googleblog.com/2018/10/control-flow-integrity-in-android-kernel.html
Great, thank you.
-Toke