[PATCH bpf 0/2] bpf fix for mixed tail calls and subprograms

[PATCH bpf 0/2] bpf fix for mixed tail calls and subprograms

[John Fastabend]

We recently tried to use mixed programs that have both tail calls and
subprograms, but it needs the attached fix. Also added a small test
addition that will cause the failure without the fix.

Thanks,
John

---

John Fastabend (2):
      bpf: Fix null ptr deref with mixed tail calls and subprogs
      bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch


 .../selftests/bpf/progs/tailcall_bpf2bpf4.c     | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--

[PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and subprogs

[John Fastabend]

The sub-programs prog->aux->poke_tab[] is populated in jit_subprogs() and
then used when emitting 'BPF_JMP|BPF_TAIL_CALL' insn->code from the
individual JITs. The poke_tab[] to use is stored in the insn->imm by
the code adding it to that array slot. The JIT then uses imm to find the
right entry for an individual instruction. In the x86 bpf_jit_comp.c
this is done by calling emit_bpf_tail_call_direct with the poke_tab[]
of the imm value.

However, we observed the below null-ptr-deref when mixing tail call
programs with subprog programs. For this to happen we just need to
mix bpf-2-bpf calls and tailcalls with some extra calls or instructions
that would be patched later by one of the fixup routines. So whats
happening?

Before the fixup_call_args() -- where the jit op is done -- various
code patching is done by do_misc_fixups(). This may increase the
insn count, for example when we patch map_lookup_up using map_gen_lookup
hook. This does two things. First, it means the instruction index,
insn_idx field, of a tail call instruction will move by a 'delta'.

In verifier code,

 struct bpf_jit_poke_descriptor desc = {
  .reason = BPF_POKE_REASON_TAIL_CALL,
  .tail_call.map = BPF_MAP_PTR(aux->map_ptr_state),
  .tail_call.key = bpf_map_key_immediate(aux),
  .insn_idx = i + delta,
 };

Then subprog start values subprog_info[i].start will be updated
with the delta and any poke descriptor index will also be updated
with the delta in adjust_poke_desc(). If we look at the adjust
subprog starts though we see its only adjusted when the delta
occurs before the new instructions,

        /* NOTE: fake 'exit' subprog should be updated as well. */
        for (i = 0; i <= env->subprog_cnt; i++) {
                if (env->subprog_info[i].start <= off)
                        continue;

Earlier subprograms are not changed because their start values
are not moved. But, adjust_poke_desc() does the offset + delta
indiscriminately. The result is poke descriptors are potentially
corrupted.

Then in jit_subprogs() we only populate the poke_tab[]
when the above insn_idx is less than the next subprogram start. From
above we corrupted our insn_idx so we might incorrectly assume a
poke descriptor is not used in a subprogram omitting it from the
subprogram. And finally when the jit runs it does the deref of poke_tab
when emitting the instruction and crashes with below. Because earlier
step omitted the poke descriptor.

The fix is straight forward with above context. Simply move same logic
from adjust_subprog_starts() into adjust_poke_descs() and only adjust
insn_idx when needed.

[   88.487438] BUG: KASAN: null-ptr-deref in do_jit+0x184a/0x3290
[   88.487455] Write of size 8 at addr 0000000000000008 by task test_progs/5295
[   88.487490] Call Trace:
[   88.487498]  dump_stack+0x93/0xc2
[   88.487515]  kasan_report.cold+0x5f/0xd8
[   88.487530]  ? do_jit+0x184a/0x3290
[   88.487542]  do_jit+0x184a/0x3290
 ...
[   88.487709]  bpf_int_jit_compile+0x248/0x810
 ...
[   88.487765]  bpf_check+0x3718/0x5140
 ...
[   88.487920]  bpf_prog_load+0xa22/0xf10

CC: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 kernel/bpf/verifier.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 94ba5163d4c5..ac8373da849c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11408,7 +11408,7 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len
 	}
 }
 
-static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
+static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len)
 {
 	struct bpf_jit_poke_descriptor *tab = prog->aux->poke_tab;
 	int i, sz = prog->aux->size_poke_tab;
@@ -11416,6 +11416,8 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
 
 	for (i = 0; i < sz; i++) {
 		desc = &tab[i];
+		if (desc->insn_idx <= off)
+			continue;
 		desc->insn_idx += len - 1;
 	}
 }
@@ -11436,7 +11438,7 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
 	if (adjust_insn_aux_data(env, new_prog, off, len))
 		return NULL;
 	adjust_subprog_starts(env, off, len);
-	adjust_poke_descs(new_prog, len);
+	adjust_poke_descs(new_prog, off, len);
 	return new_prog;
 }
 

[PATCH bpf 2/2] bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch

[John Fastabend]

This adds some extra noise to the tailcall_bpf2bpf4 tests that will cause
verifier to patch insns. This then moves around subprog start/end insn
index and poke descriptor insn index to ensure that verify and JIT will
continue to track these correctly.

Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 .../selftests/bpf/progs/tailcall_bpf2bpf4.c        |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
index 9a1b166b7fbe..0d70de5f97e2 100644
--- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
+++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
@@ -2,6 +2,13 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__uint(value_size, sizeof(__u32));
+} nop_table SEC(".maps");
+
 struct {
 	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
 	__uint(max_entries, 3);
@@ -11,9 +18,19 @@ struct {
 
 static volatile int count;
 
+__noinline
+int subprog_noise(struct __sk_buff *skb)
+{
+	__u32 key = 0;
+
+	bpf_map_lookup_elem(&nop_table, &key);
+	return 0;
+}
+
 __noinline
 int subprog_tail_2(struct __sk_buff *skb)
 {
+	subprog_noise(skb);
 	bpf_tail_call_static(skb, &jmp_table, 2);
 	return skb->len * 3;
 }

Re: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and subprogs

[Yonghong Song]

On 6/8/21 12:30 PM, John Fastabend wrote:
> The sub-programs prog->aux->poke_tab[] is populated in jit_subprogs() and
> then used when emitting 'BPF_JMP|BPF_TAIL_CALL' insn->code from the
> individual JITs. The poke_tab[] to use is stored in the insn->imm by
> the code adding it to that array slot. The JIT then uses imm to find the
> right entry for an individual instruction. In the x86 bpf_jit_comp.c
> this is done by calling emit_bpf_tail_call_direct with the poke_tab[]
> of the imm value.
> 
> However, we observed the below null-ptr-deref when mixing tail call
> programs with subprog programs. For this to happen we just need to
> mix bpf-2-bpf calls and tailcalls with some extra calls or instructions
> that would be patched later by one of the fixup routines. So whats
> happening?
> 
> Before the fixup_call_args() -- where the jit op is done -- various
> code patching is done by do_misc_fixups(). This may increase the
> insn count, for example when we patch map_lookup_up using map_gen_lookup
> hook. This does two things. First, it means the instruction index,
> insn_idx field, of a tail call instruction will move by a 'delta'.
> 
> In verifier code,
> 
>   struct bpf_jit_poke_descriptor desc = {
>    .reason = BPF_POKE_REASON_TAIL_CALL,
>    .tail_call.map = BPF_MAP_PTR(aux->map_ptr_state),
>    .tail_call.key = bpf_map_key_immediate(aux),
>    .insn_idx = i + delta,
>   };
> 
> Then subprog start values subprog_info[i].start will be updated
> with the delta and any poke descriptor index will also be updated
> with the delta in adjust_poke_desc(). If we look at the adjust
> subprog starts though we see its only adjusted when the delta
> occurs before the new instructions,
> 
>          /* NOTE: fake 'exit' subprog should be updated as well. */
>          for (i = 0; i <= env->subprog_cnt; i++) {
>                  if (env->subprog_info[i].start <= off)
>                          continue;
> 
> Earlier subprograms are not changed because their start values
> are not moved. But, adjust_poke_desc() does the offset + delta
> indiscriminately. The result is poke descriptors are potentially
> corrupted.
> 
> Then in jit_subprogs() we only populate the poke_tab[]
> when the above insn_idx is less than the next subprogram start. From
> above we corrupted our insn_idx so we might incorrectly assume a
> poke descriptor is not used in a subprogram omitting it from the
> subprogram. And finally when the jit runs it does the deref of poke_tab
> when emitting the instruction and crashes with below. Because earlier
> step omitted the poke descriptor.
> 
> The fix is straight forward with above context. Simply move same logic
> from adjust_subprog_starts() into adjust_poke_descs() and only adjust
> insn_idx when needed.
> 
> [   88.487438] BUG: KASAN: null-ptr-deref in do_jit+0x184a/0x3290
> [   88.487455] Write of size 8 at addr 0000000000000008 by task test_progs/5295
> [   88.487490] Call Trace:
> [   88.487498]  dump_stack+0x93/0xc2
> [   88.487515]  kasan_report.cold+0x5f/0xd8
> [   88.487530]  ? do_jit+0x184a/0x3290
> [   88.487542]  do_jit+0x184a/0x3290
>   ...
> [   88.487709]  bpf_int_jit_compile+0x248/0x810
>   ...
> [   88.487765]  bpf_check+0x3718/0x5140
>   ...
> [   88.487920]  bpf_prog_load+0xa22/0xf10
> 
> CC: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
> Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>

Acked-by: Yonghong Song <yhs@fb.com>

Re: [PATCH bpf 2/2] bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch

[Yonghong Song]

On 6/8/21 12:30 PM, John Fastabend wrote:
> This adds some extra noise to the tailcall_bpf2bpf4 tests that will cause
> verifier to patch insns. This then moves around subprog start/end insn
> index and poke descriptor insn index to ensure that verify and JIT will
> continue to track these correctly.
> 
> Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>

Acked-by: Yonghong Song <yhs@fb.com>

Re: [PATCH bpf 0/2] bpf fix for mixed tail calls and subprograms

[Yonghong Song]

On 6/8/21 12:29 PM, John Fastabend wrote:
> We recently tried to use mixed programs that have both tail calls and
> subprograms, but it needs the attached fix. Also added a small test
> addition that will cause the failure without the fix.
> 
> Thanks,
> John
> 
> ---
> 
> John Fastabend (2):
>        bpf: Fix null ptr deref with mixed tail calls and subprogs
>        bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch
> 
> 
>   .../selftests/bpf/progs/tailcall_bpf2bpf4.c     | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)

Don't know what happens. Apparently, the first patch made changes
in kernel/bpf/verifier.c, but it didn't show up in the above.

> 
> --
> 

Re: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and subprogs

[Maciej Fijalkowski]

On Tue, Jun 08, 2021 at 12:30:15PM -0700, John Fastabend wrote:
> The sub-programs prog->aux->poke_tab[] is populated in jit_subprogs() and
> then used when emitting 'BPF_JMP|BPF_TAIL_CALL' insn->code from the
> individual JITs. The poke_tab[] to use is stored in the insn->imm by
> the code adding it to that array slot. The JIT then uses imm to find the
> right entry for an individual instruction. In the x86 bpf_jit_comp.c
> this is done by calling emit_bpf_tail_call_direct with the poke_tab[]
> of the imm value.
> 
> However, we observed the below null-ptr-deref when mixing tail call
> programs with subprog programs. For this to happen we just need to
> mix bpf-2-bpf calls and tailcalls with some extra calls or instructions
> that would be patched later by one of the fixup routines. So whats
> happening?
> 
> Before the fixup_call_args() -- where the jit op is done -- various
> code patching is done by do_misc_fixups(). This may increase the
> insn count, for example when we patch map_lookup_up using map_gen_lookup
> hook. This does two things. First, it means the instruction index,
> insn_idx field, of a tail call instruction will move by a 'delta'.
> 
> In verifier code,
> 
>  struct bpf_jit_poke_descriptor desc = {
>   .reason = BPF_POKE_REASON_TAIL_CALL,
>   .tail_call.map = BPF_MAP_PTR(aux->map_ptr_state),
>   .tail_call.key = bpf_map_key_immediate(aux),
>   .insn_idx = i + delta,
>  };
> 
> Then subprog start values subprog_info[i].start will be updated
> with the delta and any poke descriptor index will also be updated
> with the delta in adjust_poke_desc(). If we look at the adjust
> subprog starts though we see its only adjusted when the delta
> occurs before the new instructions,
> 
>         /* NOTE: fake 'exit' subprog should be updated as well. */
>         for (i = 0; i <= env->subprog_cnt; i++) {
>                 if (env->subprog_info[i].start <= off)
>                         continue;
> 
> Earlier subprograms are not changed because their start values
> are not moved. But, adjust_poke_desc() does the offset + delta
> indiscriminately. The result is poke descriptors are potentially
> corrupted.
> 
> Then in jit_subprogs() we only populate the poke_tab[]
> when the above insn_idx is less than the next subprogram start. From
> above we corrupted our insn_idx so we might incorrectly assume a
> poke descriptor is not used in a subprogram omitting it from the
> subprogram. And finally when the jit runs it does the deref of poke_tab
> when emitting the instruction and crashes with below. Because earlier
> step omitted the poke descriptor.
> 
> The fix is straight forward with above context. Simply move same logic
> from adjust_subprog_starts() into adjust_poke_descs() and only adjust
> insn_idx when needed.
> 
> [   88.487438] BUG: KASAN: null-ptr-deref in do_jit+0x184a/0x3290
> [   88.487455] Write of size 8 at addr 0000000000000008 by task test_progs/5295
> [   88.487490] Call Trace:
> [   88.487498]  dump_stack+0x93/0xc2
> [   88.487515]  kasan_report.cold+0x5f/0xd8
> [   88.487530]  ? do_jit+0x184a/0x3290
> [   88.487542]  do_jit+0x184a/0x3290
>  ...
> [   88.487709]  bpf_int_jit_compile+0x248/0x810
>  ...
> [   88.487765]  bpf_check+0x3718/0x5140
>  ...
> [   88.487920]  bpf_prog_load+0xa22/0xf10
> 
> CC: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
> Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  kernel/bpf/verifier.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 94ba5163d4c5..ac8373da849c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11408,7 +11408,7 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len
>  	}
>  }
>  
> -static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
> +static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len)
>  {
>  	struct bpf_jit_poke_descriptor *tab = prog->aux->poke_tab;
>  	int i, sz = prog->aux->size_poke_tab;
> @@ -11416,6 +11416,8 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
>  
>  	for (i = 0; i < sz; i++) {
>  		desc = &tab[i];

Can we have a comment below that would say something like:
"don't update taicall's insn idx if the patching is being done on higher
insns" ?

What I'm saying is that after a long break from that code I find 'off' as
a confusing name. It's the offset within the flat-structured bpf prog (so
the prog that is not yet sliced onto subprogs). Maybe we could find a
better name for that, like "curr_insn_idx". I'm not sure what's your view
on that.

OTOH I'm aware that whole content of bpf_patch_insn_data operates on
'off'.

Generally sorry that I missed that, it didn't come to my mind to mix in
other helpers that include patching.

Anyway:
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

> +		if (desc->insn_idx <= off)
> +			continue;
>  		desc->insn_idx += len - 1;
>  	}
>  }
> @@ -11436,7 +11438,7 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
>  	if (adjust_insn_aux_data(env, new_prog, off, len))
>  		return NULL;
>  	adjust_subprog_starts(env, off, len);
> -	adjust_poke_descs(new_prog, len);
> +	adjust_poke_descs(new_prog, off, len);
>  	return new_prog;
>  }
>  
> 
> 

Re: [PATCH bpf 2/2] bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch

[Maciej Fijalkowski]

On Tue, Jun 08, 2021 at 12:30:33PM -0700, John Fastabend wrote:
> This adds some extra noise to the tailcall_bpf2bpf4 tests that will cause
> verifier to patch insns. This then moves around subprog start/end insn
> index and poke descriptor insn index to ensure that verify and JIT will
> continue to track these correctly.

This test is the most complicated one where I tried to document the scope
of it on the side of prog_tests/tailcalls.c. I feel that it would make it
more difficult to debug it if under any circumstances something would have
been broken with that logic.

Maybe a separate test scenario? Or is this an overkill? If so, I would
vote for moving it to tailcall_bpf2bpf1.c and have a little comment that
testing other bpf helpers mixed in is in scope of that test.

> 
> Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  .../selftests/bpf/progs/tailcall_bpf2bpf4.c        |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> index 9a1b166b7fbe..0d70de5f97e2 100644
> --- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> +++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> @@ -2,6 +2,13 @@
>  #include <linux/bpf.h>
>  #include <bpf/bpf_helpers.h>
>  
> +struct {
> +	__uint(type, BPF_MAP_TYPE_ARRAY);
> +	__uint(max_entries, 1);
> +	__uint(key_size, sizeof(__u32));
> +	__uint(value_size, sizeof(__u32));
> +} nop_table SEC(".maps");
> +
>  struct {
>  	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
>  	__uint(max_entries, 3);
> @@ -11,9 +18,19 @@ struct {
>  
>  static volatile int count;
>  
> +__noinline
> +int subprog_noise(struct __sk_buff *skb)
> +{
> +	__u32 key = 0;
> +
> +	bpf_map_lookup_elem(&nop_table, &key);
> +	return 0;
> +}
> +
>  __noinline
>  int subprog_tail_2(struct __sk_buff *skb)
>  {
> +	subprog_noise(skb);
>  	bpf_tail_call_static(skb, &jmp_table, 2);
>  	return skb->len * 3;
>  }
> 
> 

Re: [PATCH bpf 0/2] bpf fix for mixed tail calls and subprograms

[John Fastabend]

Yonghong Song wrote:
> 
> 
> On 6/8/21 12:29 PM, John Fastabend wrote:
> > We recently tried to use mixed programs that have both tail calls and
> > subprograms, but it needs the attached fix. Also added a small test
> > addition that will cause the failure without the fix.
> > 
> > Thanks,
> > John
> > 
> > ---
> > 
> > John Fastabend (2):
> >        bpf: Fix null ptr deref with mixed tail calls and subprogs
> >        bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch
> > 
> > 
> >   .../selftests/bpf/progs/tailcall_bpf2bpf4.c     | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> 
> Don't know what happens. Apparently, the first patch made changes
> in kernel/bpf/verifier.c, but it didn't show up in the above.

Agh its how I applied the patches and cover-letter :/ I moved them between
trees (bpf-next -> bpf) and lost the diff. I can resubmit if anyone
cares.

Re: [PATCH bpf 2/2] bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch

[Maciej Fijalkowski]

On Wed, Jun 09, 2021 at 09:26:01AM -0700, John Fastabend wrote:
> Maciej Fijalkowski wrote:
> > On Tue, Jun 08, 2021 at 12:30:33PM -0700, John Fastabend wrote:
> > > This adds some extra noise to the tailcall_bpf2bpf4 tests that will cause
> > > verifier to patch insns. This then moves around subprog start/end insn
> > > index and poke descriptor insn index to ensure that verify and JIT will
> > > continue to track these correctly.
> > 
> > This test is the most complicated one where I tried to document the scope
> > of it on the side of prog_tests/tailcalls.c. I feel that it would make it
> > more difficult to debug it if under any circumstances something would have
> > been broken with that logic.
> > 
> > Maybe a separate test scenario? Or is this an overkill? If so, I would
> > vote for moving it to tailcall_bpf2bpf1.c and have a little comment that
> > testing other bpf helpers mixed in is in scope of that test.
> 
> I like pushing it into the complex test to get the most instruction
> patching combinations possible.

Makes sense after a second thought, that was the intention of that test
case, to squeeze out the feature out here.

I still would ask to have it commented on the prog_tests/tailcalls.c side,
WDYT?

> 
> > 
> > > 
> > > Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> > > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > > ---
> > >  .../selftests/bpf/progs/tailcall_bpf2bpf4.c        |   17 +++++++++++++++++
> > >  1 file changed, 17 insertions(+)
> > > 
> > > diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > > index 9a1b166b7fbe..0d70de5f97e2 100644
> > > --- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > > +++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > > @@ -2,6 +2,13 @@
> > >  #include <linux/bpf.h>
> > >  #include <bpf/bpf_helpers.h>
> > >  
> > > +struct {
> > > +	__uint(type, BPF_MAP_TYPE_ARRAY);
> > > +	__uint(max_entries, 1);
> > > +	__uint(key_size, sizeof(__u32));
> > > +	__uint(value_size, sizeof(__u32));
> > > +} nop_table SEC(".maps");
> > > +
> > >  struct {
> > >  	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
> > >  	__uint(max_entries, 3);
> > > @@ -11,9 +18,19 @@ struct {
> > >  
> > >  static volatile int count;
> > >  
> > > +__noinline
> > > +int subprog_noise(struct __sk_buff *skb)
> > > +{
> > > +	__u32 key = 0;
> > > +
> > > +	bpf_map_lookup_elem(&nop_table, &key);
> > > +	return 0;
> > > +}
> > > +
> > >  __noinline
> > >  int subprog_tail_2(struct __sk_buff *skb)
> > >  {
> > > +	subprog_noise(skb);
> > >  	bpf_tail_call_static(skb, &jmp_table, 2);
> > >  	return skb->len * 3;
> > >  }
> > > 
> > > 
> 
> 

Re: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and subprogs

[John Fastabend]

Maciej Fijalkowski wrote:
> On Tue, Jun 08, 2021 at 12:30:15PM -0700, John Fastabend wrote:
> > The sub-programs prog->aux->poke_tab[] is populated in jit_subprogs() and
> > then used when emitting 'BPF_JMP|BPF_TAIL_CALL' insn->code from the
> > individual JITs. The poke_tab[] to use is stored in the insn->imm by
> > the code adding it to that array slot. The JIT then uses imm to find the
> > right entry for an individual instruction. In the x86 bpf_jit_comp.c
> > this is done by calling emit_bpf_tail_call_direct with the poke_tab[]
> > of the imm value.
> > 
> > However, we observed the below null-ptr-deref when mixing tail call
> > programs with subprog programs. For this to happen we just need to
> > mix bpf-2-bpf calls and tailcalls with some extra calls or instructions
> > that would be patched later by one of the fixup routines. So whats
> > happening?
> > 
> > Before the fixup_call_args() -- where the jit op is done -- various
> > code patching is done by do_misc_fixups(). This may increase the
> > insn count, for example when we patch map_lookup_up using map_gen_lookup
> > hook. This does two things. First, it means the instruction index,
> > insn_idx field, of a tail call instruction will move by a 'delta'.
> > 
> > In verifier code,
> > 
> >  struct bpf_jit_poke_descriptor desc = {
> >   .reason = BPF_POKE_REASON_TAIL_CALL,
> >   .tail_call.map = BPF_MAP_PTR(aux->map_ptr_state),
> >   .tail_call.key = bpf_map_key_immediate(aux),
> >   .insn_idx = i + delta,
> >  };
> > 
> > Then subprog start values subprog_info[i].start will be updated
> > with the delta and any poke descriptor index will also be updated
> > with the delta in adjust_poke_desc(). If we look at the adjust
> > subprog starts though we see its only adjusted when the delta
> > occurs before the new instructions,
> > 
> >         /* NOTE: fake 'exit' subprog should be updated as well. */
> >         for (i = 0; i <= env->subprog_cnt; i++) {
> >                 if (env->subprog_info[i].start <= off)
> >                         continue;
> > 
> > Earlier subprograms are not changed because their start values
> > are not moved. But, adjust_poke_desc() does the offset + delta
> > indiscriminately. The result is poke descriptors are potentially
> > corrupted.
> > 
> > Then in jit_subprogs() we only populate the poke_tab[]
> > when the above insn_idx is less than the next subprogram start. From
> > above we corrupted our insn_idx so we might incorrectly assume a
> > poke descriptor is not used in a subprogram omitting it from the
> > subprogram. And finally when the jit runs it does the deref of poke_tab
> > when emitting the instruction and crashes with below. Because earlier
> > step omitted the poke descriptor.
> > 
> > The fix is straight forward with above context. Simply move same logic
> > from adjust_subprog_starts() into adjust_poke_descs() and only adjust
> > insn_idx when needed.
> > 
> > [   88.487438] BUG: KASAN: null-ptr-deref in do_jit+0x184a/0x3290
> > [   88.487455] Write of size 8 at addr 0000000000000008 by task test_progs/5295
> > [   88.487490] Call Trace:
> > [   88.487498]  dump_stack+0x93/0xc2
> > [   88.487515]  kasan_report.cold+0x5f/0xd8
> > [   88.487530]  ? do_jit+0x184a/0x3290
> > [   88.487542]  do_jit+0x184a/0x3290
> >  ...
> > [   88.487709]  bpf_int_jit_compile+0x248/0x810
> >  ...
> > [   88.487765]  bpf_check+0x3718/0x5140
> >  ...
> > [   88.487920]  bpf_prog_load+0xa22/0xf10
> > 
> > CC: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> > Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
> > Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> >  kernel/bpf/verifier.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 94ba5163d4c5..ac8373da849c 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -11408,7 +11408,7 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len
> >  	}
> >  }
> >  
> > -static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
> > +static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len)
> >  {
> >  	struct bpf_jit_poke_descriptor *tab = prog->aux->poke_tab;
> >  	int i, sz = prog->aux->size_poke_tab;
> > @@ -11416,6 +11416,8 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 len)
> >  
> >  	for (i = 0; i < sz; i++) {
> >  		desc = &tab[i];
> 
> Can we have a comment below that would say something like:
> "don't update taicall's insn idx if the patching is being done on higher
> insns" ?
> 
> What I'm saying is that after a long break from that code I find 'off' as
> a confusing name. It's the offset within the flat-structured bpf prog (so
> the prog that is not yet sliced onto subprogs). Maybe we could find a
> better name for that, like "curr_insn_idx". I'm not sure what's your view
> on that.
> 
> OTOH I'm aware that whole content of bpf_patch_insn_data operates on
> 'off'.

I'm not necessarily opposed to a comment there but we don't have a comment
above for the same operation on start offsets. I'll think about it for
a follow up patch assuming no one shouts.

> 
> Generally sorry that I missed that, it didn't come to my mind to mix in
> other helpers that include patching.


Thanks for testing.

> 
> Anyway:
> Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Tested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> 
> > +		if (desc->insn_idx <= off)
> > +			continue;
> >  		desc->insn_idx += len - 1;
> >  	}
> >  }
> > @@ -11436,7 +11438,7 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
> >  	if (adjust_insn_aux_data(env, new_prog, off, len))
> >  		return NULL;
> >  	adjust_subprog_starts(env, off, len);
> > -	adjust_poke_descs(new_prog, len);
> > +	adjust_poke_descs(new_prog, off, len);
> >  	return new_prog;
> >  }
> >  
> > 
> > 

Re: [PATCH bpf 2/2] bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch

[John Fastabend]

Maciej Fijalkowski wrote:
> On Tue, Jun 08, 2021 at 12:30:33PM -0700, John Fastabend wrote:
> > This adds some extra noise to the tailcall_bpf2bpf4 tests that will cause
> > verifier to patch insns. This then moves around subprog start/end insn
> > index and poke descriptor insn index to ensure that verify and JIT will
> > continue to track these correctly.
> 
> This test is the most complicated one where I tried to document the scope
> of it on the side of prog_tests/tailcalls.c. I feel that it would make it
> more difficult to debug it if under any circumstances something would have
> been broken with that logic.
> 
> Maybe a separate test scenario? Or is this an overkill? If so, I would
> vote for moving it to tailcall_bpf2bpf1.c and have a little comment that
> testing other bpf helpers mixed in is in scope of that test.

I like pushing it into the complex test to get the most instruction
patching combinations possible.

> 
> > 
> > Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> >  .../selftests/bpf/progs/tailcall_bpf2bpf4.c        |   17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > index 9a1b166b7fbe..0d70de5f97e2 100644
> > --- a/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > +++ b/tools/testing/selftests/bpf/progs/tailcall_bpf2bpf4.c
> > @@ -2,6 +2,13 @@
> >  #include <linux/bpf.h>
> >  #include <bpf/bpf_helpers.h>
> >  
> > +struct {
> > +	__uint(type, BPF_MAP_TYPE_ARRAY);
> > +	__uint(max_entries, 1);
> > +	__uint(key_size, sizeof(__u32));
> > +	__uint(value_size, sizeof(__u32));
> > +} nop_table SEC(".maps");
> > +
> >  struct {
> >  	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
> >  	__uint(max_entries, 3);
> > @@ -11,9 +18,19 @@ struct {
> >  
> >  static volatile int count;
> >  
> > +__noinline
> > +int subprog_noise(struct __sk_buff *skb)
> > +{
> > +	__u32 key = 0;
> > +
> > +	bpf_map_lookup_elem(&nop_table, &key);
> > +	return 0;
> > +}
> > +
> >  __noinline
> >  int subprog_tail_2(struct __sk_buff *skb)
> >  {
> > +	subprog_noise(skb);
> >  	bpf_tail_call_static(skb, &jmp_table, 2);
> >  	return skb->len * 3;
> >  }
> > 
> > 

Re: [PATCH bpf 0/2] bpf fix for mixed tail calls and subprograms

[Yonghong Song]

On 6/9/21 9:17 AM, John Fastabend wrote:
> Yonghong Song wrote:
>>
>>
>> On 6/8/21 12:29 PM, John Fastabend wrote:
>>> We recently tried to use mixed programs that have both tail calls and
>>> subprograms, but it needs the attached fix. Also added a small test
>>> addition that will cause the failure without the fix.
>>>
>>> Thanks,
>>> John
>>>
>>> ---
>>>
>>> John Fastabend (2):
>>>         bpf: Fix null ptr deref with mixed tail calls and subprogs
>>>         bpf: selftest to verify mixing bpf2bpf calls and tailcalls with insn patch
>>>
>>>
>>>    .../selftests/bpf/progs/tailcall_bpf2bpf4.c     | 17 +++++++++++++++++
>>>    1 file changed, 17 insertions(+)
>>
>> Don't know what happens. Apparently, the first patch made changes
>> in kernel/bpf/verifier.c, but it didn't show up in the above.
> 
> Agh its how I applied the patches and cover-letter :/ I moved them between
> trees (bpf-next -> bpf) and lost the diff. I can resubmit if anyone
> cares.

You don't need to resubmit just because of this. This cover letter
may not be merged eventually.