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

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Message-ID: <162318061518.323820.4329181800429686297.stgit@john-XPS-13-9370>
Date:   Tue, 08 Jun 2021 12:30:15 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     ast@...nel.org, daniel@...earbox.net, andriin@...com
Cc:     john.fastabend@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, maciej.fijalkowski@...el.com
Subject: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and
 subprogs

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@...el.com>
Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
Reviewed-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: John Fastabend <john.fastabend@...il.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;
 }