From: John Fastabend john.fastabend@gmail.com
[ Upstream commit 7506d211b932870155bcb39e3dd9e39fab45a7c7 ]
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.
[ 82.396354] bpf_testmod: version magic '5.12.0-rc2alu+ SMP preempt mod_unload ' should be '5.12.0+ SMP preempt mod_unload ' [ 82.623001] loop10: detected capacity change from 0 to 8 [ 88.487424] ================================================================== [ 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 [ 88.487471] CPU: 7 PID: 5295 Comm: test_progs Tainted: G I 5.12.0+ #386 [ 88.487483] Hardware name: Dell Inc. Precision 5820 Tower/002KVM, BIOS 1.9.2 01/24/2019 [ 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
Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms") Reported-by: Jussi Maki joamaki@gmail.com Signed-off-by: John Fastabend john.fastabend@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Reviewed-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Sasha Levin sashal@kernel.org --- 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 c6a27574242d..6e2ebcb0d66f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -11459,7 +11459,7 @@ 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; @@ -11467,6 +11467,8 @@ 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; } } @@ -11487,7 +11489,7 @@ 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; }