bpf: disallow 40-bytes extra stack for bpf_fastcall patterns

Hou Tao reported an issue with bpf_fastcall patterns allowing extra
stack space above MAX_BPF_STACK limit. This extra stack allowance is
not integrated properly with the following verifier parts:
- backtracking logic still assumes that stack can't exceed
  MAX_BPF_STACK;
- bpf_verifier_env->scratched_stack_slots assumes only 64 slots are
  available.

Here is an example of an issue with precision tracking
(note stack slot -8 tracked as precise instead of -520):

    0: (b7) r1 = 42                       ; R1_w=42
    1: (b7) r2 = 42                       ; R2_w=42
    2: (7b) *(u64 *)(r10 -512) = r1       ; R1_w=42 R10=fp0 fp-512_w=42
    3: (7b) *(u64 *)(r10 -520) = r2       ; R2_w=42 R10=fp0 fp-520_w=42
    4: (85) call bpf_get_smp_processor_id#8       ; R0_w=scalar(...)
    5: (79) r2 = *(u64 *)(r10 -520)       ; R2_w=42 R10=fp0 fp-520_w=42
    6: (79) r1 = *(u64 *)(r10 -512)       ; R1_w=42 R10=fp0 fp-512_w=42
    7: (bf) r3 = r10                      ; R3_w=fp0 R10=fp0
    8: (0f) r3 += r2
    mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
    mark_precise: frame0: regs=r2 stack= before 7: (bf) r3 = r10
    mark_precise: frame0: regs=r2 stack= before 6: (79) r1 = *(u64 *)(r10 -512)
    mark_precise: frame0: regs=r2 stack= before 5: (79) r2 = *(u64 *)(r10 -520)
    mark_precise: frame0: regs= stack=-8 before 4: (85) call bpf_get_smp_processor_id#8
    mark_precise: frame0: regs= stack=-8 before 3: (7b) *(u64 *)(r10 -520) = r2
    mark_precise: frame0: regs=r2 stack= before 2: (7b) *(u64 *)(r10 -512) = r1
    mark_precise: frame0: regs=r2 stack= before 1: (b7) r2 = 42
    9: R2_w=42 R3_w=fp42
    9: (95) exit

This patch disables the additional allowance for the moment.
Also, two test cases are removed:
- bpf_fastcall_max_stack_ok:
  it fails w/o additional stack allowance;
- bpf_fastcall_max_stack_fail:
  this test is no longer necessary, stack size follows
  regular rules, pattern invalidation is checked by other
  test cases.

Reported-by: Hou Tao <houtao@huaweicloud.com>
Closes: https://lore.kernel.org/bpf/20241023022752.172005-1-houtao@huaweicloud.com/
Fixes: 5b5f51bff1b6 ("bpf: no_caller_saved_registers attribute for helper calls")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20241029193911.1575719-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 2 insertions, 12 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3254c27085b8..bb99bada7e2e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6804,20 +6804,10 @@ static int check_stack_slot_within_bounds(struct bpf_verifier_env *env,
                                           struct bpf_func_state *state,
                                           enum bpf_access_type t)
 {
-	struct bpf_insn_aux_data *aux = &env->insn_aux_data[env->insn_idx];
-	int min_valid_off, max_bpf_stack;
-
-	/* If accessing instruction is a spill/fill from bpf_fastcall pattern,
-	 * add room for all caller saved registers below MAX_BPF_STACK.
-	 * In case if bpf_fastcall rewrite won't happen maximal stack depth
-	 * would be checked by check_max_stack_depth_subprog().
-	 */
-	max_bpf_stack = MAX_BPF_STACK;
-	if (aux->fastcall_pattern)
-		max_bpf_stack += CALLER_SAVED_REGS * BPF_REG_SIZE;
+	int min_valid_off;
 
 	if (t == BPF_WRITE || env->allow_uninit_stack)
-		min_valid_off = -max_bpf_stack;
+		min_valid_off = -MAX_BPF_STACK;
 	else
 		min_valid_off = -state->allocated_stack;