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 <[email protected]>
Closes: https://lore.kernel.org/bpf/[email protected]/
Fixes: 5b5f51b ("bpf: no_caller_saved_registers attribute for helper calls")
Signed-off-by: Eduard Zingerman <[email protected]>

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;
 

tools/testing/selftests/bpf/progs/verifier_bpf_fastcall.c

@@ -790,61 +790,6 @@ __naked static void cumulative_stack_depth_subprog(void)
 	:: __imm(bpf_get_smp_processor_id) : __clobber_all);
 }
 
-SEC("raw_tp")
-__arch_x86_64
-__log_level(4)
-__msg("stack depth 512")
-__xlated("0: r1 = 42")
-__xlated("1: *(u64 *)(r10 -512) = r1")
-__xlated("2: w0 = ")
-__xlated("3: r0 = &(void __percpu *)(r0)")
-__xlated("4: r0 = *(u32 *)(r0 +0)")
-__xlated("5: exit")
-__success
-__naked int bpf_fastcall_max_stack_ok(void)
-{
-	asm volatile(
-	"r1 = 42;"
-	"*(u64 *)(r10 - %[max_bpf_stack]) = r1;"
-	"*(u64 *)(r10 - %[max_bpf_stack_8]) = r1;"
-	"call %[bpf_get_smp_processor_id];"
-	"r1 = *(u64 *)(r10 - %[max_bpf_stack_8]);"
-	"exit;"
-	:
-	: __imm_const(max_bpf_stack, MAX_BPF_STACK),
-	  __imm_const(max_bpf_stack_8, MAX_BPF_STACK + 8),
-	  __imm(bpf_get_smp_processor_id)
-	: __clobber_all
-	);
-}
-
-SEC("raw_tp")
-__arch_x86_64
-__log_level(4)
-__msg("stack depth 520")
-__failure
-__naked int bpf_fastcall_max_stack_fail(void)
-{
-	asm volatile(
-	"r1 = 42;"
-	"*(u64 *)(r10 - %[max_bpf_stack]) = r1;"
-	"*(u64 *)(r10 - %[max_bpf_stack_8]) = r1;"
-	"call %[bpf_get_smp_processor_id];"
-	"r1 = *(u64 *)(r10 - %[max_bpf_stack_8]);"
-	/* call to prandom blocks bpf_fastcall rewrite */
-	"*(u64 *)(r10 - %[max_bpf_stack_8]) = r1;"
-	"call %[bpf_get_prandom_u32];"
-	"r1 = *(u64 *)(r10 - %[max_bpf_stack_8]);"
-	"exit;"
-	:
-	: __imm_const(max_bpf_stack, MAX_BPF_STACK),
-	  __imm_const(max_bpf_stack_8, MAX_BPF_STACK + 8),
-	  __imm(bpf_get_smp_processor_id),
-	  __imm(bpf_get_prandom_u32)
-	: __clobber_all
-	);
-}
-
 SEC("cgroup/getsockname_unix")
 __xlated("0: r2 = 1")
 /* bpf_cast_to_kern_ctx is replaced by a single assignment */