[bpf-next v3 11/12] bpf: do check_nocsr_stack_contract() for ARG_ANYTHING helper params

[Eduard Zingerman <eddyz87@gmail.com>]

There is a number of BPF helper functions that use ARG_ANYTHING to
mark parameters that are used as memory addresses.
An address of BPF stack slot could be passed as such parameter
for two such helper functions:
- bpf_probe_read_kernel
- bpf_probe_read_kernel_str

This might lead to a surprising behavior in combination with nocsr
rewrites, e.g. consider the program below:

     1: r1 = 1;
        /* nocsr pattern with stack offset -16 */
     2: *(u64 *)(r10 - 16) = r1;
     3: call %[bpf_get_smp_processor_id];
     4: r1 = *(u64 *)(r10 - 16);
     5: r1 = r10;
     6: r1 += -8;
     7: r2 = 1;
     8: r3 = r10;
     9: r3 += -16;
        /* bpf_probe_read_kernel(dst: &fp[-8], size: 1, src: &fp[-16]) */
    10: call %[bpf_probe_read_kernel];
    11: exit;

Here nocsr rewrite logic would remove instructions (2) and (4).
However, (2) writes a value that is later read by a call at (10).

Function check_func_arg() is called from check_helper_call() and is
responsible for memory access checks, when helper argument type is
declared as ARG_PTR_TO_... .

However, for ARG_ANYTHING this function returns early and does not
call check_stack_{read,write}().

This patch opts to add a check_nocsr_stack_contract() to the
ARG_ANYTHING return path if passed parameter is a pointer to stack.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 438daf36a694..77affc563a64 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8684,11 +8684,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		return err;
 
 	if (arg_type == ARG_ANYTHING) {
+		/* return value depends on env->allow_ptr_leaks */
 		if (is_pointer_value(env, regno)) {
 			verbose(env, "R%d leaks addr into helper function\n",
 				regno);
 			return -EACCES;
 		}
+		if (reg->type == PTR_TO_STACK)
+			check_nocsr_stack_contract(env, cur_func(env), insn_idx,
+						   reg->smin_value + reg->off);
 		return 0;
 	}
 
-- 
2.45.2