bpf: Clobber stack slot when writing over spilled PTR_TO_BTF_ID

When support was added for spilled PTR_TO_BTF_ID to be accessed by
helper memory access, the stack slot was not overwritten to STACK_MISC
(and that too is only safe when env->allow_ptr_leaks is true).

This means that helpers who take ARG_PTR_TO_MEM and write to it may
essentially overwrite the value while the verifier continues to track
the slot for spilled register.

This can cause issues when PTR_TO_BTF_ID is spilled to stack, and then
overwritten by helper write access, which can then be passed to BPF
helpers or kfuncs.

Handle this by falling back to the case introduced in a later commit,
which will also handle PTR_TO_BTF_ID along with other pointer types,
i.e. cd17d38f ("bpf: Permits pointers on stack for helper calls").

Finally, include a comment on why REG_LIVE_WRITTEN is not being set when
clobber is set to true. In short, the reason is that while when clobber
is unset, we know that we won't be writing, when it is true, we *may*
write to any of the stack slots in that range. It may be a partial or
complete write, to just one or many stack slots.

We cannot be sure, hence to be conservative, we leave things as is and
never set REG_LIVE_WRITTEN for any stack slot. However, clobber still
needs to reset them to STACK_MISC assuming writes happened. However read
marks still need to be propagated upwards from liveness point of view,
as parent stack slot's contents may still continue to matter to child
states.

Cc: Yonghong Song <yhs@meta.com>
Fixes: 1d68f22b ("bpf: Handle spilled PTR_TO_BTF_ID properly when checking stack_boundary")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221103191013.1236066-4-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -5154,10 +5154,6 @@ static int check_stack_range_initialized(
 			goto mark;
 		}
 
-		if (is_spilled_reg(&state->stack[spi]) &&
-		    base_type(state->stack[spi].spilled_ptr.type) == PTR_TO_BTF_ID)
-			goto mark;
-
 		if (is_spilled_reg(&state->stack[spi]) &&
 		    (state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
 		     env->allow_ptr_leaks)) {
@@ -5188,6 +5184,11 @@ static int check_stack_range_initialized(
 		mark_reg_read(env, &state->stack[spi].spilled_ptr,
 			      state->stack[spi].spilled_ptr.parent,
 			      REG_LIVE_READ64);
+		/* We do not set REG_LIVE_WRITTEN for stack slot, as we can not
+		 * be sure that whether stack slot is written to or not. Hence,
+		 * we must still conservatively propagate reads upwards even if
+		 * helper may write to the entire memory range.
+		 */
 	}
 	return update_stack_depth(env, state, min_off);
 }