Commit 97e6d7da authored Mar 19, 2022 by Kumar Kartikeya Dwivedi Committed by Alexei Starovoitov Apr 06, 2022

bpf: Check PTR_TO_MEM | MEM_RDONLY in check_helper_mem_access



The commit being fixed was aiming to disallow users from incorrectly
obtaining writable pointer to memory that is only meant to be read. This
is enforced now using a MEM_RDONLY flag.

For instance, in case of global percpu variables, when the BTF type is
not struct (e.g. bpf_prog_active), the verifier marks register type as
PTR_TO_MEM | MEM_RDONLY from bpf_this_cpu_ptr or bpf_per_cpu_ptr
helpers. However, when passing such pointer to kfunc, global funcs, or
BPF helpers, in check_helper_mem_access, there is no expectation
MEM_RDONLY flag will be set, hence it is checked as pointer to writable
memory. Later, verifier sets up argument type of global func as
PTR_TO_MEM | PTR_MAYBE_NULL, so user can use a global func to get around
the limitations imposed by this flag.

This check will also cover global non-percpu variables that may be
introduced in kernel BTF in future.

Also, we update the log message for PTR_TO_BUF case to be similar to
PTR_TO_MEM case, so that the reason for error is clear to user.

Fixes: 34d3a78c ("bpf: Make per_cpu_ptr return rdonly PTR_TO_MEM.")
Reviewed-by: Hao Luo <haoluo@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220319080827.73251-3-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent be77354a

kernel/bpf/verifier.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -4871,13 +4871,23 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
		return check_map_access(env, regno, reg->off, access_size,
		zero_size_allowed);
		case PTR_TO_MEM:
		if (type_is_rdonly_mem(reg->type)) {
		if (meta && meta->raw_mode) {
		verbose(env, "R%d cannot write into %s\n", regno,
		reg_type_str(env, reg->type));
		return -EACCES;
		}
		}
		return check_mem_region_access(env, regno, reg->off,
		access_size, reg->mem_size,
		zero_size_allowed);
		case PTR_TO_BUF:
		if (type_is_rdonly_mem(reg->type)) {
		if (meta && meta->raw_mode)
		if (meta && meta->raw_mode) {
		verbose(env, "R%d cannot write into %s\n", regno,
		reg_type_str(env, reg->type));
		return -EACCES;
		}

		max_access = &env->prog->aux->max_rdonly_access;
		} else {