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: 34d3a78c681e ("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>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e9807e6e1090ecdf52e70c3c32be0b321c1c0feb..d953e62b5268d2da5bfe92f014d96786964c87a2 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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 {