bpf: Check PTR_TO_MEM | MEM_RDONLY in check_helper_mem_access

BugLink: https://bugs.launchpad.net/bugs/1981862
commit 97e6d7dab1ca4648821c790a2b7913d6d5d549db upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bd622c47ea2946e31b7c4eedbf15fcffd7b7c03b..92de95006c2d1f54ed439e26b47f63c66d39e8c7 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4602,13 +4602,23 @@ 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;
+                       }
 
                        buf_info = "rdonly";
                        max_access = &env->prog->aux->max_rdonly_access;