From: Kumar Kartikeya Dwivedi memxor@gmail.com
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 --- kernel/bpf/verifier.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -4847,13 +4847,23 @@ static int check_helper_mem_access(struc 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;