On Mon, Nov 8, 2021 at 4:31 PM Hao Luo <haoluo@xxxxxxxxxx> wrote: > > There are currently two ways to modify a kernel memory in bpf programs: > 1. declare a ksym of scalar type and directly modify its memory. > 2. Pass a RDONLY_BUF into a helper function which will override > its arguments. For example, bpf_d_path, bpf_snprintf. > > This patchset fixes these two problem. For the first, we introduce a > new reg type PTR_TO_RDONLY_MEM for the scalar typed ksym, which forbids > writing. For the second, we introduce a new arg type ARG_CONST_PTR_TO_MEM > to differentiate the arg types that only read the memory from those > that may write the memory. The previous ARG_PTR_TO_MEM is now only > compatible with writable memories. If a helper doesn't write into its > argument, it can use ARG_CONST_PTR_TO_MEM, which is also compatible > with read-only memories. > > In v2, Andrii suggested using the name "ARG_PTR_TO_RDONLY_MEM", but I > find it is sort of misleading. Because the new arg_type is compatible > with both write and read-only memory. So I chose ARG_CONST_PTR_TO_MEM > instead. I find ARG_CONST_PTR_TO_MEM misleading. It's the difference between `char * const` (const pointer to mutable memory) vs `const char *` (pointer to an immutable memory). We need the latter semantics, and that *is* PTR_TO_RDONLY_MEM in BPF verifier terms. Drawing further analogies from C, you can pass `char *` (pointer to mutable memory) to any function that expects `const char *`, because it's safe to do so, but not the other way. So I don't think it's confusing at all that it is PTR_TO_RDONLY_MEM and that you can pass PTR_TO_MEM register to a helper that expects ARG_PTR_TO_RDONLY_MEM. > > Hao Luo (3): > bpf: Prevent write to ksym memory > bpf: Introduce ARG_CONST_PTR_TO_MEM > bpf/selftests: Test PTR_TO_RDONLY_MEM > > include/linux/bpf.h | 20 +++++- > include/uapi/linux/bpf.h | 4 +- > kernel/bpf/btf.c | 2 +- > kernel/bpf/cgroup.c | 2 +- > kernel/bpf/helpers.c | 12 ++-- > kernel/bpf/ringbuf.c | 2 +- > kernel/bpf/syscall.c | 2 +- > kernel/bpf/verifier.c | 60 +++++++++++++---- > kernel/trace/bpf_trace.c | 26 ++++---- > net/core/filter.c | 64 +++++++++---------- > tools/include/uapi/linux/bpf.h | 4 +- > .../selftests/bpf/prog_tests/ksyms_btf.c | 14 ++++ > .../bpf/progs/test_ksyms_btf_write_check.c | 29 +++++++++ > 13 files changed, 168 insertions(+), 73 deletions(-) > create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_btf_write_check.c > > -- > 2.34.0.rc0.344.g81b53c2807-goog >
