Re: [PATCH bpf] bpf: Add extra path pointer check to d_path helper

Jiri Olsa
2023-06-06T07:08:57Z

On Mon, Jun 05, 2023 at 04:00:16PM -0700, Alexei Starovoitov wrote:
> On Sun, Jun 4, 2023 at 7:01 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Anastasios reported crash on stable 5.15 kernel with following
> > bpf attached to lsm hook:
> >
> > SEC("lsm.s/bprm_creds_for_exec")
> > int BPF_PROG(bprm_creds_for_exec, struct linux_binprm *bprm)
> > {
> > struct path *path = &bprm->executable->f_path;
> > char p[128] = { 0 };
> >
> > bpf_d_path(path, p, 128);
> > return 0;
> > }
> >
> > but bprm->executable can be NULL, so bpf_d_path call will crash:
> >
> > BUG: kernel NULL pointer dereference, address: 0000000000000018
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> > Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
> > ...
> > RIP: 0010:d_path+0x22/0x280
> > ...
> > Call Trace:
> > <TASK>
> > bpf_d_path+0x21/0x60
> > bpf_prog_db9cf176e84498d9_bprm_creds_for_exec+0x94/0x99
> > bpf_trampoline_6442506293_0+0x55/0x1000
> > bpf_lsm_bprm_creds_for_exec+0x5/0x10
> > security_bprm_creds_for_exec+0x29/0x40
> > bprm_execve+0x1c1/0x900
> > do_execveat_common.isra.0+0x1af/0x260
> > __x64_sys_execve+0x32/0x40
> >
> > It's problem for all stable trees with bpf_d_path helper, which was
> > added in 5.9.
> >
> > This issue is fixed in current bpf code, where we identify and mark
> > trusted pointers, so the above code would fail to load.
> >
> > For the sake of the stable trees and to workaround potentially broken
> > verifier in the future, adding the code that reads the path object from
> > the passed pointer and verifies it's valid in kernel space.
> >
> > Cc: stable@vger.kernel.org # v5.9+
> > Fixes: 6e22ab9da793 ("bpf: Add d_path helper")
> > Suggested-by: Alexei Starovoitov <ast@kernel.org>
> > Reported-by: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > kernel/trace/bpf_trace.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 9a050e36dc6c..aecd98ee73dc 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -900,12 +900,22 @@ static const struct bpf_func_proto bpf_send_signal_thread_proto = {
> >
> > BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> > {
> > + struct path copy;
> > long len;
> > char *p;
> >
> > if (!sz)
> > return 0;
> >
> > + /*
> > + * The path pointer is verified as trusted and safe to use,
> > + * but let's double check it's valid anyway to workaround
> > + * potentially broken verifier.
> > + */
> > + len = copy_from_kernel_nofault(&copy, path, sizeof(*path));
> > + if (len < 0)
> > + return len;
> > +
> > p = d_path(path, buf, sz);
>
> Since we copied it anyway, let's use a stable copy here?
> Otherwise somebody might send a patch to remove 'dead code'.

ok, will change that

jirka