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

Alexei Starovoitov
2023-06-05T23:00:32Z

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'.