Re: [PATCH bpf-next v8 0/8] MAC and Audit policy using eBPF (KRSI)

[KP Singh]

On 28-Mar 23:30, KP Singh wrote:
> On Sat, Mar 28, 2020 at 10:50 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > On Sat, Mar 28, 2020 at 08:56:36PM +0100, KP Singh wrote:
> > > Since the attachment succeeds and the hook does not get called, it
> > > seems like "bpf" LSM is not being initialized and the hook, although
> > > present, does not get called.
> > >
> > > This indicates that "bpf" is not in CONFIG_LSM. It should, however, be
> > > there by default as we added it to default value of CONFIG_LSM and
> > > also for other DEFAULT_SECURITY_* options.
> > >
> > > Let me know if that's the case and it fixes it.
> >
> > Is the selftest expected to at least fail cleanly (i.e. not segfault)
> 
> I am not sure where the crash comes from, it does not look like it's test_lsm,
> it seems to happen in test_overhead. Both seem to run fine for me.

So I was able to reproduce the crash:

* Remove "bpf" from CONFIG_LSM

./test_progs -n 66,67
test_test_lsm:PASS:skel_load 0 nsec
test_test_lsm:PASS:attach 0 nsec
test_test_lsm:PASS:exec_cmd 0 nsec
test_test_lsm:FAIL:bprm_count bprm_count = 0
test_test_lsm:FAIL:heap_mprotect want errno=EPERM, got 0
#66 test_lsm:FAIL
Caught signal #11!
Stack trace:
./test_progs(crash_handler+0x1f)[0x55b7f9867acf]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x13520)[0x7fcf1467e520]
/lib/x86_64-linux-gnu/libc.so.6(+0x15f73d)[0x7fcf1460a73d]
/lib/x86_64-linux-gnu/libc.so.6(__libc_calloc+0x2ca)[0x7fcf1453286a]
/usr/lib/x86_64-linux-gnu/libelf.so.1(+0x37

[snip]

* The crash went away when I removed the heap_mprotect call, now the BPF
  hook attached did not allow this operation, so it had no side-effects.
  Which lead me to believe the crash could be a side-effect of this
  operation. So I did:

--- a/tools/testing/selftests/bpf/prog_tests/test_lsm.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_lsm.c
@@ -29,7 +29,7 @@ int heap_mprotect(void)
        if (buf == NULL)
                return -ENOMEM;

-       ret = mprotect(buf, sz, PROT_READ | PROT_EXEC);
+       ret = mprotect(buf, sz, PROT_READ | PROT_WRITE | PROT_EXEC);
        free(buf);
        return ret;
 }

and the crash went away. Which made me realize that the free
operation does not like memory without PROT_WRITE, So I did this:

diff --git a/tools/testing/selftests/bpf/prog_tests/test_lsm.c b/tools/testing/selftests/bpf/prog_tests/test_lsm.c
index fcd839e88540..78f125cc09b3 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_lsm.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_lsm.c
@@ -30,7 +30,7 @@ int heap_mprotect(void)
                return -ENOMEM;

        ret = mprotect(buf, sz, PROT_READ | PROT_EXEC);
-       free(buf);
+       // free(buf);
        return ret;
 }

and the crash went away as well. So it indeed was a combination of:

* CONFIG_LSM not enabling the hook
* mprotect marking the memory as non-writeable
* free being called on the memory.

I will send a v9 which has the PROT_WRITE on the mprotect. Thanks
for noticing this!

- KP

> 
> - KP
> 
> > when the BPF LSF is not built into the kernel?
> >
> > --
> > Kees Cook