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SubjectRe: [PATCH bpf-next v1 06/13] bpf: lsm: Init Hooks and create files in securityfs
On 23-Dec 22:28, Andrii Nakryiko wrote:
> On Fri, Dec 20, 2019 at 7:43 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > The LSM creates files in securityfs for each hook registered with the
> > LSM.
> >
> > /sys/kernel/security/bpf/<h_name>
> >
> > The list of LSM hooks are maintained in an internal header "hooks.h"
> > Eventually, this list should either be defined collectively in
> > include/linux/lsm_hooks.h or auto-generated from it.
> >
> > * Creation of a file for the hook in the securityfs.
> > * Allocation of a bpf_lsm_hook data structure which stores
> > a pointer to the dentry of the newly created file in securityfs.
> > * Creation of a typedef for the hook so that BTF information
> > can be generated for the LSM hooks to:
> >
> > - Make them "Compile Once, Run Everywhere".
> > - Pass the right arguments when the attached programs are run.
> > - Verify the accesses made by the program by using the BTF
> > information.
> >
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > ---
> > include/linux/bpf_lsm.h | 12 +
> > security/bpf/Makefile | 4 +-
> > security/bpf/include/bpf_lsm.h | 63 ++
> > security/bpf/include/fs.h | 23 +
> > security/bpf/include/hooks.h | 1015 ++++++++++++++++++++++++++++++++
> > security/bpf/lsm.c | 138 ++++-
> > security/bpf/lsm_fs.c | 82 +++
> > 7 files changed, 1333 insertions(+), 4 deletions(-)
> > create mode 100644 include/linux/bpf_lsm.h
> > create mode 100644 security/bpf/include/bpf_lsm.h
> > create mode 100644 security/bpf/include/fs.h
> > create mode 100644 security/bpf/include/hooks.h
> > create mode 100644 security/bpf/lsm_fs.c
> >
>
> [...]
>
> > +
> > +/*
> > + * The hooks can have an int or void return type, these macros allow having a
> > + * single implementation of DEFINE_LSM_HOOK irrespective of the return type.
> > + */
> > +#define LSM_HOOK_RET(ret, x) LSM_HOOK_RET_##ret(x)
> > +#define LSM_HOOK_RET_int(x) x
> > +#define LSM_HOOK_RET_void(x)
> > +
> > +/*
> > + * This macro defines the body of a LSM hook which runs the eBPF programs that
> > + * are attached to the hook and returns the error code from the eBPF programs if
> > + * the return type of the hook is int.
> > + */
> > +#define DEFINE_LSM_HOOK(hook, ret, proto, args) \
> > +typedef ret (*lsm_btf_##hook)(proto); \
> > +static ret bpf_lsm_##hook(proto) \
> > +{ \
> > + return LSM_HOOK_RET(ret, LSM_RUN_PROGS(hook##_type, args)); \
> > }
>
> I'm probably missing something, but according to LSM_HOOK_RET
> definition for when ret==void, bpf_lsm_##hook will be a noop and won't
> call any BPF program. Did I miss some additional macro magic?
>

Good catch! You're right. These macros will not be there in v2 as
we move to using trampolines based callbacks.

> >
> > +/*
> > + * Define the body of each of the LSM hooks defined in hooks.h.
> > + */
> > +#define BPF_LSM_HOOK(hook, ret, args, proto) \
> > + DEFINE_LSM_HOOK(hook, ret, BPF_LSM_ARGS(args), BPF_LSM_ARGS(proto))
> > +#include "hooks.h"
> > +#undef BPF_LSM_HOOK
> > +#undef DEFINE_LSM_HOOK
> > +
> > +/*
> > + * Initialize the bpf_lsm_hooks_list for each of the hooks defined in hooks.h.
> > + * The list contains information for each of the hook and can be indexed by the
> > + * its type to initialize security FS, attach, detach and execute eBPF programs
> > + * for the hook.
> > + */
> > +struct bpf_lsm_hook bpf_lsm_hooks_list[] = {
> > + #define BPF_LSM_HOOK(h, ...) \
> > + [h##_type] = { \
> > + .h_type = h##_type, \
> > + .mutex = __MUTEX_INITIALIZER( \
> > + bpf_lsm_hooks_list[h##_type].mutex), \
> > + .name = #h, \
> > + .btf_hook_func = \
> > + (void *)(lsm_btf_##h)(bpf_lsm_##h), \
>
> this btf_hook_func, is it assigned just so that type information for
> bpf_lsm_xxx typedefs are preserved, is that right? It doesn't seem to
> be ever called or read. If I'm not missing anything, check out
> Martin's latest STRUCT_OPS patch set. He defines EMIT_TYPE_INFO(type)
> macro, which will ensure that BTF for specified type is emitted into
> vmlinux BTF, without actually using any extra space, defining extra
> fields or static variables, etc. I suggest using the same for the
> cleanest result.
>
> One more thing regarding lsm_bpf_ typedefs. Currently you are defining
> them as a pointer to func_proto, matching LSM hook. There is an
> alternative approach, which has few benefits over using func_proto. If
> instead you define a struct, where each argument of func prototype is
> represented as 8-byte aligned field, this will contain all the
> necessary information for BPF verifier to do its job (just like
> func_proto). But in addition to that, when vmlinux.h is generated, it
> will contain a nice struct bpf_lsm_<hook_name> with correct structure
> to be used **directly** in BPF program, as a single context argument.
> So with vmlinux.h, users won't have to re-define all the argument
> types and names in their BPF_TRACE_x definition. Let me provide
> concrete example from your cover letter. This is what you provide as
> an example:

Is this also doable for the new approach suggsted by Alexei
and prototyped in?

https://lore.kernel.org/bpf/CAEf4BzYiUZtSJKh-UBL0jwyo6d=Cne2YtEyGU8ONykmSUSsuNA@mail.gmail.com/T/#m7c7ec0e7d8e803c6c357495d9eea59028a67cac6

which uses trampolines. The new approach gets rid of any type
generation and macros in security/bpf/lsm_hooks.h. Maybe the
btf_vmlinux can be augmented at runtime to generate context struct
upon attachment?

>
> BPF_TRACE_3("lsm/file_mprotect", mprotect_audit,
> struct vm_area_struct *, vma,
> unsigned long, reqprot, unsigned long, prot) {...}
>
> on kernel side, you'll have:
>
> typedef int (*bpf_lsm_file_mprotect)(struct vm_area_struct *vma,
> unsigned long reqprot,
> unsigned long prot);
>
> So you can see that user has to go and copy/paste all the arguments
> and their types and paste them in this verbose BPF_TRACE_3 macro to
> define correct BPF program.
>
> Now, imagine that instead of typedef above, we define equivalent struct:
>
> struct bpf_lsm_file_mprotect {
> struct vm_area_struct *vma;
> unsigned long reqprot;
> unsigned long prot;
> };
>
> This type will get dumped into vmlinux.h, which can be used from BPF
> user code as such:
>
> SEC("lsm/file_mprotect")
> int mprotect_audito(struct bpf_lsm_file_mprotect *ctx)
> {
> ... here you can use ctx->vma, ctx->reqprot, ctx->prot ...
> }
>
>
> Meanwhile, there will be just minimal changes to BPF verifier to use
> such struct instead of func_proto for verification of LSM programs.
>
> We currently have similar issue with raw_tp programs and I've been
> thinking about switching that to structs instead of func_proto, so we
> might as well coordinate that and reuse the same logic in BPF
> verifier.
>
> Thoughts?

Thanks for the explanation!

Using structs is definitely better if we chose to go with static type
generation.

- KP

>
>
>
> > + },
> > + #include "hooks.h"
> > + #undef BPF_LSM_HOOK
> > +};
> > +
>
> [...]

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