Messages in this thread | | | Subject | Re: BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM] | From | Daniel Borkmann <> | Date | Wed, 12 Feb 2020 14:27:15 +0100 |
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On 2/12/20 3:45 AM, Alexei Starovoitov wrote: > On Wed, Feb 12, 2020 at 01:09:07AM +0100, Daniel Borkmann wrote: >> >> Another approach could be to have a special nop inside call_int_hook() >> macro which would then get patched to avoid these situations. Somewhat >> similar like static keys where it could be defined anywhere in text but >> with updating of call_int_hook()'s RC for the verdict. > > Sounds nice in theory. I couldn't quite picture how that would look > in the code, so I hacked: > diff --git a/security/security.c b/security/security.c > index 565bc9b67276..ce4bc1e5e26c 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -28,6 +28,7 @@ > #include <linux/string.h> > #include <linux/msg.h> > #include <net/flow.h> > +#include <linux/jump_label.h> > > #define MAX_LSM_EVM_XATTR 2 > > @@ -678,12 +679,26 @@ static void __init lsm_early_task(struct task_struct *task) > * This is a hook that returns a value. > */ > > +#define LSM_HOOK_NAME(FUNC) \ > + DEFINE_STATIC_KEY_FALSE(bpf_lsm_key_##FUNC); > +#include <linux/lsm_hook_names.h> > +#undef LSM_HOOK_NAME > +__diag_push(); > +__diag_ignore(GCC, 8, "-Wstrict-prototypes", ""); > +#define LSM_HOOK_NAME(FUNC) \ > + int bpf_lsm_call_##FUNC() {return 0;} > +#include <linux/lsm_hook_names.h> > +#undef LSM_HOOK_NAME > +__diag_pop(); > + > #define call_void_hook(FUNC, ...) \ > do { \ > struct security_hook_list *P; \ > \ > hlist_for_each_entry(P, &security_hook_heads.FUNC, list) \ > P->hook.FUNC(__VA_ARGS__); \ > + if (static_branch_unlikely(&bpf_lsm_key_##FUNC)) \ > + (void)bpf_lsm_call_##FUNC(__VA_ARGS__); \ > } while (0) > > #define call_int_hook(FUNC, IRC, ...) ({ \ > @@ -696,6 +711,8 @@ static void __init lsm_early_task(struct task_struct *task) > if (RC != 0) \ > break; \ > } \ > + if (RC == IRC && static_branch_unlikely(&bpf_lsm_key_##FUNC)) \ > + RC = bpf_lsm_call_##FUNC(__VA_ARGS__); \
Nit: the `RC == IRC` test could be moved behind the static_branch_unlikely() so that it would be bypassed when not enabled.
> } while (0); \ > RC; \ > }) > > The assembly looks good from correctness and performance points. > union security_list_options can be split into lsm_hook_names.h too > to avoid __diag_ignore. Is that what you have in mind? > I don't see how one can improve call_int_hook() macro without > full refactoring of linux/lsm_hooks.h > imo static_key doesn't have to be there in the first set. We can add this > optimization later.
Yes, like the above diff looks good, and then we'd dynamically attach the program at bpf_lsm_call_##FUNC()'s fexit hook for a direct jump, so all the security_blah() internals could stay as-is which then might also address Jann's concerns wrt concrete annotation as well as potential locking changes inside security_blah(). Agree that patching out via static key could be optional but since you were talking about avoiding indirect jumps..
Thanks, Daniel
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