Messages in this thread | | | Date | Mon, 6 Jul 2020 09:15:51 +0100 | From | Will Deacon <> | Subject | Re: ptrace: seccomp: Return value when the call was already invalid |
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On Sat, Jul 04, 2020 at 09:56:50PM -0700, Kees Cook wrote: > On Sat, Jul 04, 2020 at 01:33:56PM +0100, Will Deacon wrote: > > On Fri, Jul 03, 2020 at 08:52:05AM -0700, Kees Cook wrote: > > > On Fri, Jul 03, 2020 at 04:44:27PM +0100, Will Deacon wrote: > > > > On Fri, Jul 03, 2020 at 08:17:19AM -0700, Kees Cook wrote: > > > > > On Fri, Jul 03, 2020 at 09:39:14AM +0100, Will Deacon wrote: > > > > > > diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c > > > > > > index 5f5b868292f5..a13661f44818 100644 > > > > > > --- a/arch/arm64/kernel/syscall.c > > > > > > +++ b/arch/arm64/kernel/syscall.c > > > > > > @@ -121,12 +121,10 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr, > > > > > > user_exit(); > > > > > > > > > > > > if (has_syscall_work(flags)) { > > > > > > - /* set default errno for user-issued syscall(-1) */ > > > > > > - if (scno == NO_SYSCALL) > > > > > > - regs->regs[0] = -ENOSYS; > > > > > > - scno = syscall_trace_enter(regs); > > > > > > - if (scno == NO_SYSCALL) > > > > > > + if (syscall_trace_enter(regs)) > > > > > > goto trace_exit; > > > > > > + > > > > > > + scno = regs->syscallno; > > > > > > } > > > > > > > > > > > > invoke_syscall(regs, scno, sc_nr, syscall_table); > > > > > > > > > > What effect do either of these patches have on the existing seccomp > > > > > selftests: tools/testing/selftests/seccomp/seccomp_bpf ? > > > > > > > > Tests! Thanks, I'll have a look. > > > > > > Thanks! > > > > > > (And either way, that this behavioral difference went unnoticed means we > > > need to add a test to the selftests for this patch.) > > > > Unsurprisingly, I don't think the tests go near this. I get 75/77 passes > > on arm64 defconfig with or without these changes. > > (What doesn't pass for you? I tried to go find kernelci.org test output, > but it doesn't appear to actually run selftests yet?) > > Anyway, good that the test output doesn't change, bad that seccomp has > missed a corner of this architecture interface. (i.e. the entire > TRACE_syscall fixture is dedicated to exercising the changing/skipping > interface, but I see now that it doesn't at all exercise any area of > ENOSYS results.) > > > We could add a test, but then we'd have to agree on what it's supposed to > > be doing ;) > > Well, if you look at change_syscall() in seccomp_bpf.c (once you stop > screaming) you'll likely share my desire to have more things that are > common across architectures. ;) So, to that end, yes, please, let's > define what we'd like to see, and then build out the (likely wildly > different per-architecture expectations). If I read this thread > correctly, we need to test: > > syscall(-1), direct, returns ENOSYS > syscall(-10), direct, returns ENOSYS > syscall(-1), SECCOMP_RET_TRACE+PTRACE_CONT, returns ENOSYS > syscall(-10), SECCOMP_RET_TRACE+PTRACE_CONT, returns ENOSYS > syscall(-1), ptrace+PTRACE_SYSCALL, returns ENOSYS > syscall(-10), ptrace+PTRACE_SYSCALL, returns ENOSYS > > do we need to double-check that registers before/after are otherwise > unchanged too? (I *think* just looking at syscall return should be > sufficient to catch the visible results.)
There's also the case where the tracer sets the system call to -1 to skip it.
Will
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