Hi Geert, On Wed, Dec 12, 2018 at 04:07:11PM +0300, Dmitry V. Levin wrote: > On Wed, Dec 12, 2018 at 01:54:05PM +0100, Geert Uytterhoeven wrote: > > On Wed, Dec 12, 2018 at 1:37 PM Dmitry V. Levin <l...@altlinux.org> wrote: > > > On Wed, Dec 12, 2018 at 01:27:14PM +0100, Geert Uytterhoeven wrote: > > > > On Wed, Dec 12, 2018 at 1:04 PM Dmitry V. Levin <l...@altlinux.org> > > > > wrote: > > > > > On Wed, Dec 12, 2018 at 10:43:33AM +0100, Geert Uytterhoeven wrote: > > > > > > On Wed, Dec 12, 2018 at 10:27 AM Dmitry V. Levin > > > > > > <l...@altlinux.org> wrote: > > > > > > > On Wed, Dec 12, 2018 at 10:01:29AM +0100, Geert Uytterhoeven > > > > > > > wrote: > > > > > > > > On Wed, Dec 12, 2018 at 9:55 AM Dmitry V. Levin > > > > > > > > <l...@altlinux.org> wrote: > > > > > > > > > On Mon, Dec 10, 2018 at 04:30:25PM +0300, Dmitry V. Levin > > > > > > > > > wrote: > > > > > > > > > > On Mon, Dec 10, 2018 at 02:06:28PM +0100, Geert > > > > > > > > > > Uytterhoeven wrote: > > > > > > > > > > > On Mon, Dec 10, 2018 at 1:41 PM Dmitry V. Levin > > > > > > > > > > > <l...@altlinux.org> wrote: > > > > > > > > > > > > On Mon, Dec 10, 2018 at 09:45:42AM +0100, Geert > > > > > > > > > > > > Uytterhoeven wrote: > > > > > > > > > > > > > On Mon, Dec 10, 2018 at 5:30 AM Dmitry V. Levin > > > > > > > > > > > > > <l...@altlinux.org> wrote: > > > > > > > > > > > > > > syscall_get_* functions are required to be > > > > > > > > > > > > > > implemented on all > > > > > > > > > > > > > > architectures in order to extend the generic ptrace > > > > > > > > > > > > > > API with > > > > > > > > > > > > > > PTRACE_GET_SYSCALL_INFO request. > > > > > > > > > > > > > > > > > > > > > > > > > > > > This introduces asm/syscall.h on m68k implementing > > > > > > > > > > > > > > all 5 syscall_get_* > > > > > > > > > > > > > > functions as documented in asm-generic/syscall.h: > > > > > > > > > > > > > > syscall_get_nr, > > > > > > > > > > > > > > syscall_get_arguments, syscall_get_error, > > > > > > > > > > > > > > syscall_get_return_value, > > > > > > > > > > > > > > and syscall_get_arch. > > > > > > > > > > > > > > > > > > > > > > > > > > > > Cc: Geert Uytterhoeven <ge...@linux-m68k.org> > > > > > > > > > > > > > > Cc: Oleg Nesterov <o...@redhat.com> > > > > > > > > > > > > > > Cc: Andy Lutomirski <l...@kernel.org> > > > > > > > > > > > > > > Cc: Elvira Khabirova <lineprin...@altlinux.org> > > > > > > > > > > > > > > Cc: Eugene Syromyatnikov <e...@redhat.com> > > > > > > > > > > > > > > Cc: linux-m...@lists.linux-m68k.org > > > > > > > > > > > > > > Signed-off-by: Dmitry V. Levin <l...@altlinux.org> > > > > > > > > > > > > > > --- > > > > > > > > > > > > > > > > > > > > > > > > > > > > Notes: > > > > > > > > > > > > > > v5: added syscall_get_nr, > > > > > > > > > > > > > > syscall_get_arguments, syscall_get_error, > > > > > > > > > > > > > > and syscall_get_return_value > > > > > > > > > > > > > > v1: added syscall_get_arch > > > > > > > > > > > > > > > > > > > > > > > > > > > --- /dev/null > > > > > > > > > > > > > > +++ b/arch/m68k/include/asm/syscall.h > > > > > > > > > > > > > > @@ -0,0 +1,39 @@ > > > > > > > > > > > > > > > > > > > > > > > > > > > +static inline void > > > > > > > > > > > > > > +syscall_get_arguments(struct task_struct *task, > > > > > > > > > > > > > > struct pt_regs *regs, > > > > > > > > > > > > > > + unsigned int i, unsigned int > > > > > > > > > > > > > > n, unsigned long *args) > > > > > > > > > > > > > > +{ > > > > > > > > > > > > > > + BUG_ON(i + n > 6); > > > > > > > > > > > > > > > > > > > > > > > > > > Does this have to crash the kernel? > > > > > > > > > > > > > > > > > > > > > > > > This is what most of other architectures do, but we > > > > > > > > > > > > could choose > > > > > > > > > > > > a softer approach, e.g. use WARN_ON_ONCE instead. > > > > > > > > > > > > > > > > > > > > > > > > > Perhaps you can return an error code instead? > > > > > > > > > > > > > > > > > > > > > > > > That would be problematic given the signature of this > > > > > > > > > > > > function > > > > > > > > > > > > and the nature of the potential bug which would most > > > > > > > > > > > > likely be a usage error. > > > > > > > > > > > > > > > > > > > > > > Of course to handle that, the function's signature need > > > > > > > > > > > to be changed. > > > > > > > > > > > Changing it has the advantage that the error handling can > > > > > > > > > > > be done at the > > > > > > > > > > > caller, in common code, instead of duplicating it for all > > > > > > > > > > > architectures, possibly > > > > > > > > > > > leading to different semantics. > > > > > > > > > > > > > > > > > > > > Given that *all* current users of syscall_get_arguments > > > > > > > > > > specify i == 0 > > > > > > > > > > (and there is an architecture that has BUG_ON(i)), > > > > > > > > > > it should be really a usage error to get into situation > > > > > > > > > > where i + n > 6, > > > > > > > > > > I wish a BUILD_BUG_ON could be used here instead. > > > > > > > > > > > > > > > > > > > > I don't think it worths pushing the change of API just to > > > > > > > > > > convert > > > > > > > > > > a "cannot happen" assertion into an error that would have > > > > > > > > > > to be dealt with > > > > > > > > > > on the caller side. > > > > > > > > > > > > > > > > > > I suggest the following BUG_ON replacement for > > > > > > > > > syscall_get_arguments: > > > > > > > > > > > > > > > > > > #define SYSCALL_MAX_ARGS 6 > > > > > > > > > > > > > > > > > > static inline void > > > > > > > > > syscall_get_arguments(struct task_struct *task, struct > > > > > > > > > pt_regs *regs, > > > > > > > > > unsigned int i, unsigned int n, > > > > > > > > > unsigned long *args) > > > > > > > > > { > > > > > > > > > /* > > > > > > > > > * Ideally there should have been > > > > > > > > > * BUILD_BUG_ON(i + n > SYSCALL_MAX_ARGS); > > > > > > > > > * instead of these checks. > > > > > > > > > */ > > > > > > > > > if (unlikely(i > SYSCALL_MAX_ARGS)) { > > > > > > > > > WARN_ONCE(1, "i > SYSCALL_MAX_ARGS"); > > > > > > > > > return; > > > > > > > > > > > > > > > > Does this have security implications, as args is an output > > > > > > > > parameter? > > > > > > > > I.e. if you don't fill the array, the caller will use whatever > > > > > > > > is on the stack. > > > > > > > > Can this ever be passed to userspace, leaking data? > > > > > > > > > > > > > > In the current kernel code n is always less or equal to 6, > > > > > > > but in theory future changes can potentially break the assertion > > > > > > > and this could lead to leaking data to userspace. > > > > > > > > > > > > OK. > > > > > > > > > > > > > Do you think we should rather be defensive and add some memsets, > > > > > > > e.g. > > > > > > > > > > > > > > if (unlikely(i > SYSCALL_MAX_ARGS)) { > > > > > > > WARN_ONCE(1, "i > SYSCALL_MAX_ARGS"); > > > > > > > memset(args, 0, n * sizeof(args[0])); > > > > > > > return; > > > > > > > } > > > > > > > if (unlikely(n > SYSCALL_MAX_ARGS - i)) { > > > > > > > unsigned int extra = n - (SYSCALL_MAX_ARGS - i); > > > > > > > > > > > > > > WARN_ONCE(1, "i + n > SYSCALL_MAX_ARGS"); > > > > > > > n = SYSCALL_MAX_ARGS - i; > > > > > > > memset(&args[n], 0, extra * sizeof(args[0])); > > > > > > > } > > > > > > > ? > > > > > > > > > > > > Yes please. > > > > > > > > > > > > But please handle all of that in the generic code, so it doesn't > > > > > > have to be > > > > > > replicated across all architectures. > > > > > > > > > > > > E.g. make syscall_get_arguments() a wrapper in generic code, calling > > > > > > __syscall_get_arguments() in architecture-specific code. > > > > > > > > > > > > And make the latter return int, so it can indicate other failures. > > > > > > > > > > Other failures? What syscall_get_arguments is expected to do > > > > > if __syscall_get_arguments returned, say, -1? > > > > > > > > Fail. Just like in case of other generic ill conditions it can detect > > > > itself. > > > > > > Sorry, I don't quite follow. syscall_get_arguments() has no return code, > > > > Which may be an indicator for a different problem. > > What is e.g. populate_seccomp_data() supposed to do if > > syscall_get_arguments() fails? > > Well, syscall_get_arguments() is not supposed to fail if invoked properly. > > Currently populate_seccomp_data() does this: > struct task_struct *task = current; > struct pt_regs *regs = task_pt_regs(task); > unsigned long args[6]; > ... > syscall_get_arguments(task, regs, 0, 6, args); > > I don't see how this could fail. > > > > so all it can possibly do is to zero out args[], e.g. > > > > > > if (unlikely(__syscall_get_arguments(task, regs, i, n, args) < > > > 0)) { > > > memset(args, 0, n * sizeof(args[0])); > > > return; > > > } > > > > > > Do you mean this? > > > > Exactly. > > OK, I'll prepare the change, thanks.
I have the change ready, but I don't like it. The only architecture that could benefit from being able of signalling an error condition to syscall_get_arguments is MIPS, and even in that case the return code is not suitable because it wouldn't help to distinguish between the first 4 syscall arguments that cannot cause an error and remaining arguments that can. It looks like there is no need to make __syscall_get_arguments() to return int after all. -- ldv
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