On Thu, Nov 05, 2020 at 12:11:19PM +0100, Marco Elver wrote: > On Thu, 5 Nov 2020 at 11:52, Mark Rutland <mark.rutland@xxxxxxx> wrote: > > On Thu, Nov 05, 2020 at 10:21:33AM +0100, Marco Elver wrote: > > > Instead of removing the fault handling portion of the stack trace based > > > on the fault handler's name, just use struct pt_regs directly. > > > > > > Change kfence_handle_page_fault() to take a struct pt_regs, and plumb it > > > through to kfence_report_error() for out-of-bounds, use-after-free, or > > > invalid access errors, where pt_regs is used to generate the stack > > > trace. > > > > > > If the kernel is a DEBUG_KERNEL, also show registers for more > > > information. > > > > > > Suggested-by: Mark Rutland <mark.rutland@xxxxxxx> > > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx> > > > > Wow; I wasn't expecting this to be put together so quickly, thanks for > > doing this! > > > > From a scan, this looks good to me -- just one question below. > > > > > diff --git a/include/linux/kfence.h b/include/linux/kfence.h > > > index ed2d48acdafe..98a97f9d43cd 100644 > > > --- a/include/linux/kfence.h > > > +++ b/include/linux/kfence.h > > > @@ -171,6 +171,7 @@ static __always_inline __must_check bool kfence_free(void *addr) > > > /** > > > * kfence_handle_page_fault() - perform page fault handling for KFENCE pages > > > * @addr: faulting address > > > + * @regs: current struct pt_regs (can be NULL, but shows full stack trace) > > > * > > > * Return: > > > * * false - address outside KFENCE pool, > > > > > @@ -44,8 +44,12 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries > > > case KFENCE_ERROR_UAF: > > > case KFENCE_ERROR_OOB: > > > case KFENCE_ERROR_INVALID: > > > - is_access_fault = true; > > > - break; > > > + /* > > > + * kfence_handle_page_fault() may be called with pt_regs > > > + * set to NULL; in that case we'll simply show the full > > > + * stack trace. > > > + */ > > > + return 0; > > > > For both the above comments, when/where is kfence_handle_page_fault() > > called with regs set to NULL? I couldn't spot that in this patch, so > > unless I mised it I'm guessing that's somewhere outside of the patch > > context? > > Right, currently it's not expected to happen, but I'd like to permit > this function being called not from fault handlers, for use-cases like > this: > > https://lkml.kernel.org/r/CANpmjNNxAvembOetv15FfZ=04mpj0Qwx+1tnn22tABaHHRRv=Q@xxxxxxxxxxxxxx > > The revised recommendation when trying to get KFENCE to give us more > information about allocation/free stacks after refcount underflow > (like what Paul was trying to do) would be to call > kfence_handle_page_fault(addr, NULL). > > > If this is a case we don't expect to happen, maybe add a WARN_ON_ONCE()? > > While it's currently not expected, I don't see why we should make this > WARN and limit the potential uses of the API if it works just fine if > we pass regs set to NULL. Although arguably the name > kfence_handle_page_fault() might be confusing for such uses, for now, > until more widespread use is evident (if at all) I'd say let's keep > as-is, but simply not prevent such use-cases. Fair enough! I guess in future we could always revise that anyhow. FWIW, for this as-is: Acked-by: Mark Rutland <mark.rutland@xxxxxxx> Mark.
