On Fri, Oct 30, 2020 at 2:00 PM Marco Elver <elver@xxxxxxxxxx> wrote: > On Fri, 30 Oct 2020 at 03:49, Jann Horn <jannh@xxxxxxxxxx> wrote: > > On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@xxxxxxxxxx> wrote: > > > Add architecture specific implementation details for KFENCE and enable > > > KFENCE for the x86 architecture. In particular, this implements the > > > required interface in <asm/kfence.h> for setting up the pool and > > > providing helper functions for protecting and unprotecting pages. > > > > > > For x86, we need to ensure that the pool uses 4K pages, which is done > > > using the set_memory_4k() helper function. > > > > > > Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> > > > Co-developed-by: Marco Elver <elver@xxxxxxxxxx> > > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx> > > > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx> > > [...] > > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > [...] > > > @@ -725,6 +726,9 @@ no_context(struct pt_regs *regs, unsigned long error_code, > > > if (IS_ENABLED(CONFIG_EFI)) > > > efi_recover_from_page_fault(address); > > > > > > + if (kfence_handle_page_fault(address)) > > > + return; [...] > > Unrelated sidenote: Since we're hooking after exception fixup > > handling, the debug-only KFENCE_STRESS_TEST_FAULTS can probably still > > cause some behavioral differences through spurious faults in places > > like copy_user_enhanced_fast_string (where the exception table entries > > are used even if the *kernel* pointer, not the user pointer, causes a > > fault). But since KFENCE_STRESS_TEST_FAULTS is exclusively for KFENCE > > development, the difference might not matter. And ordering them the > > other way around definitely isn't possible, because the kernel relies > > on being able to fixup OOB reads. So there probably isn't really > > anything we can do better here; it's just something to keep in mind. > > Maybe you can add a little warning to the help text for that Kconfig > > entry that warns people about this? > > Thanks for pointing it out, but that option really is *only* to stress > kfence with concurrent allocations/frees/page faults. If anybody > enables this option for anything other than testing kfence, it's their > own fault. ;-) Sounds fair. :P > I'll try to add a generic note to the Kconfig entry, but what you > mention here seems quite x86-specific. (FWIW, I think it could currently also happen on arm64 in the rare cases where KERNEL_DS is used. But luckily Christoph Hellwig has already gotten rid of most places that did that.)
