Re: [PATCH v4 02/11] x86, kfence: enable KFENCE for x86

[Jann Horn <jannh@google.com>]

On Tue, Sep 29, 2020 at 3:38 PM Marco Elver <elver@google.com> 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.
[...]
> diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h
[...]
> +/* Protect the given page and flush TLBs. */
> +static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +{
> +       unsigned int level;
> +       pte_t *pte = lookup_address(addr, &level);
> +
> +       if (!pte || level != PG_LEVEL_4K)

Do we actually expect this to happen, or is this just a "robustness"
check? If we don't expect this to happen, there should be a WARN_ON()
around the condition.

> +               return false;
> +
> +       if (protect)
> +               set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
> +       else
> +               set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));

Hmm... do we have this helper (instead of using the existing helpers
for modifying memory permissions) to work around the allocation out of
the data section?

> +       flush_tlb_one_kernel(addr);
> +       return true;
> +}
> +
> +#endif /* _ASM_X86_KFENCE_H */
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
[...]
> @@ -701,6 +702,9 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>         }
>  #endif
>
> +       if (kfence_handle_page_fault(address))
> +               return;
> +
>         /*
>          * 32-bit:
>          *

The standard 5 lines of diff context don't really make it obvious
what's going on here. Here's a diff with more context:


        /*
         * Stack overflow?  During boot, we can fault near the initial
         * stack in the direct map, but that's not an overflow -- check
         * that we're in vmalloc space to avoid this.
         */
        if (is_vmalloc_addr((void *)address) &&
            (((unsigned long)tsk->stack - 1 - address < PAGE_SIZE) ||
             address - ((unsigned long)tsk->stack + THREAD_SIZE) < PAGE_SIZE)) {
                unsigned long stack = __this_cpu_ist_top_va(DF) -
sizeof(void *);
                /*
                 * We're likely to be running with very little stack space
                 * left.  It's plausible that we'd hit this condition but
                 * double-fault even before we get this far, in which case
                 * we're fine: the double-fault handler will deal with it.
                 *
                 * We don't want to make it all the way into the oops code
                 * and then double-fault, though, because we're likely to
                 * break the console driver and lose most of the stack dump.
                 */
                asm volatile ("movq %[stack], %%rsp\n\t"
                              "call handle_stack_overflow\n\t"
                              "1: jmp 1b"
                              : ASM_CALL_CONSTRAINT
                              : "D" ("kernel stack overflow (page fault)"),
                                "S" (regs), "d" (address),
                                [stack] "rm" (stack));
                unreachable();
        }
 #endif

+       if (kfence_handle_page_fault(address))
+               return;
+
        /*
         * 32-bit:
         *
         *   Valid to do another page fault here, because if this fault
         *   had been triggered by is_prefetch fixup_exception would have
         *   handled it.
         *
         * 64-bit:
         *
         *   Hall of shame of CPU/BIOS bugs.
         */
        if (is_prefetch(regs, error_code, address))
                return;

        if (is_errata93(regs, address))
                return;

        /*
         * Buggy firmware could access regions which might page fault, try to
         * recover from such faults.
         */
        if (IS_ENABLED(CONFIG_EFI))
                efi_recover_from_page_fault(address);

 oops:
        /*
         * Oops. The kernel tried to access some bad page. We'll have to
         * terminate things with extreme prejudice:
         */
        flags = oops_begin();



Shouldn't kfence_handle_page_fault() happen after prefetch handling,
at least? Maybe directly above the "oops" label?