Re: [PATCH v2 06/20] x86/alternative: use temporary mm for text poking

Borislav Petkov
2019-02-05T09:59:06Z

On Mon, Jan 28, 2019 at 04:34:08PM -0800, Rick Edgecombe wrote:
> From: Nadav Amit <namit@vmware.com>
>
> text_poke() can potentially compromise the security as it sets temporary

s/the //

> PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
> from other cores accidentally or maliciously, if an attacker gains the
> ability to write onto kernel memory.

Eww, sneaky. That would be a really nasty attack.

> Moreover, since remote TLBs are not flushed after the temporary PTEs are
> removed, the time-window in which the code is writable is not limited if
> the fixmap PTEs - maliciously or accidentally - are cached in the TLB.
> To address these potential security hazards, we use a temporary mm for
> patching the code.
>
> Finally, text_poke() is also not conservative enough when mapping pages,
> as it always tries to map 2 pages, even when a single one is sufficient.
> So try to be more conservative, and do not map more than needed.
>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
> arch/x86/include/asm/fixmap.h | 2 -
> arch/x86/kernel/alternative.c | 106 +++++++++++++++++++++++++++-------
> arch/x86/xen/mmu_pv.c | 2 -
> 3 files changed, 84 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index 50ba74a34a37..9da8cccdf3fb 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -103,8 +103,6 @@ enum fixed_addresses {
> #ifdef CONFIG_PARAVIRT
> FIX_PARAVIRT_BOOTMAP,
> #endif
> - FIX_TEXT_POKE1, /* reserve 2 pages for text_poke() */
> - FIX_TEXT_POKE0, /* first page is last, because allocation is backward */

Two fixmap slots less - good riddance. :)

> #ifdef CONFIG_X86_INTEL_MID
> FIX_LNW_VRTC,
> #endif
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index ae05fbb50171..76d482a2b716 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -11,6 +11,7 @@
> #include <linux/stop_machine.h>
> #include <linux/slab.h>
> #include <linux/kdebug.h>
> +#include <linux/mmu_context.h>
> #include <asm/text-patching.h>
> #include <asm/alternative.h>
> #include <asm/sections.h>
> @@ -683,41 +684,102 @@ __ro_after_init unsigned long poking_addr;
>
> static void *__text_poke(void *addr, const void *opcode, size_t len)
> {
> + bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
> + temporary_mm_state_t prev;
> + struct page *pages[2] = {NULL};
> unsigned long flags;
> - char *vaddr;
> - struct page *pages[2];
> - int i;
> + pte_t pte, *ptep;
> + spinlock_t *ptl;
> + pgprot_t prot;
>
> /*
> - * While boot memory allocator is runnig we cannot use struct
> - * pages as they are not yet initialized.
> + * While boot memory allocator is running we cannot use struct pages as
> + * they are not yet initialized.
> */
> BUG_ON(!after_bootmem);
>
> if (!core_kernel_text((unsigned long)addr)) {
> pages[0] = vmalloc_to_page(addr);
> - pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> + if (cross_page_boundary)
> + pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> } else {
> pages[0] = virt_to_page(addr);
> WARN_ON(!PageReserved(pages[0]));
> - pages[1] = virt_to_page(addr + PAGE_SIZE);
> + if (cross_page_boundary)
> + pages[1] = virt_to_page(addr + PAGE_SIZE);
> }
> - BUG_ON(!pages[0]);
> + BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

checkpatch fires a lot for this patchset and I think we should tone down
the BUG_ON() use.

WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code rather than BUG() or BUG_ON()
#116: FILE: arch/x86/kernel/alternative.c:711:
+ BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

While the below BUG_ON makes sense, this here could be a WARN_ON or so.

Which begs the next question: AFAICT, nothing looks at text_poke*()'s
retval. So why are we even bothering returning something?

> +
> local_irq_save(flags);
> - set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> - if (pages[1])
> - set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
> - vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
> - memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
> - clear_fixmap(FIX_TEXT_POKE0);
> - if (pages[1])
> - clear_fixmap(FIX_TEXT_POKE1);
> - local_flush_tlb();
> - sync_core();
> - /* Could also do a CLFLUSH here to speed up CPU recovery; but
> - that causes hangs on some VIA CPUs. */
> - for (i = 0; i < len; i++)
> - BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> +
> + /*
> + * The lock is not really needed, but this allows to avoid open-coding.
> + */
> + ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> +
> + /*
> + * This must not fail; preallocated in poking_init().
> + */
> + VM_BUG_ON(!ptep);
> +
> + /*
> + * flush_tlb_mm_range() would be called when the poking_mm is not
> + * loaded. When PCID is in use, the flush would be deferred to the time
> + * the poking_mm is loaded again. Set the PTE as non-global to prevent
> + * it from being used when we are done.
> + */
> + prot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);

So

_KERNPG_TABLE | _PAGE_NX

as this is pagetable page, AFAICT.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.