Re: [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching

Andy Lutomirski
2019-02-11T19:07:58Z

On Mon, Feb 11, 2019 at 10:05 AM Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > On Feb 10, 2019, at 9:18 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >
> >
> > On Feb 10, 2019, at 4:39 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> >
> >>> On Jan 28, 2019, at 4:34 PM, Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
> >>>
> >>> From: Nadav Amit <namit@vmware.com>
> >>>
> >>> To prevent improper use of the PTEs that are used for text patching, we
> >>> want to use a temporary mm struct. We initailize it by copying the init
> >>> mm.
> >>>
> >>> The address that will be used for patching is taken from the lower area
> >>> that is usually used for the task memory. Doing so prevents the need to
> >>> frequently synchronize the temporary-mm (e.g., when BPF programs are
> >>> installed), since different PGDs are used for the task memory.
> >>>
> >>> Finally, we randomize the address of the PTEs to harden against exploits
> >>> that use these PTEs.
> >>>
> >>> Cc: Kees Cook <keescook@chromium.org>
> >>> Cc: Dave Hansen <dave.hansen@intel.com>
> >>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >>> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> >>> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
> >>> Suggested-by: Andy Lutomirski <luto@kernel.org>
> >>> Signed-off-by: Nadav Amit <namit@vmware.com>
> >>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> >>> ---
> >>> arch/x86/include/asm/pgtable.h | 3 +++
> >>> arch/x86/include/asm/text-patching.h | 2 ++
> >>> arch/x86/kernel/alternative.c | 3 +++
> >>> arch/x86/mm/init_64.c | 36 ++++++++++++++++++++++++++++
> >>> init/main.c | 3 +++
> >>> 5 files changed, 47 insertions(+)
> >>>
> >>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> >>> index 40616e805292..e8f630d9a2ed 100644
> >>> --- a/arch/x86/include/asm/pgtable.h
> >>> +++ b/arch/x86/include/asm/pgtable.h
> >>> @@ -1021,6 +1021,9 @@ static inline void __meminit init_trampoline_default(void)
> >>> /* Default trampoline pgd value */
> >>> trampoline_pgd_entry = init_top_pgt[pgd_index(__PAGE_OFFSET)];
> >>> }
> >>> +
> >>> +void __init poking_init(void);
> >>> +
> >>> # ifdef CONFIG_RANDOMIZE_MEMORY
> >>> void __meminit init_trampoline(void);
> >>> # else
> >>> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> >>> index f8fc8e86cf01..a75eed841eed 100644
> >>> --- a/arch/x86/include/asm/text-patching.h
> >>> +++ b/arch/x86/include/asm/text-patching.h
> >>> @@ -39,5 +39,7 @@ extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
> >>> extern int poke_int3_handler(struct pt_regs *regs);
> >>> extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
> >>> extern int after_bootmem;
> >>> +extern __ro_after_init struct mm_struct *poking_mm;
> >>> +extern __ro_after_init unsigned long poking_addr;
> >>>
> >>> #endif /* _ASM_X86_TEXT_PATCHING_H */
> >>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> >>> index 12fddbc8c55b..ae05fbb50171 100644
> >>> --- a/arch/x86/kernel/alternative.c
> >>> +++ b/arch/x86/kernel/alternative.c
> >>> @@ -678,6 +678,9 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
> >>> return addr;
> >>> }
> >>>
> >>> +__ro_after_init struct mm_struct *poking_mm;
> >>> +__ro_after_init unsigned long poking_addr;
> >>> +
> >>> static void *__text_poke(void *addr, const void *opcode, size_t len)
> >>> {
> >>> unsigned long flags;
> >>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> >>> index bccff68e3267..125c8c48aa24 100644
> >>> --- a/arch/x86/mm/init_64.c
> >>> +++ b/arch/x86/mm/init_64.c
> >>> @@ -53,6 +53,7 @@
> >>> #include <asm/init.h>
> >>> #include <asm/uv/uv.h>
> >>> #include <asm/setup.h>
> >>> +#include <asm/text-patching.h>
> >>>
> >>> #include "mm_internal.h"
> >>>
> >>> @@ -1383,6 +1384,41 @@ unsigned long memory_block_size_bytes(void)
> >>> return memory_block_size_probed;
> >>> }
> >>>
> >>> +/*
> >>> + * Initialize an mm_struct to be used during poking and a pointer to be used
> >>> + * during patching.
> >>> + */
> >>> +void __init poking_init(void)
> >>> +{
> >>> + spinlock_t *ptl;
> >>> + pte_t *ptep;
> >>> +
> >>> + poking_mm = copy_init_mm();
> >>> + BUG_ON(!poking_mm);
> >>> +
> >>> + /*
> >>> + * Randomize the poking address, but make sure that the following page
> >>> + * will be mapped at the same PMD. We need 2 pages, so find space for 3,
> >>> + * and adjust the address if the PMD ends after the first one.
> >>> + */
> >>> + poking_addr = TASK_UNMAPPED_BASE;
> >>> + if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> >>> + poking_addr += (kaslr_get_random_long("Poking") & PAGE_MASK) %
> >>> + (TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
> >>> +
> >>> + if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
> >>> + poking_addr += PAGE_SIZE;
> >>
> >> Further thinking about it, I think that allocating the virtual address for
> >> poking from user address-range is problematic. The user can set watchpoints
> >> on different addresses, cause some static-keys to be enabled/disabled, and
> >> monitor the signals to derandomize the poking address.
> >
> > Hmm, I hadn’t thought about watchpoints. I’m not sure how much we care
> > about possible derandomization like this, but we certainly don’t want to
> > send signals or otherwise malfunction.
> >
> >> Andy, I think you were pushing this change. Can I go back to use a vmalloc’d
> >> address instead, or do you have a better solution?
> >
> > Hmm. If we use a vmalloc address, we have to make sure it’s not actually
> > allocated. I suppose we could allocate one once at boot and use that. We
> > also have the problem that the usual APIs for handling “user” addresses
> > might assume they’re actually in the user range, although this seems
> > unlikely to be a problem in practice. More seriously, though, the code
> > that manipulates per-mm paging structures assumes that *all* of the
> > structures up to the top level are per-mm, and, if we use anything less
> > than a private pgd, this isn’t the case.
>
> I forgot that I only had this conversation in my mind ;-)
>
> Well, I did write some code that kept some vmalloc’d area private, and it
> did require more synchronization between the pgd’s. It is still possible
> to use another top-level PGD, but … (continued below)
>
> >
> >> I prefer not to
> >> save/restore DR7, of course.
> >
> > I suspect we may want to use the temporary mm concept for EFI, too, so we
> > may want to just suck it up and save/restore DR7. But only if a watchpoint
> > is in use, of course. I have an old patch I could dust off that tracks DR7
> > to make things like this efficient.
>
> … but, if this is the case, then I will just make (un)use_temporary_mm() to
> save/restore DR7. I guess you are ok with such a solution. I will
> incorporate it into Rick’s v3.
>

I'm certainly amenable to other solutions, but this one does seem the
least messy. I looked at my old patch, and it doesn't do what you
want. I'd suggest you just add a percpu variable like cpu_dr7 and rig
up some accessors so that it stays up to date. Then you can skip the
dr7 writes if there are no watchpoints set.

Also, EFI is probably a less interesting example than rare_write.
With rare_write, especially the dynamically allocated variants that
people keep coming up with, we'll need a swath of address space fully
as large as the vmalloc area. and getting *that* right while still
using the kernel address range might be more of a mess than we really
want to deal with.

--Andy