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

[Nadav Amit]

> On Feb 10, 2019, at 9:18 PM, Andy Lutomirski <luto at amacapital.net> wrote:
> 
> 
> 
> On Feb 10, 2019, at 4:39 PM, Nadav Amit <nadav.amit at gmail.com> wrote:
> 
>>> On Jan 28, 2019, at 4:34 PM, Rick Edgecombe <rick.p.edgecombe at intel.com> wrote:
>>> 
>>> From: Nadav Amit <namit at 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 at chromium.org>
>>> Cc: Dave Hansen <dave.hansen at intel.com>
>>> Acked-by: Peter Zijlstra (Intel) <peterz at infradead.org>
>>> Reviewed-by: Masami Hiramatsu <mhiramat at kernel.org>
>>> Tested-by: Masami Hiramatsu <mhiramat at kernel.org>
>>> Suggested-by: Andy Lutomirski <luto at kernel.org>
>>> Signed-off-by: Nadav Amit <namit at vmware.com>
>>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe at 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.