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

[Nadav Amit]

> On Jan 28, 2019, at 4:34 PM, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> wrote:
> 
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> 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@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Tested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> 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.

Andy, I think you were pushing this change. Can I go back to use a vmallocâd
address instead, or do you have a better solution? I prefer not to
save/restore DR7, of course.