Re: [RFC PATCH 4/6] x86/alternatives: initializing temporary mm for patching

[Nadav Amit <namit@vmware.com>]

at 6:21 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Wed, 29 Aug 2018 01:11:45 -0700
> Nadav Amit <namit@vmware.com> wrote:
> 
>> 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: Masami Hiramatsu <mhiramat@kernel.org>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Suggested-by: Andy Lutomirski <luto@kernel.org>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> arch/x86/include/asm/pgtable.h       |  4 ++++
>> arch/x86/include/asm/text-patching.h |  2 ++
>> arch/x86/mm/init_64.c                | 35 ++++++++++++++++++++++++++++
>> include/asm-generic/pgtable.h        |  4 ++++
>> init/main.c                          |  1 +
>> 5 files changed, 46 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index e4ffa565a69f..c65d2b146ff6 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -1022,6 +1022,10 @@ 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);
>> +#define poking_init poking_init
> 
> Would we need this macro?
> 
>> +
>> # 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 e85ff65c43c3..ffe7902cc326 100644
>> --- a/arch/x86/include/asm/text-patching.h
>> +++ b/arch/x86/include/asm/text-patching.h
>> @@ -38,5 +38,7 @@ extern void *text_poke(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/mm/init_64.c b/arch/x86/mm/init_64.c
>> index dd519f372169..ed4a46a89946 100644
>> --- a/arch/x86/mm/init_64.c
>> +++ b/arch/x86/mm/init_64.c
>> @@ -33,6 +33,7 @@
>> #include <linux/nmi.h>
>> #include <linux/gfp.h>
>> #include <linux/kcore.h>
>> +#include <linux/sched/mm.h>
>> 
>> #include <asm/processor.h>
>> #include <asm/bios_ebda.h>
>> @@ -54,6 +55,7 @@
>> #include <asm/init.h>
>> #include <asm/uv/uv.h>
>> #include <asm/setup.h>
>> +#include <asm/text-patching.h>
>> 
>> #include "mm_internal.h"
>> 
>> @@ -1389,6 +1391,39 @@ 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. If anything fails during initialization, poking will be done
>> + * using the fixmap, which is unsafe, so warn the user about it.
>> + */
>> +void __init poking_init(void)
>> +{
>> +	unsigned long poking_addr;
>> +
>> +	poking_mm = copy_init_mm();
>> +	if (!poking_mm)
>> +		goto error;
>> +
>> +	/*
>> +	 * 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 +
>> +		(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;
>> +
>> +	return;
>> +error:
>> +	if (poking_mm)
>> +		mmput(poking_mm);
>> +	poking_mm = NULL;
> 
> At this point, only poking_mm == NULL case jumps into error. So we don't
> need above 3 lines.

Right. Will be fixed in the next version. 

> 
>> +	pr_err("x86/mm: error setting a separate poking address space\n");
>> +}
>> +
>> #ifdef CONFIG_SPARSEMEM_VMEMMAP
>> /*
>>  * Initialise the sparsemem vmemmap using huge-pages at the PMD level.
>> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
>> index 88ebc6102c7c..c66579d0ee67 100644
>> --- a/include/asm-generic/pgtable.h
>> +++ b/include/asm-generic/pgtable.h
>> @@ -1111,6 +1111,10 @@ static inline bool arch_has_pfn_modify_check(void)
>> 
>> #ifndef PAGE_KERNEL_EXEC
>> # define PAGE_KERNEL_EXEC PAGE_KERNEL
>> +
>> +#ifndef poking_init
>> +static inline void poking_init(void) { }
>> +#endif
> 
> Hmm, this seems a bit tricky. Maybe we can make an __weak function
> in init/main.c.

Of course - __weak is much better. Thanks!