Re: [PATCH 01/17] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"

[hpa@zytor.com]

On January 16, 2019 10:47:01 PM PST, Masami Hiramatsu <mhiramat@kernel.org> wrote:
>On Wed, 16 Jan 2019 16:32:43 -0800
>Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
>
>> From: Nadav Amit <namit@vmware.com>
>> 
>> text_mutex is currently expected to be held before text_poke() is
>> called, but we kgdb does not take the mutex, and instead *supposedly*
>> ensures the lock is not taken and will not be acquired by any other
>core
>> while text_poke() is running.
>> 
>> The reason for the "supposedly" comment is that it is not entirely
>clear
>> that this would be the case if gdb_do_roundup is zero.
>> 
>> This patch creates two wrapper functions, text_poke() and
>> text_poke_kgdb() which do or do not run the lockdep assertion
>> respectively.
>> 
>> While we are at it, change the return code of text_poke() to
>something
>> meaningful. One day, callers might actually respect it and the
>existing
>> BUG_ON() when patching fails could be removed. For kgdb, the return
>> value can actually be used.
>
>Looks good to me.
>
>Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
>
>Thank you,
>
>> 
>> 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>
>> Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in
>text_poke*()")
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Acked-by: Jiri Kosina <jkosina@suse.cz>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>> ---
>>  arch/x86/include/asm/text-patching.h |  1 +
>>  arch/x86/kernel/alternative.c        | 52
>++++++++++++++++++++--------
>>  arch/x86/kernel/kgdb.c               | 11 +++---
>>  3 files changed, 45 insertions(+), 19 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/text-patching.h
>b/arch/x86/include/asm/text-patching.h
>> index e85ff65c43c3..f8fc8e86cf01 100644
>> --- a/arch/x86/include/asm/text-patching.h
>> +++ b/arch/x86/include/asm/text-patching.h
>> @@ -35,6 +35,7 @@ extern void *text_poke_early(void *addr, const void
>*opcode, size_t len);
>>   * inconsistent instruction while you patch.
>>   */
>>  extern void *text_poke(void *addr, const void *opcode, size_t len);
>> +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;
>> diff --git a/arch/x86/kernel/alternative.c
>b/arch/x86/kernel/alternative.c
>> index ebeac487a20c..c6a3a10a2fd5 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -678,18 +678,7 @@ void *__init_or_module text_poke_early(void
>*addr, const void *opcode,
>>  	return addr;
>>  }
>>  
>> -/**
>> - * text_poke - Update instructions on a live kernel
>> - * @addr: address to modify
>> - * @opcode: source of the copy
>> - * @len: length to copy
>> - *
>> - * Only atomic text poke/set should be allowed when not doing early
>patching.
>> - * It means the size must be writable atomically and the address
>must be aligned
>> - * in a way that permits an atomic write. It also makes sure we fit
>on a single
>> - * page.
>> - */
>> -void *text_poke(void *addr, const void *opcode, size_t len)
>> +static void *__text_poke(void *addr, const void *opcode, size_t len)
>>  {
>>  	unsigned long flags;
>>  	char *vaddr;
>> @@ -702,8 +691,6 @@ void *text_poke(void *addr, const void *opcode,
>size_t len)
>>  	 */
>>  	BUG_ON(!after_bootmem);
>>  
>> -	lockdep_assert_held(&text_mutex);
>> -
>>  	if (!core_kernel_text((unsigned long)addr)) {
>>  		pages[0] = vmalloc_to_page(addr);
>>  		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
>> @@ -732,6 +719,43 @@ void *text_poke(void *addr, const void *opcode,
>size_t len)
>>  	return addr;
>>  }
>>  
>> +/**
>> + * text_poke - Update instructions on a live kernel
>> + * @addr: address to modify
>> + * @opcode: source of the copy
>> + * @len: length to copy
>> + *
>> + * Only atomic text poke/set should be allowed when not doing early
>patching.
>> + * It means the size must be writable atomically and the address
>must be aligned
>> + * in a way that permits an atomic write. It also makes sure we fit
>on a single
>> + * page.
>> + */
>> +void *text_poke(void *addr, const void *opcode, size_t len)
>> +{
>> +	lockdep_assert_held(&text_mutex);
>> +
>> +	return __text_poke(addr, opcode, len);
>> +}
>> +
>> +/**
>> + * text_poke_kgdb - Update instructions on a live kernel by kgdb
>> + * @addr: address to modify
>> + * @opcode: source of the copy
>> + * @len: length to copy
>> + *
>> + * Only atomic text poke/set should be allowed when not doing early
>patching.
>> + * It means the size must be writable atomically and the address
>must be aligned
>> + * in a way that permits an atomic write. It also makes sure we fit
>on a single
>> + * page.
>> + *
>> + * Context: should only be used by kgdb, which ensures no other core
>is running,
>> + *	    despite the fact it does not hold the text_mutex.
>> + */
>> +void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
>> +{
>> +	return __text_poke(addr, opcode, len);
>> +}
>> +
>>  static void do_sync_core(void *info)
>>  {
>>  	sync_core();
>> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
>> index 5db08425063e..1461544cba8b 100644
>> --- a/arch/x86/kernel/kgdb.c
>> +++ b/arch/x86/kernel/kgdb.c
>> @@ -758,13 +758,13 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt
>*bpt)
>>  	if (!err)
>>  		return err;
>>  	/*
>> -	 * It is safe to call text_poke() because normal kernel execution
>> +	 * It is safe to call text_poke_kgdb() because normal kernel
>execution
>>  	 * is stopped on all cores, so long as the text_mutex is not
>locked.
>>  	 */
>>  	if (mutex_is_locked(&text_mutex))
>>  		return -EBUSY;
>> -	text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
>> -		  BREAK_INSTR_SIZE);
>> +	text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
>> +		       BREAK_INSTR_SIZE);
>>  	err = probe_kernel_read(opc, (char *)bpt->bpt_addr,
>BREAK_INSTR_SIZE);
>>  	if (err)
>>  		return err;
>> @@ -783,12 +783,13 @@ int kgdb_arch_remove_breakpoint(struct
>kgdb_bkpt *bpt)
>>  	if (bpt->type != BP_POKE_BREAKPOINT)
>>  		goto knl_write;
>>  	/*
>> -	 * It is safe to call text_poke() because normal kernel execution
>> +	 * It is safe to call text_poke_kgdb() because normal kernel
>execution
>>  	 * is stopped on all cores, so long as the text_mutex is not
>locked.
>>  	 */
>>  	if (mutex_is_locked(&text_mutex))
>>  		goto knl_write;
>> -	text_poke((void *)bpt->bpt_addr, bpt->saved_instr,
>BREAK_INSTR_SIZE);
>> +	text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
>> +		       BREAK_INSTR_SIZE);
>>  	err = probe_kernel_read(opc, (char *)bpt->bpt_addr,
>BREAK_INSTR_SIZE);
>>  	if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
>>  		goto knl_write;
>> -- 
>> 2.17.1
>> 

If you are reorganizing this code, please do so so that the caller doesn't have to worry about if it should call text_poke_bp() or text_poke_early(). Right now the caller had to know that, which makes no sense.
-- 
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