Re: [PATCH v4 01/10] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"

[Masami Hiramatsu]

On Sat, 10 Nov 2018 15:17:23 -0800
Nadav Amit <namit@xxxxxxxxxx> wrote:

> 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.

Hm, this looks reasonable and good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you!

> 
> Cc: Jiri Kosina <jkosina@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/text-patching.h |  3 +-
>  arch/x86/kernel/alternative.c        | 72 +++++++++++++++++++++-------
>  arch/x86/kernel/kgdb.c               | 15 ++++--
>  3 files changed, 66 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index e85ff65c43c3..5a2600370763 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -34,7 +34,8 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>   * On the local CPU you need to be protected again NMI or MCE handlers seeing an
>   * inconsistent instruction while you patch.
>   */
> -extern void *text_poke(void *addr, const void *opcode, size_t len);
> +extern int text_poke(void *addr, const void *opcode, size_t len);
> +extern int 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..ebe9210dc92e 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -678,23 +678,12 @@ 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 int __text_poke(void *addr, const void *opcode, size_t len)
>  {
>  	unsigned long flags;
>  	char *vaddr;
>  	struct page *pages[2];
> -	int i;
> +	int i, r = 0;
>  
>  	/*
>  	 * While boot memory allocator is runnig we cannot use struct
> @@ -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);
> @@ -712,7 +699,8 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>  		WARN_ON(!PageReserved(pages[0]));
>  		pages[1] = virt_to_page(addr + PAGE_SIZE);
>  	}
> -	BUG_ON(!pages[0]);
> +	if (!pages[0])
> +		return -EFAULT;
>  	local_irq_save(flags);
>  	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
>  	if (pages[1])
> @@ -727,9 +715,57 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>  	/* Could also do a CLFLUSH here to speed up CPU recovery; but
>  	   that causes hangs on some VIA CPUs. */
>  	for (i = 0; i < len; i++)
> -		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> +		if (((char *)addr)[i] != ((char *)opcode)[i])
> +			r = -EFAULT;
>  	local_irq_restore(flags);
> -	return addr;
> +	return r;
> +}
> +
> +/**
> + * 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.
> + */
> +int text_poke(void *addr, const void *opcode, size_t len)
> +{
> +	int r;
> +
> +	lockdep_assert_held(&text_mutex);
> +
> +	r = __text_poke(addr, opcode, len);
> +
> +	/*
> +	 * TODO: change the callers to consider the return value and remove this
> +	 *       historical assertion.
> +	 */
> +	BUG_ON(r);
> +
> +	return r;
> +}
> +
> +/**
> + * 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.
> + */
> +int text_poke_kgdb(void *addr, const void *opcode, size_t len)
> +{
> +	return __text_poke(addr, opcode, len);
>  }
>  
>  static void do_sync_core(void *info)
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index 8e36f249646e..8091b2e381d4 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -763,13 +763,15 @@ 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);
> +	err = text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
> +			     BREAK_INSTR_SIZE);
> +	if (err)
> +		return err;
>  	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
>  	if (err)
>  		return err;
> @@ -788,12 +790,15 @@ 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);
> +	err = text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
> +			     BREAK_INSTR_SIZE);
> +	if (err)
> +		return err;
>  	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
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>