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Date:   Sat, 16 Oct 2021 08:42:08 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 12/13] lkdtm: Fix execute_[user]_location()



Le 15/10/2021 à 23:31, Kees Cook a écrit :
> On Thu, Oct 14, 2021 at 07:50:01AM +0200, Christophe Leroy wrote:
>> execute_location() and execute_user_location() intent
>> to copy do_nothing() text and execute it at a new location.
>> However, at the time being it doesn't copy do_nothing() function
>> but do_nothing() function descriptor which still points to the
>> original text. So at the end it still executes do_nothing() at
>> its original location allthough using a copied function descriptor.
>>
>> So, fix that by really copying do_nothing() text and build a new
>> function descriptor by copying do_nothing() function descriptor and
>> updating the target address with the new location.
>>
>> Also fix the displayed addresses by dereferencing do_nothing()
>> function descriptor.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   drivers/misc/lkdtm/perms.c     | 25 +++++++++++++++++++++----
>>   include/asm-generic/sections.h |  5 +++++
>>   2 files changed, 26 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
>> index 5266dc28df6e..96b3ebfcb8ed 100644
>> --- a/drivers/misc/lkdtm/perms.c
>> +++ b/drivers/misc/lkdtm/perms.c
>> @@ -44,19 +44,32 @@ static noinline void do_overwritten(void)
>>   	return;
>>   }
>>   
>> +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>> +{
>> +	memcpy(fdesc, do_nothing, sizeof(*fdesc));
>> +	fdesc->addr = (unsigned long)dst;
>> +	barrier();
>> +
>> +	return fdesc;
>> +}
> 
> How about collapsing the "have_function_descriptors()" check into
> setup_function_descriptor()?
> 
> static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> {
> 	if (__is_defined(HAVE_FUNCTION_DESCRIPTORS)) {
> 		memcpy(fdesc, do_nothing, sizeof(*fdesc));
> 		fdesc->addr = (unsigned long)dst;
> 		barrier();
> 		return fdesc;
> 	} else {
> 		return dst;
> 	}
> }

Ok

> 
>> +
>>   static noinline void execute_location(void *dst, bool write)
>>   {
>>   	void (*func)(void) = dst;
>> +	func_desc_t fdesc;
>> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>>   
>> -	pr_info("attempting ok execution at %px\n", do_nothing);
>> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>>   	do_nothing();
>>   
>>   	if (write == CODE_WRITE) {
>> -		memcpy(dst, do_nothing, EXEC_SIZE);
>> +		memcpy(dst, do_nothing_text, EXEC_SIZE);
>>   		flush_icache_range((unsigned long)dst,
>>   				   (unsigned long)dst + EXEC_SIZE);
>>   	}
>>   	pr_info("attempting bad execution at %px\n", func);
>> +	if (have_function_descriptors())
>> +		func = setup_function_descriptor(&fdesc, dst);
>>   	func();
>>   	pr_err("FAIL: func returned\n");
>>   }
>> @@ -67,15 +80,19 @@ static void execute_user_location(void *dst)
>>   
>>   	/* Intentionally crossing kernel/user memory boundary. */
>>   	void (*func)(void) = dst;
>> +	func_desc_t fdesc;
>> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>>   
>> -	pr_info("attempting ok execution at %px\n", do_nothing);
>> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>>   	do_nothing();
>>   
>> -	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
>> +	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
>>   				   EXEC_SIZE, FOLL_WRITE);
>>   	if (copied < EXEC_SIZE)
>>   		return;
>>   	pr_info("attempting bad execution at %px\n", func);
>> +	if (have_function_descriptors())
>> +		func = setup_function_descriptor(&fdesc, dst);
>>   	func();
>>   	pr_err("FAIL: func returned\n");
>>   }
> 
> 
>> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
>> index 76163883c6ff..d225318538bd 100644
>> --- a/include/asm-generic/sections.h
>> +++ b/include/asm-generic/sections.h
>> @@ -70,6 +70,11 @@ typedef struct {
>>   } func_desc_t;
>>   #endif
>>   
>> +static inline bool have_function_descriptors(void)
>> +{
>> +	return __is_defined(HAVE_FUNCTION_DESCRIPTORS);
>> +}
>> +
>>   /* random extra sections (if any).  Override
>>    * in asm/sections.h */
>>   #ifndef arch_is_kernel_text
> 
> This hunk seems like it should live in a separate patch.
> 

Ok I move it in a previous patch.

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