diff mbox series

[v2,13/13] lkdtm: Add a test for function descriptors protection

Message ID 08a3dfbc55e1c7a0a1917b22f0ca6cabd9895ab2.1634190022.git.christophe.leroy@csgroup.eu (mailing list archive)
State New
Headers show
Series Fix LKDTM for PPC64/IA64/PARISC | expand

Commit Message

Christophe Leroy Oct. 14, 2021, 5:50 a.m. UTC 
Add WRITE_OPD to check that you can't modify function
descriptors.

Gives the following result when function descriptors are
not protected:

	lkdtm: Performing direct entry WRITE_OPD
	lkdtm: attempting bad 16 bytes write at c00000000269b358
	lkdtm: FAIL: survived bad write
	lkdtm: do_nothing was hijacked!

Looks like a standard compiler barrier(); is not enough to force
GCC to use the modified function descriptor. Add to add a fake empty
inline assembly to force GCC to reload the function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 drivers/misc/lkdtm/core.c  |  1 +
 drivers/misc/lkdtm/lkdtm.h |  1 +
 drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
 3 files changed, 24 insertions(+)

Comments

Kees Cook Oct. 15, 2021, 9:35 p.m. UTC | #1 
On Thu, Oct 14, 2021 at 07:50:02AM +0200, Christophe Leroy wrote:
> Add WRITE_OPD to check that you can't modify function
> descriptors.
> 
> Gives the following result when function descriptors are
> not protected:
> 
> 	lkdtm: Performing direct entry WRITE_OPD
> 	lkdtm: attempting bad 16 bytes write at c00000000269b358
> 	lkdtm: FAIL: survived bad write
> 	lkdtm: do_nothing was hijacked!
> 
> Looks like a standard compiler barrier(); is not enough to force
> GCC to use the modified function descriptor. Add to add a fake empty
> inline assembly to force GCC to reload the function descriptor.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  drivers/misc/lkdtm/core.c  |  1 +
>  drivers/misc/lkdtm/lkdtm.h |  1 +
>  drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index fe6fd34b8caf..de092aa03b5d 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(WRITE_RO),
>  	CRASHTYPE(WRITE_RO_AFTER_INIT),
>  	CRASHTYPE(WRITE_KERN),
> +	CRASHTYPE(WRITE_OPD),
>  	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index c212a253edde..188bd0fd6575 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
>  void lkdtm_WRITE_RO(void);
>  void lkdtm_WRITE_RO_AFTER_INIT(void);
>  void lkdtm_WRITE_KERN(void);
> +void lkdtm_WRITE_OPD(void);
>  void lkdtm_EXEC_DATA(void);
>  void lkdtm_EXEC_STACK(void);
>  void lkdtm_EXEC_KMALLOC(void);
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 96b3ebfcb8ed..3870bc82d40d 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
>  	return;
>  }
>  
> +static noinline void do_almost_nothing(void)
> +{
> +	pr_info("do_nothing was hijacked!\n");
> +}
> +
>  static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>  {
>  	memcpy(fdesc, do_nothing, sizeof(*fdesc));
> @@ -143,6 +148,23 @@ void lkdtm_WRITE_KERN(void)
>  	do_overwritten();
>  }
>  
> +void lkdtm_WRITE_OPD(void)
> +{
> +	size_t size = sizeof(func_desc_t);
> +	void (*func)(void) = do_nothing;
> +
> +	if (!have_function_descriptors()) {
> +		pr_info("Platform doesn't have function descriptors.\n");

This should be more explicit ('xfail'):

	pr_info("XFAIL: platform doesn't use function descriptors.\n");

> +		return;
> +	}
> +	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
> +	memcpy(do_nothing, do_almost_nothing, size);
> +	pr_err("FAIL: survived bad write\n");
> +
> +	asm("" : "=m"(func));

Since this is a descriptor, I assume no icache flush is needed. Are
function descriptors strictly dcache? (Is anything besides just a
barrier needed?)

> +	func();
> +}
> +
>  void lkdtm_EXEC_DATA(void)
>  {
>  	execute_location(data_area, CODE_WRITE);
> -- 
> 2.31.1
>
Christophe Leroy Oct. 16, 2021, 6:28 a.m. UTC | #2 
Le 15/10/2021 à 23:35, Kees Cook a écrit :
> On Thu, Oct 14, 2021 at 07:50:02AM +0200, Christophe Leroy wrote:
>> Add WRITE_OPD to check that you can't modify function
>> descriptors.
>>
>> Gives the following result when function descriptors are
>> not protected:
>>
>> 	lkdtm: Performing direct entry WRITE_OPD
>> 	lkdtm: attempting bad 16 bytes write at c00000000269b358
>> 	lkdtm: FAIL: survived bad write
>> 	lkdtm: do_nothing was hijacked!
>>
>> Looks like a standard compiler barrier(); is not enough to force
>> GCC to use the modified function descriptor. Add to add a fake empty
>> inline assembly to force GCC to reload the function descriptor.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   drivers/misc/lkdtm/core.c  |  1 +
>>   drivers/misc/lkdtm/lkdtm.h |  1 +
>>   drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
>>   3 files changed, 24 insertions(+)
>>
>> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
>> index fe6fd34b8caf..de092aa03b5d 100644
>> --- a/drivers/misc/lkdtm/core.c
>> +++ b/drivers/misc/lkdtm/core.c
>> @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
>>   	CRASHTYPE(WRITE_RO),
>>   	CRASHTYPE(WRITE_RO_AFTER_INIT),
>>   	CRASHTYPE(WRITE_KERN),
>> +	CRASHTYPE(WRITE_OPD),
>>   	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
>>   	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
>>   	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
>> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
>> index c212a253edde..188bd0fd6575 100644
>> --- a/drivers/misc/lkdtm/lkdtm.h
>> +++ b/drivers/misc/lkdtm/lkdtm.h
>> @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
>>   void lkdtm_WRITE_RO(void);
>>   void lkdtm_WRITE_RO_AFTER_INIT(void);
>>   void lkdtm_WRITE_KERN(void);
>> +void lkdtm_WRITE_OPD(void);
>>   void lkdtm_EXEC_DATA(void);
>>   void lkdtm_EXEC_STACK(void);
>>   void lkdtm_EXEC_KMALLOC(void);
>> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
>> index 96b3ebfcb8ed..3870bc82d40d 100644
>> --- a/drivers/misc/lkdtm/perms.c
>> +++ b/drivers/misc/lkdtm/perms.c
>> @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
>>   	return;
>>   }
>>   
>> +static noinline void do_almost_nothing(void)
>> +{
>> +	pr_info("do_nothing was hijacked!\n");
>> +}
>> +
>>   static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>>   {
>>   	memcpy(fdesc, do_nothing, sizeof(*fdesc));
>> @@ -143,6 +148,23 @@ void lkdtm_WRITE_KERN(void)
>>   	do_overwritten();
>>   }
>>   
>> +void lkdtm_WRITE_OPD(void)
>> +{
>> +	size_t size = sizeof(func_desc_t);
>> +	void (*func)(void) = do_nothing;
>> +
>> +	if (!have_function_descriptors()) {
>> +		pr_info("Platform doesn't have function descriptors.\n");
> 
> This should be more explicit ('xfail'):
> 
> 	pr_info("XFAIL: platform doesn't use function descriptors.\n");

Ok


> 
>> +		return;
>> +	}
>> +	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
>> +	memcpy(do_nothing, do_almost_nothing, size);
>> +	pr_err("FAIL: survived bad write\n");
>> +
>> +	asm("" : "=m"(func));
> 
> Since this is a descriptor, I assume no icache flush is needed. Are
> function descriptors strictly dcache? (Is anything besides just a
> barrier needed?)

No flush is needed, the code just loads the function address from memory 
into CTR, loads R2 and branch to CTR:

	 19c:	e9 21 00 70 	ld      r9,112(r1)
	 1a0:	e9 49 00 00 	ld      r10,0(r9)
	 1a4:	7d 49 03 a6 	mtctr   r10
	 1a8:	e8 49 00 08 	ld      r2,8(r9)
	 1ac:	4e 80 04 21 	bctrl


> 
>> +	func();
>> +}
>> +
>>   void lkdtm_EXEC_DATA(void)
>>   {
>>   	execute_location(data_area, CODE_WRITE);
>> -- 
>> 2.31.1
>>
>
diff mbox series

Patch

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index fe6fd34b8caf..de092aa03b5d 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -148,6 +148,7 @@  static const struct crashtype crashtypes[] = {
 	CRASHTYPE(WRITE_RO),
 	CRASHTYPE(WRITE_RO_AFTER_INIT),
 	CRASHTYPE(WRITE_KERN),
+	CRASHTYPE(WRITE_OPD),
 	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
 	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
 	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index c212a253edde..188bd0fd6575 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -105,6 +105,7 @@  void __init lkdtm_perms_init(void);
 void lkdtm_WRITE_RO(void);
 void lkdtm_WRITE_RO_AFTER_INIT(void);
 void lkdtm_WRITE_KERN(void);
+void lkdtm_WRITE_OPD(void);
 void lkdtm_EXEC_DATA(void);
 void lkdtm_EXEC_STACK(void);
 void lkdtm_EXEC_KMALLOC(void);
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 96b3ebfcb8ed..3870bc82d40d 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,6 +44,11 @@  static noinline void do_overwritten(void)
 	return;
 }
 
+static noinline void do_almost_nothing(void)
+{
+	pr_info("do_nothing was hijacked!\n");
+}
+
 static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
 {
 	memcpy(fdesc, do_nothing, sizeof(*fdesc));
@@ -143,6 +148,23 @@  void lkdtm_WRITE_KERN(void)
 	do_overwritten();
 }
 
+void lkdtm_WRITE_OPD(void)
+{
+	size_t size = sizeof(func_desc_t);
+	void (*func)(void) = do_nothing;
+
+	if (!have_function_descriptors()) {
+		pr_info("Platform doesn't have function descriptors.\n");
+		return;
+	}
+	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
+	memcpy(do_nothing, do_almost_nothing, size);
+	pr_err("FAIL: survived bad write\n");
+
+	asm("" : "=m"(func));
+	func();
+}
+
 void lkdtm_EXEC_DATA(void)
 {
 	execute_location(data_area, CODE_WRITE);