Re: [RFC PATCH] lkdtm: Replace lkdtm_rodata_do_nothing() by do_nothing()

[Christophe Leroy]

Hi Kees,

Le 17/10/2021 à 19:19, Christophe Leroy a écrit :
> All EXEC tests are based on running a copy of do_nothing()
> except lkdtm_EXEC_RODATA which uses a different function
> called lkdtm_rodata_do_nothing().
> 
> On architectures using function descriptors, EXEC tests are
> performed using execute_location() which is a function
> that most of the time copies do_nothing() at the tested
> location then duplicates do_nothing() function descriptor
> and updates it with the address of the copy of do_nothing().
> 
> But for EXEC_RODATA test, execute_location() uses
> lkdtm_rodata_do_nothing() which is already in rodata section
> at build time instead of using a copy of do_nothing(). However
> it still uses the function descriptor of do_nothing(). There
> is a risk that running lkdtm_rodata_do_nothing() with the
> function descriptor of do_thing() is wrong.
> 
> To remove the above risk, change the approach and do the same
> as for other EXEC tests: use a copy of do_nothing(). The copy
> cannot be done during the test because RODATA area is write
> protected. Do the copy during init, before RODATA becomes
> write protected.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

Any opinion on this patch ?

Thanks
Christophe

> ---
> This applies on top of series v3 "Fix LKDTM for PPC64/IA64/PARISC"
> 
>   drivers/misc/lkdtm/Makefile | 11 -----------
>   drivers/misc/lkdtm/lkdtm.h  |  3 ---
>   drivers/misc/lkdtm/perms.c  |  9 +++++++--
>   drivers/misc/lkdtm/rodata.c | 11 -----------
>   4 files changed, 7 insertions(+), 27 deletions(-)
>   delete mode 100644 drivers/misc/lkdtm/rodata.c
> 
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index e2984ce51fe4..3d45a2b3007d 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -6,21 +6,10 @@ lkdtm-$(CONFIG_LKDTM)		+= bugs.o
>   lkdtm-$(CONFIG_LKDTM)		+= heap.o
>   lkdtm-$(CONFIG_LKDTM)		+= perms.o
>   lkdtm-$(CONFIG_LKDTM)		+= refcount.o
> -lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
>   lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
>   lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
>   lkdtm-$(CONFIG_LKDTM)		+= cfi.o
>   lkdtm-$(CONFIG_LKDTM)		+= fortify.o
>   lkdtm-$(CONFIG_PPC_BOOK3S_64)	+= powerpc.o
>   
> -KASAN_SANITIZE_rodata.o		:= n
>   KASAN_SANITIZE_stackleak.o	:= n
> -KCOV_INSTRUMENT_rodata.o	:= n
> -CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
> -
> -OBJCOPYFLAGS :=
> -OBJCOPYFLAGS_rodata_objcopy.o	:= \
> -			--rename-section .noinstr.text=.rodata,alloc,readonly,load,contents
> -targets += rodata.o rodata_objcopy.o
> -$(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
> -	$(call if_changed,objcopy)
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index 188bd0fd6575..905555d4c2cf 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -137,9 +137,6 @@ void lkdtm_REFCOUNT_SUB_AND_TEST_SATURATED(void);
>   void lkdtm_REFCOUNT_TIMING(void);
>   void lkdtm_ATOMIC_TIMING(void);
>   
> -/* rodata.c */
> -void lkdtm_rodata_do_nothing(void);
> -
>   /* usercopy.c */
>   void __init lkdtm_usercopy_init(void);
>   void __exit lkdtm_usercopy_exit(void);
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 2c6aba3ff32b..9b951ca48363 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -27,6 +27,7 @@ static const unsigned long rodata = 0xAA55AA55;
>   
>   /* This is marked __ro_after_init, so it should ultimately be .rodata. */
>   static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
> +static u8 rodata_area[EXEC_SIZE] __ro_after_init;
>   
>   /*
>    * This just returns to the caller. It is designed to be copied into
> @@ -193,8 +194,7 @@ void lkdtm_EXEC_VMALLOC(void)
>   
>   void lkdtm_EXEC_RODATA(void)
>   {
> -	execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
> -			 CODE_AS_IS);
> +	execute_location(rodata_area, CODE_AS_IS);
>   }
>   
>   void lkdtm_EXEC_USERSPACE(void)
> @@ -269,4 +269,9 @@ void __init lkdtm_perms_init(void)
>   {
>   	/* Make sure we can write to __ro_after_init values during __init */
>   	ro_after_init |= 0xAA;
> +
> +	memcpy(rodata_area, dereference_function_descriptor(do_nothing),
> +	       EXEC_SIZE);
> +	flush_icache_range((unsigned long)rodata_area,
> +			   (unsigned long)rodata_area + EXEC_SIZE);
>   }
> diff --git a/drivers/misc/lkdtm/rodata.c b/drivers/misc/lkdtm/rodata.c
> deleted file mode 100644
> index baacb876d1d9..000000000000
> --- a/drivers/misc/lkdtm/rodata.c
> +++ /dev/null
> @@ -1,11 +0,0 @@
> -// SPDX-License-Identifier: GPL-2.0
> -/*
> - * This includes functions that are meant to live entirely in .rodata
> - * (via objcopy tricks), to validate the non-executability of .rodata.
> - */
> -#include "lkdtm.h"
> -
> -void noinstr lkdtm_rodata_do_nothing(void)
> -{
> -	/* Does nothing. We just want an architecture agnostic "return". */
> -}