Re: [RFC][PATCH v2] Fortify string function strscpy.

[Francis Laniel <laniel_francis@privacyrequired.com>]

Le samedi 17 octobre 2020, 01:16:36 CEST Kees Cook a écrit :
> On Fri, Oct 16, 2020 at 02:38:09PM +0200, laniel_francis@privacyrequired.com 
wrote:
> > From: Francis Laniel <laniel_francis@privacyrequired.com>
> > 
> > Thanks to kees advices (see:
> > https://github.com/KSPP/linux/issues/96#issuecomment-709620337) I wrote a
> > LKDTM test for the fortified version of strscpy I added in the v1 of this
> > patch. The test panics due to write overflow.
> 
> Ah nice, thanks! I am reminded about this series as well:
> https://lore.kernel.org/lkml/20200120045424.16147-1-dja@axtens.net
> I think we can likely do this all at the same time, merge the
> complementary pieces, etc.

You are welcome!
Just to be sure I understand correctly: you want me to add work of Daniel 
Axtens to my local version, then add my modifications on top of his work and 
republish the whole patch set?

> 
> Notes below...
> 
> > Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
> > ---
> > 
> >  drivers/misc/lkdtm/Makefile  |  1 +
> >  drivers/misc/lkdtm/core.c    |  1 +
> >  drivers/misc/lkdtm/fortify.c | 37 +++++++++++++++++++++++++++++
> >  drivers/misc/lkdtm/lkdtm.h   | 17 ++++++++------
> 
> Yay tests! These should, however, be a separate patch.

Ok, I will separate it.
If I understand correctly: one semantic modification = one commit.

> 
> >  include/linux/string.h       | 45 ++++++++++++++++++++++++++++++++++++
> >  5 files changed, 94 insertions(+), 7 deletions(-)
> >  create mode 100644 drivers/misc/lkdtm/fortify.c
> > 
> > diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> > index c70b3822013f..d898f7b22045 100644
> > --- a/drivers/misc/lkdtm/Makefile
> > +++ b/drivers/misc/lkdtm/Makefile
> > @@ -10,6 +10,7 @@ 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
> > 
> >  KASAN_SANITIZE_stackleak.o	:= n
> >  KCOV_INSTRUMENT_rodata.o	:= n
> > 
> > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> > index a5e344df9166..979f9e3feefd 100644
> > --- a/drivers/misc/lkdtm/core.c
> > +++ b/drivers/misc/lkdtm/core.c
> > @@ -178,6 +178,7 @@ static const struct crashtype crashtypes[] = {
> > 
> >  #ifdef CONFIG_X86_32
> >  
> >  	CRASHTYPE(DOUBLE_FAULT),
> >  
> >  #endif
> > 
> > +	CRASHTYPE(FORTIFIED_STRSCPY),
> > 
> >  };
> > 
> > diff --git a/drivers/misc/lkdtm/fortify.c b/drivers/misc/lkdtm/fortify.c
> > new file mode 100644
> > index 000000000000..0397d2def66d
> > --- /dev/null
> > +++ b/drivers/misc/lkdtm/fortify.c
> > @@ -0,0 +1,37 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (c) 2020 Francis Laniel <laniel_francis@privacyrequired.com>
> > + *
> > + * Add tests related to fortified functions in this file.
> > + */
> > +#include <linux/string.h>
> > +#include <linux/slab.h>
> > +#include "lkdtm.h"
> > +
> > +
> > +/*
> > + * Calls fortified strscpy to generate a panic because there is a write
> > + * overflow (i.e. src length is greater than dst length).
> > + */
> > +void lkdtm_FORTIFIED_STRSCPY(void)
> > +{
> > +#if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) &&
> > defined(CONFIG_FORTIFY_SOURCE) +	char *src;
> > +	char dst[3];
> > +
> > +	src = kmalloc(7, GFP_KERNEL);
> > +	src[0] = 'f';
> > +	src[1] = 'o';
> > +	src[2] = 'o';
> > +	src[3] = 'b';
> > +	src[4] = 'a';
> > +	src[5] = 'r';
> > +	src[6] = '\0';
> 
> Hah, yes, I guess we need to bypass the common utilities. ;) I wonder if
> using __underlying_strcpy() might be easier.

I am sorry but I did not understand.
If we use here __underlying_strcpy() the function this will not profit from the 
protection added in fortified version of strscpy()?

> 
> > +
> > +	strscpy(dst, src, 1000);
> > +
> > +	kfree(dst);
> > +
> > +	pr_info("Fail: No overflow in above strscpy call!\n");
> > +#endif
> > +}
> 
> One thing I'd love to see is a _compile-time_ test too: but it needs to
> be a negative failure case, which Makefiles are not well suited to
> dealing with. e.g. something like:
> 
> good.o: nop.c bad.c
> 	if $(CC) .... -o bad.o bad.c $< ; then exit 1; else $(CC) ... -o good.c
> nop.c ; fi
> 
> I'm not sure how to do it.
> 

This is a good idea, I though to it but I did not see an easy way to deal with 
it.
I will investigate one it, but I cannot guarantee the next version will come 
with this feature.

> > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> > index 8878538b2c13..8e5e90eb0e00 100644
> > --- a/drivers/misc/lkdtm/lkdtm.h
> > +++ b/drivers/misc/lkdtm/lkdtm.h
> > @@ -6,7 +6,7 @@
> > 
> >  #include <linux/kernel.h>
> > 
> > -/* lkdtm_bugs.c */
> > +/* bugs.c */
> 
> oops, yes. Can you split change from the others, since it's an unrelated
> clean-up.

Understand, it will be done for next version!

> 
> >  void __init lkdtm_bugs_init(int *recur_param);
> >  void lkdtm_PANIC(void);
> >  void lkdtm_BUG(void);
> > 
> > @@ -34,7 +34,7 @@ void lkdtm_UNSET_SMEP(void);
> > 
> >  void lkdtm_DOUBLE_FAULT(void);
> >  void lkdtm_CORRUPT_PAC(void);
> > 
> > -/* lkdtm_heap.c */
> > +/* heap.c */
> > 
> >  void __init lkdtm_heap_init(void);
> >  void __exit lkdtm_heap_exit(void);
> >  void lkdtm_OVERWRITE_ALLOCATION(void);
> > 
> > @@ -46,7 +46,7 @@ void lkdtm_SLAB_FREE_DOUBLE(void);
> > 
> >  void lkdtm_SLAB_FREE_CROSS(void);
> >  void lkdtm_SLAB_FREE_PAGE(void);
> > 
> > -/* lkdtm_perms.c */
> > +/* perms.c */
> > 
> >  void __init lkdtm_perms_init(void);
> >  void lkdtm_WRITE_RO(void);
> >  void lkdtm_WRITE_RO_AFTER_INIT(void);
> > 
> > @@ -61,7 +61,7 @@ void lkdtm_EXEC_NULL(void);
> > 
> >  void lkdtm_ACCESS_USERSPACE(void);
> >  void lkdtm_ACCESS_NULL(void);
> > 
> > -/* lkdtm_refcount.c */
> > +/* refcount.c */
> > 
> >  void lkdtm_REFCOUNT_INC_OVERFLOW(void);
> >  void lkdtm_REFCOUNT_ADD_OVERFLOW(void);
> >  void lkdtm_REFCOUNT_INC_NOT_ZERO_OVERFLOW(void);
> > 
> > @@ -82,10 +82,10 @@ void lkdtm_REFCOUNT_SUB_AND_TEST_SATURATED(void);
> > 
> >  void lkdtm_REFCOUNT_TIMING(void);
> >  void lkdtm_ATOMIC_TIMING(void);
> > 
> > -/* lkdtm_rodata.c */
> > +/* rodata.c */
> > 
> >  void lkdtm_rodata_do_nothing(void);
> > 
> > -/* lkdtm_usercopy.c */
> > +/* usercopy.c */
> > 
> >  void __init lkdtm_usercopy_init(void);
> >  void __exit lkdtm_usercopy_exit(void);
> >  void lkdtm_USERCOPY_HEAP_SIZE_TO(void);
> > 
> > @@ -98,10 +98,13 @@ void lkdtm_USERCOPY_STACK_BEYOND(void);
> > 
> >  void lkdtm_USERCOPY_KERNEL(void);
> >  void lkdtm_USERCOPY_KERNEL_DS(void);
> > 
> > -/* lkdtm_stackleak.c */
> > +/* stackleak.c */
> > 
> >  void lkdtm_STACKLEAK_ERASING(void);
> >  
> >  /* cfi.c */
> >  void lkdtm_CFI_FORWARD_PROTO(void);
> > 
> > +/* fortify.c */
> > +void lkdtm_FORTIFIED_STRSCPY(void);
> > +
> > 
> >  #endif
> > 
> > diff --git a/include/linux/string.h b/include/linux/string.h
> > index b1f3894a0a3e..b661863619e0 100644
> > --- a/include/linux/string.h
> > +++ b/include/linux/string.h
> > @@ -6,6 +6,8 @@
> > 
> >  #include <linux/compiler.h>	/* for inline */
> >  #include <linux/types.h>	/* for size_t */
> >  #include <linux/stddef.h>	/* for NULL */
> > 
> > +#include <linux/bug.h>		/* for WARN_ON_ONCE */
> > +#include <linux/errno.h>	/* for E2BIG */
> > 
> >  #include <stdarg.h>
> >  #include <uapi/linux/string.h>
> > 
> > @@ -357,6 +359,49 @@ __FORTIFY_INLINE size_t strlcpy(char *p, const char
> > *q, size_t size)> 
> >  	return ret;
> >  
> >  }
> > 
> > +/* defined after fortified strlen to reuse it */
> > +extern ssize_t __real_strscpy(char *, const char *, size_t)
> > __RENAME(strscpy); +__FORTIFY_INLINE ssize_t strscpy(char *p, const char
> > *q, size_t count)
> I would name "count" as "size" to match the other helpers.
> 

I decided to keep count because it is the argument name in unfortified version 
of strscpy
I will change the name for next version to stick with all the fortified 
functions arguments.

> > +{
> > +	size_t len;
> > +	size_t p_size = __builtin_object_size(p, 0);
> > +	size_t q_size = __builtin_object_size(q, 0);
> 
> These can be using ", 1" instead of ", 0". And I'll grab the related
> changes from the mentioned series above.
> 

I looked Daniel Axtens patch and understood why it is better to use 1 instead 
of 0 so I will add it for the next version.

> > +	/*
> > +	 * If p_size and q_size cannot be known at compile time we just had to
> > +	 * trust this function caller.
> > +	 */
> > +	if (p_size == (size_t)-1 && q_size == (size_t)-1)
> > +		return __real_strscpy(p, q, count);
> > +	len = strlen(q);
> > +	if (count) {
> 
> This test isn't needed; it'll work itself out correctly. :P
> 

Indeed, if this condition is met, __real_strscpy will be called later.

> > +		/* If count is bigger than INT_MAX, strscpy returns -E2BIG. */
> > +		if (WARN_ON_ONCE(count > INT_MAX))
> > +			return -E2BIG;
> 
> This is already handled in strscpy, I'd drop this here.

I though of it at first, but since the patch modify count/size before giving it 
to __real_strscpy(), real one will never return -E2BIG due to that.
So removing this modification will lead to difference between returned value of 
fortified strscpy() and __real_strscpy().

> 
> > +		/*
> > +		 * strscpy handles read overflows by stop reading q when '\0' is
> > +		 * met.
> > +		 * We stick to this behavior here.
> > +		 */
> > +		len = (len >= count) ? count : len;
> > +		/*
> > +		 * If len can be known at compile time and is greater than
> > +		 * p_size, generate a compile time write overflow error.
> > +		 */
> > +		if (__builtin_constant_p(len) && len > p_size)
> 
> This won't work (len wasn't an argument and got assigned); you need:
> 
> 		if (__builtin_constant_p(size) && p_size < size)
> 

You are right, len is unknown at compile time... So, I will correct it for 
next version!

> > +			__write_overflow();
> > +		/* Otherwise generate a runtime write overflow error. */
> > +		if (len > p_size)
> > +			fortify_panic(__func__);
> 
> I think this just needs to be:
> 
> 		if (p_size < size)
> 			fortify_panic(__func__);
> 

I am not really sure.
If p_size is 4, size is 1000 and q is "foo\0", then what you suggested will 
panic but there is not need to panic since __real_strscpy will truncate size 
and copy just 4 bytes into p (because of '\0' in q).
Am I correct?

> > +		/*
> > +		 * Still use count as third argument to correctly compute max
> > +		 * inside strscpy.
> > +		 */
> > +		return __real_strscpy(p, q, count);
> > +	}
> > +	/* If count is 0, strscpy return -E2BIG. */
> > +	return -E2BIG;
> 
> I'd let __real_strscpy() handle this.
> 

See my three times above comment.
__real_strscpy is called only if count > 0, so it will never return -E2BIG due 
to this.
So it will lead to difference in returned value between fortified strscpy() and 
__real_strscpy().

> > +}
> > +
> > 
> >  /* defined after fortified strlen and strnlen to reuse them */
> >  __FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t
> >  count) {