Date:   Mon, 19 Oct 2020 22:51:26 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     Francis Laniel <laniel_francis@...vacyrequired.com>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-hardening@...r.kernel.org
Subject: Re: [RFC][PATCH v2] Fortify string function strscpy.

Francis Laniel <laniel_francis@...vacyrequired.com> writes:

> 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@...vacyrequired.com 
> wrote:
>> > From: Francis Laniel <laniel_francis@...vacyrequired.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?

That would make sense to me: apply my patches from the list and include
them in your next patch series. If you need to modify my patches, just
add your Signed-off-by: after mine.

I don't know if my patches still apply to a current tree: let me know if
you need any help getting them up to date.

Kind regards,
Daniel

>
>> 
>> Notes below...
>> 
>> > Signed-off-by: Francis Laniel <laniel_francis@...vacyrequired.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@...vacyrequired.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) {

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