Subject: Re: [PATCH] kasan: fix the missing underflow in memmove and memcpy with CONFIG_KASAN_GENERIC=y

From: Walter Wu <[email-masked]>
Date: Mon, 7 Oct 2019 16:18:09 +0800

On Mon, 2019-10-07 at 09:29 +0200, Dmitry Vyukov wrote:
> > > > diff --git a/mm/kasan/tags_report.c b/mm/kasan/tags_report.c
> > > > index 969ae08f59d7..19b9e364b397 100644
> > > > --- a/mm/kasan/tags_report.c
> > > > +++ b/mm/kasan/tags_report.c
> > > > @@ -36,6 +36,16 @@
> > > >
> > > >  const char *get_bug_type(struct kasan_access_info *info)
> > > >  {
> > > > +       /*
> > > > +        * if access_size < 0, then it will be larger than ULONG_MAX/2,
> > > > +        * so that this can qualify as out-of-bounds.
> > > > +        * out-of-bounds is the _least_ frequent KASAN bug type. So saying
> > > > +        * out-of-bounds has downsides of both approaches and won't prevent
> > > > +        * duplicate reports by syzbot.
> > > > +        */
> > > > +       if ((long)info->access_size < 0)
> > > > +               return "out-of-bounds";
> > >
> > >
> > > wait, no :)
> > > I meant we change it to heap-out-of-bounds and explain why we are
> > > saying this is a heap-out-of-bounds.
> > > The current comment effectively says we are doing non useful thing for
> > > no reason, it does not eliminate any of my questions as a reader of
> > > this code :)
> > >
> > Ok, the current comment may not enough to be understood why we use OOB
> > to represent size<0 bug. We can modify it as below :)
> >
> > If access_size < 0, then it has two reasons to be defined as
> > out-of-bounds.
> > 1) Casting negative numbers to size_t would indeed turn up as a "large"
> > size_t and its value will be larger than ULONG_MAX/2, so that this can
> > qualify as out-of-bounds.
> > 2) Don't generate new bug type in order to prevent duplicate reports by
> > some systems, e.g. syzbot."
> 
> Looks good to me. I think it should provide enough hooks for future
> readers to understand why we do this.
> 
Thanks for your review and suggestion again.
If no other questions, We will send this patchset.

The patchsets help to produce KASAN report when size is negative numbers
in memory operation function. It is helpful for programmer to solve the 
undefined behavior issue. Patch 1 based on Dmitry's review and
suggestion, patch 2 is a test in order to verify the patch 1. 

[1]https://bugzilla.kernel.org/show_bug.cgi?id=199341 
[2]https://lore.kernel.org/linux-arm-kernel/[email-masked]/ 

Walter Wu (2): 
kasan: detect invalid size in memory operation function 
kasan: add test for invalid size in memmove

 lib/test_kasan.c          | 18 ++++++++++++++++++
 mm/kasan/common.c         | 13 ++++++++-----
 mm/kasan/generic.c        |  5 +++++
 mm/kasan/generic_report.c | 12 ++++++++++++
 mm/kasan/tags.c           |  5 +++++
 mm/kasan/tags_report.c    | 12 ++++++++++++
 6 files changed, 60 insertions(+), 5 deletions(-)

commit 5b3b68660b3d420fd2bd792f2d9fd3ccb8877ef7
Author: Walter-zh Wu <[email-masked]>
Date:   Fri Oct 4 18:38:31 2019 +0800

    kasan: detect invalid size in memory operation function
    
    It is an undefined behavior to pass a negative numbers to
memset()/memcpy()/memmove()
    , so need to be detected by KASAN.
    
    If size is negative numbers, then it has two reasons to be defined
as out-of-bounds bug type.
    1) Casting negative numbers to size_t would indeed turn up as a
large
    size_t and its value will be larger than ULONG_MAX/2, so that this
can
    qualify as out-of-bounds.
    2) Don't generate new bug type in order to prevent duplicate reports
by
    some systems, e.g. syzbot.
    
    KASAN report:
    
     BUG: KASAN: out-of-bounds in kmalloc_memmove_invalid_size+0x70/0xa0
     Read of size 18446744073709551608 at addr ffffff8069660904 by task
cat/72
    
     CPU: 2 PID: 72 Comm: cat Not tainted
5.4.0-rc1-next-20191004ajb-00001-gdb8af2f372b2-dirty #1
     Hardware name: linux,dummy-virt (DT)
     Call trace:
      dump_backtrace+0x0/0x288
      show_stack+0x14/0x20
      dump_stack+0x10c/0x164
      print_address_description.isra.9+0x68/0x378
      __kasan_report+0x164/0x1a0
      kasan_report+0xc/0x18
      check_memory_region+0x174/0x1d0
      memmove+0x34/0x88
      kmalloc_memmove_invalid_size+0x70/0xa0
    
    [1] https://bugzilla.kernel.org/show_bug.cgi?id=199341
    
    Signed-off-by: Walter Wu <[email-masked]>
    Reported -by: Dmitry Vyukov <[email-masked]>
    Suggested-by: Dmitry Vyukov <[email-masked]>

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 6814d6d6a023..6ef0abd27f06 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -102,7 +102,8 @@ EXPORT_SYMBOL(__kasan_check_write);
 #undef memset
 void *memset(void *addr, int c, size_t len)
 {
-	check_memory_region((unsigned long)addr, len, true, _RET_IP_);
+	if (!check_memory_region((unsigned long)addr, len, true, _RET_IP_))
+		return NULL;
 
 	return __memset(addr, c, len);
 }
@@ -110,8 +111,9 @@ void *memset(void *addr, int c, size_t len)
 #undef memmove
 void *memmove(void *dest, const void *src, size_t len)
 {
-	check_memory_region((unsigned long)src, len, false, _RET_IP_);
-	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
+	if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
+	!check_memory_region((unsigned long)dest, len, true, _RET_IP_))
+		return NULL;
 
 	return __memmove(dest, src, len);
 }
@@ -119,8 +121,9 @@ void *memmove(void *dest, const void *src, size_t
len)
 #undef memcpy
 void *memcpy(void *dest, const void *src, size_t len)
 {
-	check_memory_region((unsigned long)src, len, false, _RET_IP_);
-	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
+	if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
+	!check_memory_region((unsigned long)dest, len, true, _RET_IP_))
+		return NULL;
 
 	return __memcpy(dest, src, len);
 }
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index 616f9dd82d12..02148a317d27 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -173,6 +173,11 @@ static __always_inline bool
check_memory_region_inline(unsigned long addr,
 	if (unlikely(size == 0))
 		return true;
 
+	if (unlikely((long)size < 0)) {
+		kasan_report(addr, size, write, ret_ip);
+		return false;
+	}
+
 	if (unlikely((void *)addr <
 		kasan_shadow_to_mem((void *)KASAN_SHADOW_START))) {
 		kasan_report(addr, size, write, ret_ip);
diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
index 36c645939bc9..ed0eb94cb811 100644
--- a/mm/kasan/generic_report.c
+++ b/mm/kasan/generic_report.c
@@ -107,6 +107,18 @@ static const char *get_wild_bug_type(struct
kasan_access_info *info)
 
 const char *get_bug_type(struct kasan_access_info *info)
 {
+	/*
+	 * If access_size is negative numbers, then it has two reasons
+	 * to be defined as out-of-bounds bug type.
+	 * 1) Casting negative numbers to size_t would indeed turn up as
+	 * a 'large' size_t and its value will be larger than ULONG_MAX/2,
+	 * so that this can qualify as out-of-bounds.
+	 * 2) Don't generate new bug type in order to prevent duplicate
reports
+	 * by some systems, e.g. syzbot.
+	 */
+	if ((long)info->access_size < 0)
+		return "out-of-bounds";
+
 	if (addr_has_shadow(info->access_addr))
 		return get_shadow_bug_type(info);
 	return get_wild_bug_type(info);
diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
index 0e987c9ca052..b829535a3ad7 100644
--- a/mm/kasan/tags.c
+++ b/mm/kasan/tags.c
@@ -86,6 +86,11 @@ bool check_memory_region(unsigned long addr, size_t
size, bool write,
 	if (unlikely(size == 0))
 		return true;
 
+	if (unlikely((long)size < 0)) {
+		kasan_report(addr, size, write, ret_ip);
+		return false;
+	}
+
 	tag = get_tag((const void *)addr);
 
 	/*
diff --git a/mm/kasan/tags_report.c b/mm/kasan/tags_report.c
index 969ae08f59d7..012fbe3a793f 100644
--- a/mm/kasan/tags_report.c
+++ b/mm/kasan/tags_report.c
@@ -36,6 +36,18 @@
 
 const char *get_bug_type(struct kasan_access_info *info)
 {
+	/*
+	 * If access_size is negative numbers, then it has two reasons
+	 * to be defined as out-of-bounds bug type.
+	 * 1) Casting negative numbers to size_t would indeed turn up as
+	 * a 'large' size_t and its value will be larger than ULONG_MAX/2,
+	 * so that this can qualify as out-of-bounds.
+	 * 2) Don't generate new bug type in order to prevent duplicate
reports
+	 * by some systems, e.g. syzbot.
+	 */
+	if ((long)info->access_size < 0)
+		return "out-of-bounds";
+
 #ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
 	struct kasan_alloc_meta *alloc_meta;
 	struct kmem_cache *cache;

commit fb5cf7bd16e939d1feef229af0211a8616c9ea03
Author: Walter-zh Wu <[email-masked]>
Date:   Fri Oct 4 18:32:03 2019 +0800

    kasan: add test for invalid size in memmove
    
    Test size is negative vaule in memmove in order to verify
    if it correctly get KASAN report.
    
    Signed-off-by: Walter Wu <[email-masked]>

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 49cc4d570a40..06942cf585cc 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -283,6 +283,23 @@ static noinline void __init
kmalloc_oob_in_memset(void)
 	kfree(ptr);
 }
 
+static noinline void __init kmalloc_memmove_invalid_size(void)
+{
+	char *ptr;
+	size_t size = 64;
+
+	pr_info("invalid size in memmove\n");
+	ptr = kmalloc(size, GFP_KERNEL);
+	if (!ptr) {
+		pr_err("Allocation failed\n");
+		return;
+	}
+
+	memset((char *)ptr, 0, 64);
+	memmove((char *)ptr, (char *)ptr + 4, -2);
+	kfree(ptr);
+}
+
 static noinline void __init kmalloc_uaf(void)
 {
 	char *ptr;
@@ -773,6 +790,7 @@ static int __init kmalloc_tests_init(void)
 	kmalloc_oob_memset_4();
 	kmalloc_oob_memset_8();
 	kmalloc_oob_memset_16();
+	kmalloc_memmove_invalid_size();
 	kmalloc_uaf();
 	kmalloc_uaf_memset();
 	kmalloc_uaf2();

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