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

From: Walter Wu <[email-masked]>
Date: Thu, 3 Oct 2019 21:51:21 +0800

On Thu, 2019-10-03 at 17:38 +0800, Walter Wu wrote:
> On Thu, 2019-10-03 at 08:26 +0200, Dmitry Vyukov wrote:
> > On Thu, Oct 3, 2019 at 4:18 AM Walter Wu <[email-masked]> wrote:
> > >
> > > On Wed, 2019-10-02 at 15:57 +0200, Dmitry Vyukov wrote:
> > > > On Wed, Oct 2, 2019 at 2:15 PM Walter Wu <[email-masked]> wrote:
> > > > >
> > > > > On Mon, 2019-09-30 at 12:36 +0800, Walter Wu wrote:
> > > > > > On Fri, 2019-09-27 at 21:41 +0200, Dmitry Vyukov wrote:
> > > > > > > On Fri, Sep 27, 2019 at 4:22 PM Walter Wu <[email-masked]> wrote:
> > > > > > > >
> > > > > > > > On Fri, 2019-09-27 at 15:07 +0200, Dmitry Vyukov wrote:
> > > > > > > > > On Fri, Sep 27, 2019 at 5:43 AM Walter Wu <[email-masked]> wrote:
> > > > > > > > > >
> > > > > > > > > > memmove() and memcpy() have missing underflow issues.
> > > > > > > > > > When -7 <= size < 0, then KASAN will miss to catch the underflow issue.
> > > > > > > > > > It looks like shadow start address and shadow end address is the same,
> > > > > > > > > > so it does not actually check anything.
> > > > > > > > > >
> > > > > > > > > > The following test is indeed not caught by KASAN:
> > > > > > > > > >
> > > > > > > > > >         char *p = kmalloc(64, GFP_KERNEL);
> > > > > > > > > >         memset((char *)p, 0, 64);
> > > > > > > > > >         memmove((char *)p, (char *)p + 4, -2);
> > > > > > > > > >         kfree((char*)p);
> > > > > > > > > >
> > > > > > > > > > It should be checked here:
> > > > > > > > > >
> > > > > > > > > > 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_);
> > > > > > > > > >
> > > > > > > > > >         return __memmove(dest, src, len);
> > > > > > > > > > }
> > > > > > > > > >
> > > > > > > > > > We fix the shadow end address which is calculated, then generic KASAN
> > > > > > > > > > get the right shadow end address and detect this underflow issue.
> > > > > > > > > >
> > > > > > > > > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=199341
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Walter Wu <[email-masked]>
> > > > > > > > > > Reported-by: Dmitry Vyukov <[email-masked]>
> > > > > > > > > > ---
> > > > > > > > > >  lib/test_kasan.c   | 36 ++++++++++++++++++++++++++++++++++++
> > > > > > > > > >  mm/kasan/generic.c |  8 ++++++--
> > > > > > > > > >  2 files changed, 42 insertions(+), 2 deletions(-)
> > > > > > > > > >
> > > > > > > > > > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> > > > > > > > > > index b63b367a94e8..8bd014852556 100644
> > > > > > > > > > --- a/lib/test_kasan.c
> > > > > > > > > > +++ b/lib/test_kasan.c
> > > > > > > > > > @@ -280,6 +280,40 @@ static noinline void __init kmalloc_oob_in_memset(void)
> > > > > > > > > >         kfree(ptr);
> > > > > > > > > >  }
> > > > > > > > > >
> > > > > > > > > > +static noinline void __init kmalloc_oob_in_memmove_underflow(void)
> > > > > > > > > > +{
> > > > > > > > > > +       char *ptr;
> > > > > > > > > > +       size_t size = 64;
> > > > > > > > > > +
> > > > > > > > > > +       pr_info("underflow out-of-bounds 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_oob_in_memmove_overflow(void)
> > > > > > > > > > +{
> > > > > > > > > > +       char *ptr;
> > > > > > > > > > +       size_t size = 64;
> > > > > > > > > > +
> > > > > > > > > > +       pr_info("overflow out-of-bounds in memmove\n");
> > > > > > > > > > +       ptr = kmalloc(size, GFP_KERNEL);
> > > > > > > > > > +       if (!ptr) {
> > > > > > > > > > +               pr_err("Allocation failed\n");
> > > > > > > > > > +               return;
> > > > > > > > > > +       }
> > > > > > > > > > +
> > > > > > > > > > +       memset((char *)ptr, 0, 64);
> > > > > > > > > > +       memmove((char *)ptr + size, (char *)ptr, 2);
> > > > > > > > > > +       kfree(ptr);
> > > > > > > > > > +}
> > > > > > > > > > +
> > > > > > > > > >  static noinline void __init kmalloc_uaf(void)
> > > > > > > > > >  {
> > > > > > > > > >         char *ptr;
> > > > > > > > > > @@ -734,6 +768,8 @@ static int __init kmalloc_tests_init(void)
> > > > > > > > > >         kmalloc_oob_memset_4();
> > > > > > > > > >         kmalloc_oob_memset_8();
> > > > > > > > > >         kmalloc_oob_memset_16();
> > > > > > > > > > +       kmalloc_oob_in_memmove_underflow();
> > > > > > > > > > +       kmalloc_oob_in_memmove_overflow();
> > > > > > > > > >         kmalloc_uaf();
> > > > > > > > > >         kmalloc_uaf_memset();
> > > > > > > > > >         kmalloc_uaf2();
> > > > > > > > > > diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> > > > > > > > > > index 616f9dd82d12..34ca23d59e67 100644
> > > > > > > > > > --- a/mm/kasan/generic.c
> > > > > > > > > > +++ b/mm/kasan/generic.c
> > > > > > > > > > @@ -131,9 +131,13 @@ static __always_inline bool memory_is_poisoned_n(unsigned long addr,
> > > > > > > > > >                                                 size_t size)
> > > > > > > > > >  {
> > > > > > > > > >         unsigned long ret;
> > > > > > > > > > +       void *shadow_start = kasan_mem_to_shadow((void *)addr);
> > > > > > > > > > +       void *shadow_end = kasan_mem_to_shadow((void *)addr + size - 1) + 1;
> > > > > > > > > >
> > > > > > > > > > -       ret = memory_is_nonzero(kasan_mem_to_shadow((void *)addr),
> > > > > > > > > > -                       kasan_mem_to_shadow((void *)addr + size - 1) + 1);
> > > > > > > > > > +       if ((long)size < 0)
> > > > > > > > > > +               shadow_end = kasan_mem_to_shadow((void *)addr + size);
> > > > > > > > >
> > > > > > > > > Hi Walter,
> > > > > > > > >
> > > > > > > > > Thanks for working on this.
> > > > > > > > >
> > > > > > > > > If size<0, does it make sense to continue at all? We will still check
> > > > > > > > > 1PB of shadow memory? What happens when we pass such huge range to
> > > > > > > > > memory_is_nonzero?
> > > > > > > > > Perhaps it's better to produce an error and bail out immediately if size<0?
> > > > > > > >
> > > > > > > > I agree with what you said. when size<0, it is indeed an unreasonable
> > > > > > > > behavior, it should be blocked from continuing to do.
> > > > > > > >
> > > > > > > >
> > > > > > > > > Also, what's the failure mode of the tests? Didn't they badly corrupt
> > > > > > > > > memory? We tried to keep tests such that they produce the KASAN
> > > > > > > > > reports, but don't badly corrupt memory b/c/ we need to run all of
> > > > > > > > > them.
> > > > > > > >
> > > > > > > > Maybe we should first produce KASAN reports and then go to execute
> > > > > > > > memmove() or do nothing? It looks like it’s doing the following.or?
> > > > > > > >
> > > > > > > > void *memmove(void *dest, const void *src, size_t len)
> > > > > > > >  {
> > > > > > > > +       if (long(len) <= 0)
> > > > > > >
> > > > > > > /\/\/\/\/\/\
> > > > > > >
> > > > > > > This check needs to be inside of check_memory_region, otherwise we
> > > > > > > will have similar problems in all other places that use
> > > > > > > check_memory_region.
> > > > > > Thanks for your reminder.
> > > > > >
> > > > > >  bool check_memory_region(unsigned long addr, size_t size, bool write,
> > > > > >                                 unsigned long ret_ip)
> > > > > >  {
> > > > > > +       if (long(size) < 0) {
> > > > > > +               kasan_report_invalid_size(src, dest, len, _RET_IP_);
> > > > > > +               return false;
> > > > > > +       }
> > > > > > +
> > > > > >         return check_memory_region_inline(addr, size, write, ret_ip);
> > > > > >  }
> > > > > >
> > > > > > > But check_memory_region already returns a bool, so we could check that
> > > > > > > bool and return early.
> > > > > >
> > > > > > When size<0, we should only show one KASAN report, and should we only
> > > > > > limit to return when size<0 is true? If yse, then __memmove() will do
> > > > > > nothing.
> > > > > >
> > > > > >
> > > > > >  void *memmove(void *dest, const void *src, size_t len)
> > > > > >  {
> > > > > > -       check_memory_region((unsigned long)src, len, false, _RET_IP_);
> > > > > > +       if(!check_memory_region((unsigned long)src, len, false,
> > > > > > _RET_IP_)
> > > > > > +               && long(size) < 0)
> > > > > > +               return;
> > > > > > +
> > > > > >         check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> > > > > >
> > > > > >         return __memmove(dest, src, len);
> > > > > >
> > > > > > >
> > > > > Hi Dmitry,
> > > > >
> > > > > What do you think the following code is better than the above one.
> > > > > In memmmove/memset/memcpy, they need to determine whether size < 0 is
> > > > > true. we directly determine whether size is negative in memmove and
> > > > > return early. it avoid to generate repeated KASAN report. Is it better?
> > > > >
> > > > > void *memmove(void *dest, const void *src, size_t len)
> > > > > {
> > > > > +       if (long(size) < 0) {
> > > > > +               kasan_report_invalid_size(src, dest, len, _RET_IP_);
> > > > > +               return;
> > > > > +       }
> > > > > +
> > > > >         check_memory_region((unsigned long)src, len, false, _RET_IP_);
> > > > >         check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> > > > >
> > > > >
> > > > > check_memory_region() still has to check whether the size is negative.
> > > > > but memmove/memset/memcpy generate invalid size KASAN report will not be
> > > > > there.
> > > >
> > > >
> > > > If check_memory_region() will do the check, why do we need to
> > > > duplicate it inside of memmove and all other range functions?
> > > >
> > > Yes, I know it has duplication, but if we don't have to determine size<0
> > > in memmove, then all check_memory_region return false will do nothing,
> > 
> > But they will produce a KASAN report, right? They are asked to check
> > if 18446744073709551614 bytes are good. 18446744073709551614 bytes
> > can't be good.
> > 
> > 
> > > it includes other memory corruption behaviors, this is my original
> > > concern.
> > >
> > > > I would do:
> > > >
> > > > void *memmove(void *dest, const void *src, size_t len)
> > > > {
> > > >         if (check_memory_region((unsigned long)src, len, false, _RET_IP_))
> > > >                 return;
> > > if check_memory_region return TRUE is to do nothing, but it is no memory
> > > corruption? Should it return early when check_memory_region return a
> > > FALSE?
> > 
> > Maybe. I just meant the overall idea: check_memory_region should
> > detect that 18446744073709551614 bytes are bad, print an error, return
> > an indication that bytes were bad, memmove should return early if the
> > range is bad.
> > 
> ok, i will send new patch.
> Thanks for your review.
> 
how about this?

commit fd64691026e7ccb8d2946d0804b0621ac177df38
Author: Walter Wu <[email-masked]>
Date:   Fri Sep 27 09:54:18 2019 +0800

    kasan: detect invalid size in memory operation function
    
    It is an undefined behavior to pass a negative value to
memset()/memcpy()/memmove()
    , so need to be detected by KASAN.
    
    KASAN report:
    
     BUG: KASAN: invalid size 18446744073709551614 in
kmalloc_memmove_invalid_size+0x70/0xa0
    
     CPU: 1 PID: 91 Comm: cat Not tainted
5.3.0-rc1ajb-00001-g31943bbc21ce-dirty #7
     Hardware name: linux,dummy-virt (DT)
     Call trace:
      dump_backtrace+0x0/0x278
      show_stack+0x14/0x20
      dump_stack+0x108/0x15c
      print_address_description+0x64/0x368
      __kasan_report+0x108/0x1a4
      kasan_report+0xc/0x18
      check_memory_region+0x15c/0x1b8
      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]>

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index b63b367a94e8..e4e517a51860 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -280,6 +280,23 @@ 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;
@@ -734,6 +751,7 @@ 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();
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 2277b82902d8..5fd377af7457 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);
 }
@@ -110,7 +111,8 @@ 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_);
+	if(!check_memory_region((unsigned long)src, len, false, _RET_IP_))
+		return NULL;
 	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
 
 	return __memmove(dest, src, len);
@@ -119,7 +121,8 @@ 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_);
+	if(!check_memory_region((unsigned long)src, len, false, _RET_IP_))
+		return NULL;
 	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
 
 	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/report.c b/mm/kasan/report.c
index 0e5f965f1882..0cd317ef30f5 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -68,11 +68,16 @@ __setup("kasan_multi_shot", kasan_set_multi_shot);
 
 static void print_error_description(struct kasan_access_info *info)
 {
-	pr_err("BUG: KASAN: %s in %pS\n",
-		get_bug_type(info), (void *)info->ip);
-	pr_err("%s of size %zu at addr %px by task %s/%d\n",
-		info->is_write ? "Write" : "Read", info->access_size,
-		info->access_addr, current->comm, task_pid_nr(current));
+	if ((long)info->access_size < 0) {
+		pr_err("BUG: KASAN: invalid size %zu in %pS\n",
+			info->access_size, (void *)info->ip);
+	} else {
+		pr_err("BUG: KASAN: %s in %pS\n",
+			get_bug_type(info), (void *)info->ip);
+		pr_err("%s of size %zu at addr %px by task %s/%d\n",
+			info->is_write ? "Write" : "Read", info->access_size,
+			info->access_addr, current->comm, task_pid_nr(current));
+	}
 }
 
 static DEFINE_SPINLOCK(report_lock);
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);
 
 	/*

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