[PATCH] cxgb4: fix undefined behavior in mem.c

[PATCH] cxgb4: fix undefined behavior in mem.c

[Shaobo He]

In function `c4iw_dealloc_mw`, variable mhp's value is printed after
freed, which triggers undefined behavior according to this post:
https://trust-in-soft.com/dangling-pointer-indeterminate/.

This commit fixes it by swapping the order of `kfree` and `pr_debug`.

Signed-off-by: Shaobo He <shaobo@cs.utah.edu>
---
 drivers/infiniband/hw/cxgb4/mem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/cxgb4/mem.c b/drivers/infiniband/hw/cxgb4/mem.c
index 7b76e6f..bb8e0bc 100644
--- a/drivers/infiniband/hw/cxgb4/mem.c
+++ b/drivers/infiniband/hw/cxgb4/mem.c
@@ -684,8 +684,8 @@ int c4iw_dealloc_mw(struct ib_mw *mw)
 			  mhp->wr_waitp);
 	kfree_skb(mhp->dereg_skb);
 	c4iw_put_wr_wait(mhp->wr_waitp);
-	kfree(mhp);
 	pr_debug("ib_mw %p mmid 0x%x ptr %p\n", mw, mmid, mhp);
+	kfree(mhp);
 	return 0;
 }
 
-- 
2.7.4

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Bart Van Assche]

On Thu, 2019-02-28 at 15:38 -0700, Shaobo He wrote:
> In function `c4iw_dealloc_mw`, variable mhp's value is printed after
> freed, which triggers undefined behavior according to this post:
> https://trust-in-soft.com/dangling-pointer-indeterminate/.
> 
> This commit fixes it by swapping the order of `kfree` and `pr_debug`.
> 
> Signed-off-by: Shaobo He <shaobo@cs.utah.edu>
> ---
>  drivers/infiniband/hw/cxgb4/mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/hw/cxgb4/mem.c b/drivers/infiniband/hw/cxgb4/mem.c
> index 7b76e6f..bb8e0bc 100644
> --- a/drivers/infiniband/hw/cxgb4/mem.c
> +++ b/drivers/infiniband/hw/cxgb4/mem.c
> @@ -684,8 +684,8 @@ int c4iw_dealloc_mw(struct ib_mw *mw)
>  			  mhp->wr_waitp);
>  	kfree_skb(mhp->dereg_skb);
>  	c4iw_put_wr_wait(mhp->wr_waitp);
> -	kfree(mhp);
>  	pr_debug("ib_mw %p mmid 0x%x ptr %p\n", mw, mmid, mhp);
> +	kfree(mhp);
>  	return 0;
>  }

Please quote the relevant paragraphs from the C standard. All I have found
about free() in ISO/IEC 9899:2017 is the following:

Description
The free function causes the space pointed to by ptr to be deallocated, that
is, made available for further allocation. If ptr is a null pointer, no
action occurs. Otherwise, if the argument does not match a pointer earlier
returned by a memory management function, or if the space has been
deallocated by a call to free or realloc, the behavior is undefined.

That is not sufficient to claim that the above code triggers undefined
behavior.

Bart.

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Shaobo He]

I can't afford a pdf version of the C standard. So I looked at the draft version 
used in the link I put in the commit message. It says (in 6.2.4:2),

```
The lifetime of an object is the portion of program execution during which 
storage is guaranteed to be reserved for it. An object exists, has a constant 
address, and retains its last-stored value throughout its lifetime. If an object 
is referred to outside of its lifetime, the behavior is undefined. The value of 
a pointer becomes indeterminate when the object it points to (or just past) 
reaches the end of its lifetime.
```
I couldn't find the definition of lifetime over a dynamically allocated object 
in the draft of C standard. I refer to this link 
(https://en.cppreference.com/w/c/language/lifetime) which suggests that the 
lifetime of an allocated object ends after the deallocation function is called 
upon it.

I think maybe the more problematic issue is that the value of a freed pointer is 
intermediate.

Shaobo
On 2/28/19 3:56 PM, Bart Van Assche wrote:
> On Thu, 2019-02-28 at 15:38 -0700, Shaobo He wrote:
>> In function `c4iw_dealloc_mw`, variable mhp's value is printed after
>> freed, which triggers undefined behavior according to this post:
>> https://trust-in-soft.com/dangling-pointer-indeterminate/.
>>
>> This commit fixes it by swapping the order of `kfree` and `pr_debug`.
>>
>> Signed-off-by: Shaobo He <shaobo@cs.utah.edu>
>> ---
>>   drivers/infiniband/hw/cxgb4/mem.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/infiniband/hw/cxgb4/mem.c b/drivers/infiniband/hw/cxgb4/mem.c
>> index 7b76e6f..bb8e0bc 100644
>> --- a/drivers/infiniband/hw/cxgb4/mem.c
>> +++ b/drivers/infiniband/hw/cxgb4/mem.c
>> @@ -684,8 +684,8 @@ int c4iw_dealloc_mw(struct ib_mw *mw)
>>   			  mhp->wr_waitp);
>>   	kfree_skb(mhp->dereg_skb);
>>   	c4iw_put_wr_wait(mhp->wr_waitp);
>> -	kfree(mhp);
>>   	pr_debug("ib_mw %p mmid 0x%x ptr %p\n", mw, mmid, mhp);
>> +	kfree(mhp);
>>   	return 0;
>>   }
> 
> Please quote the relevant paragraphs from the C standard. All I have found
> about free() in ISO/IEC 9899:2017 is the following:
> 
> Description
> The free function causes the space pointed to by ptr to be deallocated, that
> is, made available for further allocation. If ptr is a null pointer, no
> action occurs. Otherwise, if the argument does not match a pointer earlier
> returned by a memory management function, or if the space has been
> deallocated by a call to free or realloc, the behavior is undefined.
> 
> That is not sufficient to claim that the above code triggers undefined
> behavior.
> 
> Bart.
> 

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Bart Van Assche]

On Thu, 2019-02-28 at 16:18 -0700, Shaobo He wrote:
> I can't afford a pdf version of the C standard. So I looked at the draft version 
> used in the link I put in the commit message. It says (in 6.2.4:2),
> 
> ```
> The lifetime of an object is the portion of program execution during which 
> storage is guaranteed to be reserved for it. An object exists, has a constant 
> address, and retains its last-stored value throughout its lifetime. If an object 
> is referred to outside of its lifetime, the behavior is undefined. The value of 
> a pointer becomes indeterminate when the object it points to (or just past) 
> reaches the end of its lifetime.
> ```
> I couldn't find the definition of lifetime over a dynamically allocated object 
> in the draft of C standard. I refer to this link 
> (https://en.cppreference.com/w/c/language/lifetime) which suggests that the 
> lifetime of an allocated object ends after the deallocation function is called 
> upon it.
> 
> I think maybe the more problematic issue is that the value of a freed pointer is 
> intermediate.

In another section of the same draft I found the following:

J.2 Undefined behavior [ ... ] The value of a pointer that refers to space
deallocated by a call to the free or realloc function is used (7.22.3).

Since the C standard explicitly refers to free() and realloc(), does that
mean that that statement about undefined behavior does not apply to munmap()
(for user space code) nor to kfree() (for kernel code)?

Bart.

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Shaobo He]

Good catch. But if we agree on that memory management functions are those 
specified by the C standard, would it be OK to ignore so-called use after free 
or double free bugs for the kernel as C standard does not apply to kfree?

On 2/28/19 4:33 PM, Bart Van Assche wrote:
> On Thu, 2019-02-28 at 16:18 -0700, Shaobo He wrote:
>> I can't afford a pdf version of the C standard. So I looked at the draft version
>> used in the link I put in the commit message. It says (in 6.2.4:2),
>>
>> ```
>> The lifetime of an object is the portion of program execution during which
>> storage is guaranteed to be reserved for it. An object exists, has a constant
>> address, and retains its last-stored value throughout its lifetime. If an object
>> is referred to outside of its lifetime, the behavior is undefined. The value of
>> a pointer becomes indeterminate when the object it points to (or just past)
>> reaches the end of its lifetime.
>> ```
>> I couldn't find the definition of lifetime over a dynamically allocated object
>> in the draft of C standard. I refer to this link
>> (https://en.cppreference.com/w/c/language/lifetime) which suggests that the
>> lifetime of an allocated object ends after the deallocation function is called
>> upon it.
>>
>> I think maybe the more problematic issue is that the value of a freed pointer is
>> intermediate.
> 
> In another section of the same draft I found the following:
> 
> J.2 Undefined behavior [ ... ] The value of a pointer that refers to space
> deallocated by a call to the free or realloc function is used (7.22.3).
> 
> Since the C standard explicitly refers to free() and realloc(), does that
> mean that that statement about undefined behavior does not apply to munmap()
> (for user space code) nor to kfree() (for kernel code)?
> 
> Bart.
> 

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Doug Ledford]

[-- Attachment #1: Type: text/plain, Size: 3158 bytes --]

On Thu, 2019-02-28 at 16:57 -0700, Shaobo He wrote:
> Good catch. But if we agree on that memory management functions are those 
> specified by the C standard, would it be OK to ignore so-called use after free 
> or double free bugs for the kernel as C standard does not apply to kfree?

No, most kernel use-after-free bugs are real bugs.  This one might be
technically a bug by certain readings of the standard, but it's a non-
issue.  Real use-after-free bugs don't just look at the value of a local
stack variable to get the memory's old address (which is what this does,
and the same could be achieved and be totally in spec by doing this:

    old_ptr = mhp;
    kfree(mhp);
    pr_debug("%p\n", old_ptr);)

Real use after free things would actually dereference the pointer to
either read or write from the old memory region.  That leads to data
corruption or kernel data leaks.  Plus, in this case, the purpose of
printing the literal value of mhp is simply to provide a unique name for
tracing purposes.  Since kfree() doesn't alter the local stack variable,
the name is still present in the local stack variable at the point you
call pr_debug().

It could be fixed.  It's not like this patch is wrong.  But I wouldn't
submit it this late in the -rc cycle, I'd just take it for next.

> On 2/28/19 4:33 PM, Bart Van Assche wrote:
> > On Thu, 2019-02-28 at 16:18 -0700, Shaobo He wrote:
> > > I can't afford a pdf version of the C standard. So I looked at the draft version
> > > used in the link I put in the commit message. It says (in 6.2.4:2),
> > > 
> > > ```
> > > The lifetime of an object is the portion of program execution during which
> > > storage is guaranteed to be reserved for it. An object exists, has a constant
> > > address, and retains its last-stored value throughout its lifetime. If an object
> > > is referred to outside of its lifetime, the behavior is undefined. The value of
> > > a pointer becomes indeterminate when the object it points to (or just past)
> > > reaches the end of its lifetime.
> > > ```
> > > I couldn't find the definition of lifetime over a dynamically allocated object
> > > in the draft of C standard. I refer to this link
> > > (https://en.cppreference.com/w/c/language/lifetime) which suggests that the
> > > lifetime of an allocated object ends after the deallocation function is called
> > > upon it.
> > > 
> > > I think maybe the more problematic issue is that the value of a freed pointer is
> > > intermediate.
> > 
> > In another section of the same draft I found the following:
> > 
> > J.2 Undefined behavior [ ... ] The value of a pointer that refers to space
> > deallocated by a call to the free or realloc function is used (7.22.3).
> > 
> > Since the C standard explicitly refers to free() and realloc(), does that
> > mean that that statement about undefined behavior does not apply to munmap()
> > (for user space code) nor to kfree() (for kernel code)?
> > 
> > Bart.
> > 

-- 
Doug Ledford <dledford@redhat.com>
    GPG KeyID: B826A3330E572FDD
    Key fingerprint = AE6B 1BDA 122B 23B4 265B  1274 B826 A333 0E57 2FDD

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Shaobo He]

Yes, why wouldn't they be real bugs? I was simply pointing out the irrational 
conclusion if the C standard is strictly applied to kernel code.

I think the spirit of the C standard is that one shouldn't rely on the 
assumption that the value of a freed pointer does not change, even though in 
practice any compiler developers probably won't bother to implement the logic to 
change the pointer value or optimizations leveraging it even they are allowed 
to. In other words, the original code may be a little bit problematic in the 
spirit of the C standard whereas the patch simply makes it totally valid.

If it can be finally submitted, that would be great. If not, I'm totally fine.

Shaobo
On 2019/3/1 7:26, Doug Ledford wrote:
> On Thu, 2019-02-28 at 16:57 -0700, Shaobo He wrote:
>> Good catch. But if we agree on that memory management functions are those
>> specified by the C standard, would it be OK to ignore so-called use after free
>> or double free bugs for the kernel as C standard does not apply to kfree?
> 
> No, most kernel use-after-free bugs are real bugs.  This one might be
> technically a bug by certain readings of the standard, but it's a non-
> issue.  Real use-after-free bugs don't just look at the value of a local
> stack variable to get the memory's old address (which is what this does,
> and the same could be achieved and be totally in spec by doing this:
> 
>      old_ptr = mhp;
>      kfree(mhp);
>      pr_debug("%p\n", old_ptr);)
> 
> Real use after free things would actually dereference the pointer to
> either read or write from the old memory region.  That leads to data
> corruption or kernel data leaks.  Plus, in this case, the purpose of
> printing the literal value of mhp is simply to provide a unique name for
> tracing purposes.  Since kfree() doesn't alter the local stack variable,
> the name is still present in the local stack variable at the point you
> call pr_debug().
> 
> It could be fixed.  It's not like this patch is wrong.  But I wouldn't
> submit it this late in the -rc cycle, I'd just take it for next.
> 
>> On 2/28/19 4:33 PM, Bart Van Assche wrote:
>>> On Thu, 2019-02-28 at 16:18 -0700, Shaobo He wrote:
>>>> I can't afford a pdf version of the C standard. So I looked at the draft version
>>>> used in the link I put in the commit message. It says (in 6.2.4:2),
>>>>
>>>> ```
>>>> The lifetime of an object is the portion of program execution during which
>>>> storage is guaranteed to be reserved for it. An object exists, has a constant
>>>> address, and retains its last-stored value throughout its lifetime. If an object
>>>> is referred to outside of its lifetime, the behavior is undefined. The value of
>>>> a pointer becomes indeterminate when the object it points to (or just past)
>>>> reaches the end of its lifetime.
>>>> ```
>>>> I couldn't find the definition of lifetime over a dynamically allocated object
>>>> in the draft of C standard. I refer to this link
>>>> (https://en.cppreference.com/w/c/language/lifetime) which suggests that the
>>>> lifetime of an allocated object ends after the deallocation function is called
>>>> upon it.
>>>>
>>>> I think maybe the more problematic issue is that the value of a freed pointer is
>>>> intermediate.
>>>
>>> In another section of the same draft I found the following:
>>>
>>> J.2 Undefined behavior [ ... ] The value of a pointer that refers to space
>>> deallocated by a call to the free or realloc function is used (7.22.3).
>>>
>>> Since the C standard explicitly refers to free() and realloc(), does that
>>> mean that that statement about undefined behavior does not apply to munmap()
>>> (for user space code) nor to kfree() (for kernel code)?
>>>
>>> Bart.
>>>
> 

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Christopher Lameter]

On Thu, 28 Feb 2019, Shaobo He wrote:

> I think maybe the more problematic issue is that the value of a freed pointer
> is intermediate.

The pointer is not affected by freeing the data it points to. Thus it
definitely has the same value as before and is not indeterminate.

The pointer points now to an area of memory that could now be in use for
different purposes so maybe it could be taken as a dangerous situation.

But situations like that are common in code.

Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Jason Gunthorpe]

On Thu, Feb 28, 2019 at 03:38:38PM -0700, Shaobo He wrote:
> In function `c4iw_dealloc_mw`, variable mhp's value is printed after
> freed, which triggers undefined behavior according to this post:
> https://trust-in-soft.com/dangling-pointer-indeterminate/.
> 
> This commit fixes it by swapping the order of `kfree` and `pr_debug`.
> 
> Signed-off-by: Shaobo He <shaobo@cs.utah.edu>
> ---
>  drivers/infiniband/hw/cxgb4/mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Discussion aside, this is a worthwile fix. I rewrote the commit
message to avoid referencing 'undefined behavior' though, this is just
a straight up bug in the logging. Another thread could get the same
pointer value for the mhp before the print creating a confusing log.

    cxgb4: kfree mhp after the debug print
    
    In function `c4iw_dealloc_mw`, variable mhp's value is printed after
    freed, it is clearer to have the print before the kfree.
    
    Otherwise racing threads could allocate another mhp with the same pointer
    value and create confusing tracing.

Jason