diff mbox

net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

Message ID 1471959668-18209-2-git-send-email-luis.henriques@canonical.com
State Rejected, archived
Delegated to: David Miller
Headers show

Commit Message

Luis Henriques Aug. 23, 2016, 1:41 p.m. UTC 
From: Avijit Kanti Das <avijitnsec@codeaurora.org>

memset() the structure ethtool_wolinfo that has padded bytes
but the padded bytes have not been zeroed out.

Change-Id: If3fd2d872a1b1ab9521d937b86a29fc468a8bbfe
Signed-off-by: Avijit Kanti Das <avijitnsec@codeaurora.org>
---
 net/core/ethtool.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Comments

Joe Perches Aug. 23, 2016, 2:06 p.m. UTC | #1 
On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
> From: Avijit Kanti Das <avijitnsec@codeaurora.org>
> 
> memset() the structure ethtool_wolinfo that has padded bytes
> but the padded bytes have not been zeroed out.

I expect there are more of these in the kernel tree.

While this patch is strictly true and the behavior is not
guaranteed by spec, what compilers do not memset then set
the specified member?  Every time I've looked, gcc does.
> diff --git a/net/core/ethtool.c b/net/core/ethtool.c
[]
> @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
>  
>  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
>  {
> -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> +	struct ethtool_wolinfo wol;
>  
>  	if (!dev->ethtool_ops->get_wol)
>  		return -EOPNOTSUPP;
>  
> +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> +	wol.cmd = ETHTOOL_GWOL;
>  	dev->ethtool_ops->get_wol(dev, &wol);
>  
>  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
Eric Dumazet Aug. 23, 2016, 2:21 p.m. UTC | #2 
On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
> From: Avijit Kanti Das <avijitnsec@codeaurora.org>
> 
> memset() the structure ethtool_wolinfo that has padded bytes
> but the padded bytes have not been zeroed out.
> 
> Change-Id: If3fd2d872a1b1ab9521d937b86a29fc468a8bbfe
> Signed-off-by: Avijit Kanti Das <avijitnsec@codeaurora.org>
> ---
>  net/core/ethtool.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> index 977489820eb9..6bf6362e8114 100644
> --- a/net/core/ethtool.c
> +++ b/net/core/ethtool.c
> @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
>  
>  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
>  {
> -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> +	struct ethtool_wolinfo wol;
>  
>  	if (!dev->ethtool_ops->get_wol)
>  		return -EOPNOTSUPP;
>  
> +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> +	wol.cmd = ETHTOOL_GWOL;
>  	dev->ethtool_ops->get_wol(dev, &wol);
>  
>  	if (copy_to_user(useraddr, &wol, sizeof(wol)))

This would suggest a compiler bug to me.

I checked that my compiler does properly put zeros there, even in the
padding area.

If we can not rely on such constructs, we have hundreds of similar
patches to submit.

    3c1c:	48 c7 84 24 84 00 00 	movq   $0x0,0x84(%rsp)
    3c23:	00 00 00 00 00 
    3c28:	48 c7 84 24 8c 00 00 	movq   $0x0,0x8c(%rsp)
    3c2f:	00 00 00 00 00 
    3c34:	c7 84 24 94 00 00 00 	movl   $0x0,0x94(%rsp)
    3c3b:	00 00 00 00 
    3c3f:	c7 84 24 84 00 00 00 	movl   $0x5,0x84(%rsp)
    3c46:	05 00 00 00 
    3c4a:	4d 8b b5 18 02 00 00 	mov    0x218(%r13),%r14
    3c51:	49 8b 46 28          	mov    0x28(%r14),%rax
    3c55:	48 85 c0             	test   %rax,%rax
    3c58:	0f 84 9f 0d 00 00    	je     49fd <dev_ethtool+0x1d3d>
    3c5e:	48 8d b4 24 84 00 00 	lea    0x84(%rsp),%rsi
    3c65:	00 
    3c66:	4c 89 ef             	mov    %r13,%rdi
    3c69:	41 bc f2 ff ff ff    	mov    $0xfffffff2,%r12d
    3c6f:	ff d0                	callq  *%rax
    3c71:	be 0e 03 00 00       	mov    $0x30e,%esi
    3c76:	48 c7 c7 00 00 00 00 	mov    $0x0,%rdi
			3c79: R_X86_64_32S	.rodata.str1.8
    3c7d:	e8 00 00 00 00       	callq  3c82 <dev_ethtool+0xfc2>
			3c7e: R_X86_64_PC32	__might_fault-0x4
    3c82:	48 8d b4 24 84 00 00 	lea    0x84(%rsp),%rsi
    3c89:	00 
    3c8a:	ba 14 00 00 00       	mov    $0x14,%edx
    3c8f:	48 89 df             	mov    %rbx,%rdi
    3c92:	e8 00 00 00 00       	callq  3c97 <dev_ethtool+0xfd7>
			3c93: R_X86_64_PC32	_copy_to_user-0x4
    3c97:	48 85 c0             	test   %rax,%rax
Joe Perches Aug. 23, 2016, 3:05 p.m. UTC | #3 
On Tue, 2016-08-23 at 07:21 -0700, Eric Dumazet wrote:
> On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
> > From: Avijit Kanti Das <avijitnsec@codeaurora.org>
> > 
> > memset() the structure ethtool_wolinfo that has padded bytes
> > but the padded bytes have not been zeroed out.
[]
> > diff --git a/net/core/ethtool.c b/net/core/ethtool.c
[]
> > @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
> >  
> >  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
> >  {
> > -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> > +	struct ethtool_wolinfo wol;
> >  
> >  	if (!dev->ethtool_ops->get_wol)
> >  		return -EOPNOTSUPP;
> >  
> > +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> > +	wol.cmd = ETHTOOL_GWOL;
> >  	dev->ethtool_ops->get_wol(dev, &wol);
> >  
> >  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
> This would suggest a compiler bug to me.

A compiler does not have a standards based requirement to
initialize arbitrary padding bytes.

I believe gcc always does zero all padding anyway.

> I checked that my compiler does properly put zeros there, even in the
> padding area.
> 
> If we can not rely on such constructs, we have hundreds of similar
> patches to submit.

True.

From a practical point of view, does any compiler used for
kernel compilation (gcc/icc/llvm/any others?) not always
perform zero padding of alignment bytes?
Eric Dumazet Aug. 23, 2016, 3:36 p.m. UTC | #4 
On Tue, 2016-08-23 at 08:05 -0700, Joe Perches wrote:

> A compiler does not have a standards based requirement to
> initialize arbitrary padding bytes.
> 
> I believe gcc always does zero all padding anyway.

I would not worry for kernel code, because the amount of scrutiny there
will be enough to 'fix potential bugs' [1], but a lot of user land code
would suffer from various bugs as well that might sit there forever.

[1] Also, most call sites in the kernel are using same call stack, so
the offset of '1-7 leaked bytes' vs kernel stack is constant, making
exploits quite challenging.

Even if the current standards are lazy (are they, I did not check ?),
security needs would call for a sane compiler behavior and changing the
standards accordingly.
Joe Perches Aug. 23, 2016, 3:40 p.m. UTC | #5 
On Tue, 2016-08-23 at 19:27 +0400, Loganaden Velvindron wrote:
> Better be safe than sorry.

Better still would be to create a tool via something like
coccinelle that could be run on the entire kernel than
submit a single patch for a construct that likely occurs
dozens of times in the kernel source tree.

(and please do not send html formatted emails to lkml)
Andrey Ryabinin Aug. 23, 2016, 4:38 p.m. UTC | #6 
2016-08-23 18:36 GMT+03:00 Eric Dumazet <eric.dumazet@gmail.com>:
> On Tue, 2016-08-23 at 08:05 -0700, Joe Perches wrote:
>
>> A compiler does not have a standards based requirement to
>> initialize arbitrary padding bytes.
>>
>> I believe gcc always does zero all padding anyway.
>
> I would not worry for kernel code, because the amount of scrutiny there
> will be enough to 'fix potential bugs' [1], but a lot of user land code
> would suffer from various bugs as well that might sit there forever.
>
> [1] Also, most call sites in the kernel are using same call stack, so
> the offset of '1-7 leaked bytes' vs kernel stack is constant, making
> exploits quite challenging.
>
> Even if the current standards are lazy (are they, I did not check ?),
> security needs would call for a sane compiler behavior and changing the
> standards accordingly.
>

 C11 guarantees zeroed padding.
Edward Cree Aug. 23, 2016, 4:46 p.m. UTC | #7 
On 23/08/16 16:36, Eric Dumazet wrote:
> On Tue, 2016-08-23 at 08:05 -0700, Joe Perches wrote:
>
>> A compiler does not have a standards based requirement to
>> initialize arbitrary padding bytes.
>>
>> I believe gcc always does zero all padding anyway.
> Even if the current standards are lazy (are they, I did not check ?),
> security needs would call for a sane compiler behavior and changing the
> standards accordingly.
Sadly C99 is: section 6.2.6.1.6 (in draft N1256) says
  "When a value is stored in an object of structure or union type,
   including in a member object, the bytes of the object representation
   that correspond to any padding bytes take unspecified values."
with a footnote (42) reading
  "Thus, for example, structure assignment need not copy any padding bits."

In C11 (or, at least, draft N1570), the corresponding text is identical,
only the footnote number has changed (51).
HOWEVER, section 6.7.9.10 has changed, and now (for static objects) reads:
  "if it is an aggregate, every member is initialized (recursively)
   according to these rules, _and_any_padding_is_initialized_to_zero_bits_"
(emphasis added).
On the other hand, a sufficiently pedantic (and perverse) language lawyer
could argue that the initialiser only "specifies the initial value stored
in an object" (6.7.9.8), and that when a value is 'stored in an object'
section 6.2.6.1.6 applies to the storing process after the initial value
has been constructed.
On the gripping hand, such an interpretation would cause the new 6.7.9.10
language to have no observable effect, and thus clearly cannot be what was
intended by the C11 committee.

Aren't you glad you asked?

In any case, there is more to life than standards lawyering, compilers
that don't zero the padding bytes are as a practical matter broken even
though not in violation of the (C99) standard.  Besides, it's not as if
Linux doesn't already have requirements on its compilers that go beyond
the standards...

-Ed
Vegard Nossum Aug. 23, 2016, 5:15 p.m. UTC | #8 
On 23 August 2016 at 17:05, Joe Perches <joe@perches.com> wrote:
> On Tue, 2016-08-23 at 07:21 -0700, Eric Dumazet wrote:
>> On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
>> > From: Avijit Kanti Das <avijitnsec@codeaurora.org>
>> >
>> > memset() the structure ethtool_wolinfo that has padded bytes
>> > but the padded bytes have not been zeroed out.
> []
>> > diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> []
>> > @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
>> >
>> >  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
>> >  {
>> > -   struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
>> > +   struct ethtool_wolinfo wol;
>> >
>> >     if (!dev->ethtool_ops->get_wol)
>> >             return -EOPNOTSUPP;
>> >
>> > +   memset(&wol, 0, sizeof(struct ethtool_wolinfo));
>> > +   wol.cmd = ETHTOOL_GWOL;
>> >     dev->ethtool_ops->get_wol(dev, &wol);
>> >
>> >     if (copy_to_user(useraddr, &wol, sizeof(wol)))
>> This would suggest a compiler bug to me.
>
> A compiler does not have a standards based requirement to
> initialize arbitrary padding bytes.
>
> I believe gcc always does zero all padding anyway.
>
>> I checked that my compiler does properly put zeros there, even in the
>> padding area.
>>
>> If we can not rely on such constructs, we have hundreds of similar
>> patches to submit.
>
> True.
>
> From a practical point of view, does any compiler used for
> kernel compilation (gcc/icc/llvm/any others?) not always
> perform zero padding of alignment bytes?
>

gcc often does not do it, depends on a few factors though:

https://lkml.org/lkml/2016/5/20/389


Vegard
Ben Hutchings Aug. 23, 2016, 5:33 p.m. UTC | #9 
On Tue, 2016-08-23 at 07:21 -0700, Eric Dumazet wrote:
> On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
> > 
> > > > From: Avijit Kanti Das <avijitnsec@codeaurora.org>
> > 
> > memset() the structure ethtool_wolinfo that has padded bytes
> > but the padded bytes have not been zeroed out.
> > 
> > Change-Id: If3fd2d872a1b1ab9521d937b86a29fc468a8bbfe
> > > > Signed-off-by: Avijit Kanti Das <avijitnsec@codeaurora.org>
> > ---
> >  net/core/ethtool.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> > index 977489820eb9..6bf6362e8114 100644
> > --- a/net/core/ethtool.c
> > +++ b/net/core/ethtool.c
> > @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
> >  
> >  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
> >  {
> > -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> > +	struct ethtool_wolinfo wol;
> >  
> >  	if (!dev->ethtool_ops->get_wol)
> >  		return -EOPNOTSUPP;
> >  
> > +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> > +	wol.cmd = ETHTOOL_GWOL;
> >  	dev->ethtool_ops->get_wol(dev, &wol);
> >  
> >  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
> 
> This would suggest a compiler bug to me.

Unfortunately the C standard does not guarantee that padding bytes are
initialised (at least not for automatic storage).

[...]
> If we can not rely on such constructs, we have hundreds of similar
> patches to submit.
[...]

Many such patches have been applied and can be found with:

    git log --author=kangjielu@gmail.com

Ben.
diff mbox

Patch

diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index 977489820eb9..6bf6362e8114 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1435,11 +1435,13 @@  static int ethtool_reset(struct net_device *dev, char __user *useraddr)
 
 static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
 {
-	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
+	struct ethtool_wolinfo wol;
 
 	if (!dev->ethtool_ops->get_wol)
 		return -EOPNOTSUPP;
 
+	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
+	wol.cmd = ETHTOOL_GWOL;
 	dev->ethtool_ops->get_wol(dev, &wol);
 
 	if (copy_to_user(useraddr, &wol, sizeof(wol)))