CVE-2014-9900 fix is not upstream

CVE-2014-9900 fix is not upstream

[Luis Henriques]

Hi!

Digging through some old CVEs I came across this one that doesn't seem be
in mainline.  Was there a good reason for not being sent upstream?  Maybe it was
rejected for some reason and I failed to find the discussion.

References:
 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-9900
 http://source.android.com/security/bulletin/2016-08-01.html
 https://source.codeaurora.org/quic/la/kernel/msm-3.10/commit/?id=63c317dbee97983004dffdd9f742a20d17150071

Cheers,
--
Luís

net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Luis Henriques]

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)))

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Joe Perches]

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)))

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Eric Dumazet]

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

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Joe Perches]

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?

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Eric Dumazet]

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.

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Joe Perches]

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)

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Andrey Ryabinin]

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.

Re: CVE-2014-9900 fix is not upstream

[David Miller]

From: Luis Henriques <luis.henriques@canonical.com>
Date: Tue, 23 Aug 2016 14:41:07 +0100

> Digging through some old CVEs I came across this one that doesn't seem be
> in mainline.  Was there a good reason for not being sent upstream?  Maybe it was
> rejected for some reason and I failed to find the discussion.

Because the patch is completely bogus, and thus so is the CVE.

The variable initializer clears out the entire structure.

Until you can show compiler output from gcc that shows it not
initializing the structure I will not apply this patch because I know
that it faithfully does.

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Edward Cree]

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

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Vegard Nossum]

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

Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

[Ben Hutchings]

[-- Attachment #1: Type: text/plain, Size: 1772 bytes --]

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.

-- 
Ben Hutchings
The program is absolutely right; therefore, the computer must be wrong.

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Re: CVE-2014-9900 fix is not upstream

[Ben Hutchings]

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On Tue, 2016-08-23 at 09:40 -0700, David Miller wrote:
> From: Luis Henriques <luis.henriques@canonical.com>
> Date: Tue, 23 Aug 2016 14:41:07 +0100
> 
> > Digging through some old CVEs I came across this one that doesn't
> seem be
> > in mainline.  Was there a good reason for not being sent upstream? 
> Maybe it was
> > rejected for some reason and I failed to find the discussion.
> 
> Because the patch is completely bogus, and thus so is the CVE.
> 
> The variable initializer clears out the entire structure.
> 
> Until you can show compiler output from gcc that shows it not
> initializing the structure I will not apply this patch because I know
> that it faithfully does.

On some versions and architectures.  Can you guarantee that you will
notice when an exception appears?

Ben.

-- 
Ben Hutchings
The program is absolutely right; therefore, the computer must be wrong.

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Re: CVE-2014-9900 fix is not upstream

[David Miller]

From: Ben Hutchings <ben@decadent.org.uk>
Date: Tue, 23 Aug 2016 18:35:27 +0100

> On Tue, 2016-08-23 at 09:40 -0700, David Miller wrote:
>> From: Luis Henriques <luis.henriques@canonical.com>
>> Date: Tue, 23 Aug 2016 14:41:07 +0100
>> 
>> > Digging through some old CVEs I came across this one that doesn't
>> seem be
>> > in mainline.  Was there a good reason for not being sent upstream? 
>> Maybe it was
>> > rejected for some reason and I failed to find the discussion.
>> 
>> Because the patch is completely bogus, and thus so is the CVE.
>> 
>> The variable initializer clears out the entire structure.
>> 
>> Until you can show compiler output from gcc that shows it not
>> initializing the structure I will not apply this patch because I know
>> that it faithfully does.
> 
> On some versions and architectures.  Can you guarantee that you will
> notice when an exception appears?

Again, show me the assembler output exhibiting the lack of
initialization, for this specific structure and situation.

That's all that I'm asking.

Re: CVE-2014-9900 fix is not upstream

[Al Viro]

On Tue, Aug 23, 2016 at 11:24:06AM -0700, David Miller wrote:

> > On some versions and architectures.  Can you guarantee that you will
> > notice when an exception appears?
> 
> Again, show me the assembler output exhibiting the lack of
> initialization, for this specific structure and situation.
> 
> That's all that I'm asking.

... and then we can file a bug report against the sodding compiler.  Note
that
struct ethtool_wolinfo {
        __u32   cmd;
        __u32   supported;
        __u32   wolopts;
        __u8    sopass[SOPASS_MAX];	// 6, actually
};
is not going to *have* padding.  Not on anything even remotely sane.
If array of 6 char as member of a struct requires 64bit alignment on some
architecture, I would really like some of what the designers of that ABI
must have been smoking.

Initializer might be allowed to leave padding uninitialized.  But all fields
_must_ be initialized, the missing initializers treated exactly as they
would've been for a static-duration object (C99 6.7.8p19).  And that is
going to cover everything in that sucker.  It's not a function of compiler -
only of C ABI on given target.

Re: CVE-2014-9900 fix is not upstream

[Joe Perches]

On Tue, 2016-08-23 at 21:09 +0100, Al Viro wrote:
> On Tue, Aug 23, 2016 at 11:24:06AM -0700, David Miller wrote:
> > > On some versions and architectures.  Can you guarantee that you will
> > > notice when an exception appears?
> > Again, show me the assembler output exhibiting the lack of
> > initialization, for this specific structure and situation.
> > 
> > That's all that I'm asking.
> ... and then we can file a bug report against the sodding compiler.  Note
> that
> struct ethtool_wolinfo {
>         __u32   cmd;
>         __u32   supported;
>         __u32   wolopts;
>         __u8    sopass[SOPASS_MAX];	// 6, actually
> };
> is not going to *have* padding.  Not on anything even remotely sane.
> If array of 6 char as member of a struct requires 64bit alignment on some
> architecture, I would really like some of what the designers of that ABI
> must have been smoking.

try this on x86-64

$ pahole -C ethtool_wolinfo vmlinux
struct ethtool_wolinfo {
	__u32                      cmd;                  /*     0     4 */
	__u32                      supported;            /*     4     4 */
	__u32                      wolopts;              /*     8     4 */
	__u8                       sopass[6];            /*    12     6 */

	/* size: 20, cachelines: 1, members: 4 */
	/* padding: 2 */
	/* last cacheline: 20 bytes */
};

Re: CVE-2014-9900 fix is not upstream

[Lennart Sorensen]

On Tue, Aug 23, 2016 at 01:34:05PM -0700, Joe Perches wrote:
> On Tue, 2016-08-23 at 21:09 +0100, Al Viro wrote:
> > On Tue, Aug 23, 2016 at 11:24:06AM -0700, David Miller wrote:
> > ... and then we can file a bug report against the sodding compiler.  Note
> > that
> > struct ethtool_wolinfo {
> >         __u32   cmd;
> >         __u32   supported;
> >         __u32   wolopts;
> >         __u8    sopass[SOPASS_MAX];	// 6, actually
> > };
> > is not going to *have* padding.  Not on anything even remotely sane.
> > If array of 6 char as member of a struct requires 64bit alignment on some
> > architecture, I would really like some of what the designers of that ABI
> > must have been smoking.
> 
> try this on x86-64
> 
> $ pahole -C ethtool_wolinfo vmlinux
> struct ethtool_wolinfo {
> 	__u32                      cmd;                  /*     0     4 */
> 	__u32                      supported;            /*     4     4 */
> 	__u32                      wolopts;              /*     8     4 */
> 	__u8                       sopass[6];            /*    12     6 */
> 
> 	/* size: 20, cachelines: 1, members: 4 */
> 	/* padding: 2 */
> 	/* last cacheline: 20 bytes */
> };

That would be padding after the structure elements.

I think what was meant is that it won't add padding in the middle of the
structure due to alignment, ie it isn't doing:

struct ethtool_wolinfo {
	__u32                      cmd;                  /*     0     4 */
	__u32                      supported;            /*     4     4 */
	__u32                      wolopts;              /*     8     4 */
	<4 bytes padding here>
	__u8                       sopass[6];            /*    16     6 */
};

which would have 4 bytes of padding in the middle between wolopts
and sopass.

I would not think it is the compilers job to worry about what is after
your structure elements, since you shouldn't be going there.

-- 
Len Sorensen

Re: CVE-2014-9900 fix is not upstream

[Al Viro]

On Tue, Aug 23, 2016 at 04:49:33PM -0400, Lennart Sorensen wrote:
> That would be padding after the structure elements.
> 
> I think what was meant is that it won't add padding in the middle of the
> structure due to alignment, ie it isn't doing:
> 
> struct ethtool_wolinfo {
> 	__u32                      cmd;                  /*     0     4 */
> 	__u32                      supported;            /*     4     4 */
> 	__u32                      wolopts;              /*     8     4 */
> 	<4 bytes padding here>
> 	__u8                       sopass[6];            /*    16     6 */
> };
> 
> which would have 4 bytes of padding in the middle between wolopts
> and sopass.
> 
> I would not think it is the compilers job to worry about what is after
> your structure elements, since you shouldn't be going there.

Sadly, sizeof is what we use when copying that sucker to userland.  So these
padding bits in the end would've leaked, true enough, and the case is somewhat
weaker.  And any normal architecture will have those, but then any such
architecture will have no more trouble zeroing a 32bit value than 16bit one.

Re: CVE-2014-9900 fix is not upstream

[Lennart Sorensen]

On Tue, Aug 23, 2016 at 10:25:45PM +0100, Al Viro wrote:
> Sadly, sizeof is what we use when copying that sucker to userland.  So these
> padding bits in the end would've leaked, true enough, and the case is somewhat
> weaker.  And any normal architecture will have those, but then any such
> architecture will have no more trouble zeroing a 32bit value than 16bit one.

Hmm, good point.  Too bad I don't see a compiler option of "zero all
padding in structs".  Certainly generating the code should not really
be that different.

I see someone did request it 2 years ago:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63479

-- 
Len Sorensen

Re: CVE-2014-9900 fix is not upstream

[Hannes Frederic Sowa]

On 24.08.2016 16:03, Lennart Sorensen wrote:
> On Tue, Aug 23, 2016 at 10:25:45PM +0100, Al Viro wrote:
>> Sadly, sizeof is what we use when copying that sucker to userland.  So these
>> padding bits in the end would've leaked, true enough, and the case is somewhat
>> weaker.  And any normal architecture will have those, but then any such
>> architecture will have no more trouble zeroing a 32bit value than 16bit one.
> 
> Hmm, good point.  Too bad I don't see a compiler option of "zero all
> padding in structs".  Certainly generating the code should not really
> be that different.
> 
> I see someone did request it 2 years ago:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63479

I don't think this is sufficient. Basically if you write one field in a
struct after a memset again, the compiler is allowed by the standard to
write padding bytes again, causing them to be undefined.

If we want to go down this route, probably the only option is to add
__attribute__((pack)) those structs to just have no padding at all, thus
breaking uapi.

E.g. the x11 protocol implementation specifies padding bytes in their
binary representation of the wire protocol to limit the leaking:

https://cgit.freedesktop.org/xorg/proto/xproto/tree/Xproto.h

... which would be another option.

Bye,
Hannes

Re: CVE-2014-9900 fix is not upstream

[Johannes Berg]

> If we want to go down this route, probably the only option is to add
> __attribute__((pack)) those structs to just have no padding at all,
> thus breaking uapi.
> 

We could also spell out the padding bytes as reserved, i.e. instead of

struct ethtool_wolinfo {
        __u32   cmd;
        __u32   supported;
        __u32   wolopts;
        __u8    sopass[SOPASS_MAX];     // 6, actually
};

we could do

struct ethtool_wolinfo {
        __u32   cmd;
        __u32   supported;
        __u32   wolopts;
        __u8    sopass[SOPASS_MAX];     // 6, actually
	__u8	reserved[2];
};

and then the compiler has to properly treat it, since it's no longer
unnamed padding.

Maybe somebody can come up with a smart BUILD_BUG_ON() to ensure such
structs have no padding.

That would allow us to keep the C99 initializers (which is nice) and
not have to worry about this.

johannes

Re: CVE-2014-9900 fix is not upstream

[Johannes Berg]

> struct ethtool_wolinfo {
>         __u32   cmd;
>         __u32   supported;
>         __u32   wolopts;
>         __u8    sopass[SOPASS_MAX];     // 6, actually
> };
> 
> we could do
> 
> struct ethtool_wolinfo {
>         __u32   cmd;
>         __u32   supported;
>         __u32   wolopts;
>         __u8    sopass[SOPASS_MAX];     // 6, actually
> 	__u8	reserved[2];
> };
> 
> and then the compiler has to properly treat it, since it's no longer
> unnamed padding.
> 

Although, on some architectures, that could actually break the ABI by
changing the size, oh well.

johannes

Re: CVE-2014-9900 fix is not upstream

[One Thousand Gnomes]

> > I see someone did request it 2 years ago:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63479  
> 
> I don't think this is sufficient. Basically if you write one field in a
> struct after a memset again, the compiler is allowed by the standard to
> write padding bytes again, causing them to be undefined.

The question is simply what gcc actually does. The rest is C language
lawyering and since the kernel isn't written to the C language spec but
to gcc only gcc matters.

Alan