[PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Juergen Gross]

xencall_alloc_buffer() is used throughout Xen tools for allocating
hypercall buffers. Allocation is done at page granularity. For simple
administration each allocated set of pages contains a small header
holding the number of pages of that set. The hypercall buffer is
located directly after the 4 byte sized header, leading to a wrong
alignment for e.g. pointers.

Repair that by using a 16 byte sized header enforcing the same
alignment as malloc().

Signed-off-by: Juergen Gross <jgross@suse.com>
---
Not sure whether this wants to be backported. In case the answer is
"yes" I think it should be part of 4.11.
---
 tools/libs/call/buffer.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
index 2d8fc29ac6..0b6af2db60 100644
--- a/tools/libs/call/buffer.c
+++ b/tools/libs/call/buffer.c
@@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, void *p, size_t nr_pages)
 
 struct allocation_header {
     int nr_pages;
+    int pad[3];
 };
 
 void *xencall_alloc_buffer(xencall_handle *xcall, size_t size)
-- 
2.13.7


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Re: [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Andrew Cooper]

On 08/06/18 10:51, Juergen Gross wrote:
> xencall_alloc_buffer() is used throughout Xen tools for allocating
> hypercall buffers. Allocation is done at page granularity. For simple
> administration each allocated set of pages contains a small header
> holding the number of pages of that set. The hypercall buffer is
> located directly after the 4 byte sized header, leading to a wrong
> alignment for e.g. pointers.
>
> Repair that by using a 16 byte sized header enforcing the same
> alignment as malloc().
>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> Not sure whether this wants to be backported. In case the answer is
> "yes" I think it should be part of 4.11.
> ---
>  tools/libs/call/buffer.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
> index 2d8fc29ac6..0b6af2db60 100644
> --- a/tools/libs/call/buffer.c
> +++ b/tools/libs/call/buffer.c
> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, void *p, size_t nr_pages)
>  
>  struct allocation_header {
>      int nr_pages;

This hunk reveals another bug...

There are a rather large number of size_t => int truncations in the
code, which is surely going to catch people by surprise.  (Again - I was
certain I commented on this during the original review of this library.)

> +    int pad[3];

This brings it to 16 byte alignment, but is that enough?  An xsave state
block in x86 for example has 64 byte alignment.  I suppose we mostly
care about ARM here, as its memcpy() will be most impacted by this
misalignment.

Irrespective, might be easier to accomplish with an
__attribute__((__aligned__($N))) rather than explicit padding?

~Andrew

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Re: [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Juergen Gross]

On 08/06/18 12:09, Andrew Cooper wrote:
> On 08/06/18 10:51, Juergen Gross wrote:
>> xencall_alloc_buffer() is used throughout Xen tools for allocating
>> hypercall buffers. Allocation is done at page granularity. For simple
>> administration each allocated set of pages contains a small header
>> holding the number of pages of that set. The hypercall buffer is
>> located directly after the 4 byte sized header, leading to a wrong
>> alignment for e.g. pointers.
>>
>> Repair that by using a 16 byte sized header enforcing the same
>> alignment as malloc().
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> Not sure whether this wants to be backported. In case the answer is
>> "yes" I think it should be part of 4.11.
>> ---
>>  tools/libs/call/buffer.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
>> index 2d8fc29ac6..0b6af2db60 100644
>> --- a/tools/libs/call/buffer.c
>> +++ b/tools/libs/call/buffer.c
>> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, void *p, size_t nr_pages)
>>  
>>  struct allocation_header {
>>      int nr_pages;
> 
> This hunk reveals another bug...
> 
> There are a rather large number of size_t => int truncations in the
> code, which is surely going to catch people by surprise.  (Again - I was
> certain I commented on this during the original review of this library.)

On the practical side I don't think this really matters here. Passing
more than 2GB as a hypercall buffer is nothing we want to support...

> 
>> +    int pad[3];
> 
> This brings it to 16 byte alignment, but is that enough?  An xsave state
> block in x86 for example has 64 byte alignment.  I suppose we mostly
> care about ARM here, as its memcpy() will be most impacted by this
> misalignment.

Special purpose buffers can still be directly allocated via
xencall_alloc_buffer_pages(). I don't think we'll need alignment better
than malloc().

> Irrespective, might be easier to accomplish with an
> __attribute__((__aligned__($N))) rather than explicit padding?

I don't mind which way to go here. I'll leave the decision for the
maintainers. :-)


Juergen

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Re: [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Juergen Gross]

On 08/06/18 12:25, Juergen Gross wrote:
> On 08/06/18 12:09, Andrew Cooper wrote:
>> On 08/06/18 10:51, Juergen Gross wrote:
>>> xencall_alloc_buffer() is used throughout Xen tools for allocating
>>> hypercall buffers. Allocation is done at page granularity. For simple
>>> administration each allocated set of pages contains a small header
>>> holding the number of pages of that set. The hypercall buffer is
>>> located directly after the 4 byte sized header, leading to a wrong
>>> alignment for e.g. pointers.
>>>
>>> Repair that by using a 16 byte sized header enforcing the same
>>> alignment as malloc().
>>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>> Not sure whether this wants to be backported. In case the answer is
>>> "yes" I think it should be part of 4.11.
>>> ---
>>>  tools/libs/call/buffer.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
>>> index 2d8fc29ac6..0b6af2db60 100644
>>> --- a/tools/libs/call/buffer.c
>>> +++ b/tools/libs/call/buffer.c
>>> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, void *p, size_t nr_pages)
>>>  
>>>  struct allocation_header {
>>>      int nr_pages;
>>
>> This hunk reveals another bug...
>>
>> There are a rather large number of size_t => int truncations in the
>> code, which is surely going to catch people by surprise.  (Again - I was
>> certain I commented on this during the original review of this library.)
> 
> On the practical side I don't think this really matters here. Passing
> more than 2GB as a hypercall buffer is nothing we want to support...
> 
>>
>>> +    int pad[3];
>>
>> This brings it to 16 byte alignment, but is that enough?  An xsave state
>> block in x86 for example has 64 byte alignment.  I suppose we mostly
>> care about ARM here, as its memcpy() will be most impacted by this
>> misalignment.
> 
> Special purpose buffers can still be directly allocated via
> xencall_alloc_buffer_pages(). I don't think we'll need alignment better
> than malloc().
> 
>> Irrespective, might be easier to accomplish with an
>> __attribute__((__aligned__($N))) rather than explicit padding?
> 
> I don't mind which way to go here. I'll leave the decision for the
> maintainers. :-)

Ping?


Juergen


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Re: [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Wei Liu]

On Tue, Jul 10, 2018 at 03:25:07PM +0200, Juergen Gross wrote:
> On 08/06/18 12:25, Juergen Gross wrote:
> > On 08/06/18 12:09, Andrew Cooper wrote:
> >> On 08/06/18 10:51, Juergen Gross wrote:
> >>> xencall_alloc_buffer() is used throughout Xen tools for allocating
> >>> hypercall buffers. Allocation is done at page granularity. For simple
> >>> administration each allocated set of pages contains a small header
> >>> holding the number of pages of that set. The hypercall buffer is
> >>> located directly after the 4 byte sized header, leading to a wrong
> >>> alignment for e.g. pointers.
> >>>
> >>> Repair that by using a 16 byte sized header enforcing the same
> >>> alignment as malloc().
> >>>
> >>> Signed-off-by: Juergen Gross <jgross@suse.com>
> >>> ---
> >>> Not sure whether this wants to be backported. In case the answer is
> >>> "yes" I think it should be part of 4.11.
> >>> ---
> >>>  tools/libs/call/buffer.c | 1 +
> >>>  1 file changed, 1 insertion(+)
> >>>
> >>> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
> >>> index 2d8fc29ac6..0b6af2db60 100644
> >>> --- a/tools/libs/call/buffer.c
> >>> +++ b/tools/libs/call/buffer.c
> >>> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, void *p, size_t nr_pages)
> >>>  
> >>>  struct allocation_header {
> >>>      int nr_pages;
> >>
> >> This hunk reveals another bug...
> >>
> >> There are a rather large number of size_t => int truncations in the
> >> code, which is surely going to catch people by surprise.  (Again - I was
> >> certain I commented on this during the original review of this library.)
> > 
> > On the practical side I don't think this really matters here. Passing
> > more than 2GB as a hypercall buffer is nothing we want to support...
> > 
> >>
> >>> +    int pad[3];
> >>
> >> This brings it to 16 byte alignment, but is that enough?  An xsave state
> >> block in x86 for example has 64 byte alignment.  I suppose we mostly
> >> care about ARM here, as its memcpy() will be most impacted by this
> >> misalignment.
> > 
> > Special purpose buffers can still be directly allocated via
> > xencall_alloc_buffer_pages(). I don't think we'll need alignment better
> > than malloc().
> > 
> >> Irrespective, might be easier to accomplish with an
> >> __attribute__((__aligned__($N))) rather than explicit padding?
> > 
> > I don't mind which way to go here. I'll leave the decision for the
> > maintainers. :-)
> 
> Ping?

Either is fine. I will apply this patch as-is.

Wei.

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