Re: [Xen-devel] [PATCH] xen/disk: don't leak stack data via response ring

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.06.17 at 20:46, <sstabellini@xxxxxxxxxx> wrote:
> On Wed, 21 Jun 2017, Jan Beulich wrote:
>> >>> On 20.06.17 at 23:48, <sstabellini@xxxxxxxxxx> wrote:
>> > On Tue, 20 Jun 2017, Jan Beulich wrote:
>> >> @@ -36,13 +33,7 @@ struct blkif_x86_32_request_discard {
>> >>      blkif_sector_t sector_number;    /* start sector idx on disk (r/w 
> only)  */
>> >>      uint64_t       nr_sectors;       /* # of contiguous sectors to 
>> >> discard 
>   */
>> >>  };
>> >> -struct blkif_x86_32_response {
>> >> -    uint64_t        id;              /* copied from request */
>> >> -    uint8_t         operation;       /* copied from request */
>> >> -    int16_t         status;          /* BLKIF_RSP_???       */
>> >> -};
>> >>  typedef struct blkif_x86_32_request blkif_x86_32_request_t;
>> >> -typedef struct blkif_x86_32_response blkif_x86_32_response_t;
>> >>  #pragma pack(pop)
>> >>  
>> >>  /* x86_64 protocol version */
>> >> @@ -62,20 +53,14 @@ struct blkif_x86_64_request_discard {
>> >>      blkif_sector_t sector_number;    /* start sector idx on disk (r/w 
> only)  */
>> >>      uint64_t       nr_sectors;       /* # of contiguous sectors to 
>> >> discard 
>   */
>> >>  };
>> >> -struct blkif_x86_64_response {
>> >> -    uint64_t       __attribute__((__aligned__(8))) id;
>> >> -    uint8_t         operation;       /* copied from request */
>> >> -    int16_t         status;          /* BLKIF_RSP_???       */
>> >> -};
>> >>
>> >>  typedef struct blkif_x86_64_request blkif_x86_64_request_t;
>> >> -typedef struct blkif_x86_64_response blkif_x86_64_response_t;
>> >>  
>> >>  DEFINE_RING_TYPES(blkif_common, struct blkif_common_request,
>> >> -                  struct blkif_common_response);
>> >> +                  struct blkif_response);
>> >>  DEFINE_RING_TYPES(blkif_x86_32, struct blkif_x86_32_request,
>> >> -                  struct blkif_x86_32_response);
>> >> +                  struct blkif_response QEMU_PACKED);
>> > 
>> > In my test, the previous sizes and alignments of the response structs
>> > were (on both x86_32 and x86_64):
>> > 
>> > sizeof(blkif_x86_32_response)=12   sizeof(blkif_x86_64_response)=16
>> > align(blkif_x86_32_response)=4     align(blkif_x86_64_response)=8
>> > 
>> > While with these changes are now, when compiled on x86_64:
>> > sizeof(blkif_x86_32_response)=11   sizeof(blkif_x86_64_response)=16
>> > align(blkif_x86_32_response)=1     align(blkif_x86_64_response)=8
>> > 
>> > when compiled on x86_32:
>> > sizeof(blkif_x86_32_response)=11   sizeof(blkif_x86_64_response)=12
>> > align(blkif_x86_32_response)=1     align(blkif_x86_64_response)=4
>> > 
>> > Did I do my tests wrong?
>> > 
>> > QEMU_PACKED is not the same as #pragma pack(push, 4). In fact, it is the
>> > same as #pragma pack(push, 1), causing the struct to be densely packed,
>> > leaving no padding whatsever.
>> > 
>> > In addition, without __attribute__((__aligned__(8))),
>> > blkif_x86_64_response won't be 8 bytes aligned when built on x86_32.
>> > 
>> > Am I missing something?
>> 
>> Well, you're mixing attribute application upon structure
>> declaration with attribute application upon structure use. It's
>> the latter here, and hence the attribute doesn't affect
>> structure layout at all. All it does is avoid the _containing_
>> 32-bit union to become 8-byte aligned (and tail padding to be
>> inserted).
> 
> Thanks for the explanation. I admit it's the first time I see the
> aligned attribute being used at structure usage only. I think it's the
> first time QEMU_PACKED is used this way in QEMU too.
> 
> Anyway, even taking that into account, things are still not completely
> right: the alignment of struct blkif_x86_32_response QEMU_PACKED is 4
> bytes as you wrote, but the size of struct blkif_x86_32_response is
> still 16 bytes instead of 12 bytes in my test. I suspect it worked for
> you because the other member of the union (blkif_x86_32_request) is
> larger than that. However, I think is not a good idea to rely on this
> implementation detail. The implementation of DEFINE_RING_TYPES should be
> opaque from our point of view. We shouldn't have to know that there is a
> union there.

I don't follow - why should we not rely on this? It is a fundamental
aspect of the shared ring model that requests and responses share
space.

> Moreover, the other problem is still unaddressed: the size and alignment
> of blkif_x86_64_response when built on x86_32 are 12 and 4 instead of 16
> and 8 bytes. Is that working also because it's relying on the other
> member of the union to enforce the right alignment and bigger size?

Yes. For these as well as your comments further up - sizeof() and
alignof() are completely uninteresting as long as we don't
instantiate objects of those types _and then use them for
communication_. The patch specifically removes instantiation,
switching to a purely pointer based approach. And that is ...

> I think it's a bad idea to rely on that, especially given that this
> obscure but important detail is not even mentioned in the commit message.

... what the commit message explicitly says (except that of course
the reason for doing so is not the sizeof/alignof aspect, but the
security issue we mean to fix). Everything else is just mechanical
detail of how the goal is being achieved. I don't really mind
extending the commit message (admitting that I'm known to not
write the best ones), but I also don't really see the need.

In the end I'm surprised the qemu side is proving so much more
difficult to get accepted compared to the Linux one.

Jan

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