There are a few largely cosmetic things that were discussed in the
context of the XSA, but which weren't really XSA material.

1: common: map_vcpu_info() cosmetics
2: evtchn/fifo: don't enforce higher than necessary alignment

I realize both changes are controversial. For the first one
discussion was about the choice of error code. Neither EINVAL nor
EFAULT represent the fact that it is a choice of implementation
to not support mis-aligned structures. If ENXIO isn't liked, the
best I can suggest are EOPNOTSUPP or (as previously suggested)
EPERM. I think it ought to be possible to settle on an error
code everyone can live with.

For the second one the question was whether the relaxation is
useful to do. The original reason for wanting to make the change
remains: The original code here should not be used as an excuse
to introduce similar over-alignment requirements elsewhere. I
can live with the change getting rejected, but if so I'd like to
request that some alternative be submitted to ensure that the
immediate goal can still be reached.

Jan

Use ENXIO instead of EINVAL to cover the two cases of the address not
satisfying the requirements. This will make an issue here better stand
out at the call site.

Also add a missing compat-mode related size check: If the sizes
differed, other code in the function would need changing. Accompany this
by a change to the initial sizeof() expression, tying it to the type of
the variable we're actually after (matching e.g. the alignof() added by
XSA-327).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1241,17 +1241,18 @@ int map_vcpu_info(struct vcpu *v, unsign
     struct page_info *page;
     unsigned int align;
 
-    if ( offset > (PAGE_SIZE - sizeof(vcpu_info_t)) )
-        return -EINVAL;
+    if ( offset > (PAGE_SIZE - sizeof(*new_info)) )
+        return -ENXIO;
 
 #ifdef CONFIG_COMPAT
+    BUILD_BUG_ON(sizeof(*new_info) != sizeof(new_info->compat));
     if ( has_32bit_shinfo(d) )
         align = alignof(new_info->compat);
     else
 #endif
         align = alignof(*new_info);
     if ( offset & (align - 1) )
-        return -EINVAL;
+        return -ENXIO;
 
     if ( !mfn_eq(v->vcpu_info_mfn, INVALID_MFN) )
         return -EINVAL;

Hi Jan,

On 22/12/2020 08:14, Jan Beulich wrote:
> Use ENXIO instead of EINVAL to cover the two cases of the address not
> satisfying the requirements. This will make an issue here better stand
> out at the call site.
> 
> Also add a missing compat-mode related size check: If the sizes
> differed, other code in the function would need changing. Accompany this
> by a change to the initial sizeof() expression, tying it to the type of
> the variable we're actually after (matching e.g. the alignof() added by
> XSA-327).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Acked-by: Julien Grall <jgrall@amazon.com>

Cheers,

-- 
Julien Grall

Neither the code nor the original commit provide any justification for
the need to 8-byte align the struct in all cases. Enforce just as much
alignment as the structure actually needs - 4 bytes - by using alignof()
instead of a literal number.

While relaxation of the requirements is intended here, the primary goal
is to simply get rid of the hard coded number as well its lack of
connection to the structure that is is meant to apply to.

Take the opportunity and also
- add so far missing validation that native and compat mode layouts of
  the structures actually match,
- tie sizeof() expressions to the types of the fields we're actually
  after, rather than specifying the type explicitly (which in the
  general case risks a disconnect, even if there's close to zero risk in
  this particular case),
- use ENXIO instead of EINVAL for the two cases of the address not
  satisfying the requirements, which will make an issue here better
  stand out at the call site.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v2: Add comment to public header. Re-base.
---
I question the need for the array_index_nospec() here. Or else I'd
expect map_vcpu_info() would also need the same.

--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -567,6 +567,16 @@ static void setup_ports(struct domain *d
     }
 }
 
+#ifdef CONFIG_COMPAT
+
+#include <compat/event_channel.h>
+
+#define xen_evtchn_fifo_control_block evtchn_fifo_control_block
+CHECK_evtchn_fifo_control_block;
+#undef xen_evtchn_fifo_control_block
+
+#endif
+
 int evtchn_fifo_init_control(struct evtchn_init_control *init_control)
 {
     struct domain *d = current->domain;
@@ -586,19 +596,20 @@ int evtchn_fifo_init_control(struct evtc
         return -ENOENT;
 
     /* Must not cross page boundary. */
-    if ( offset > (PAGE_SIZE - sizeof(evtchn_fifo_control_block_t)) )
-        return -EINVAL;
+    if ( offset > (PAGE_SIZE - sizeof(*v->evtchn_fifo->control_block)) )
+        return -ENXIO;
 
     /*
      * Make sure the guest controlled value offset is bounded even during
      * speculative execution.
      */
     offset = array_index_nospec(offset,
-                           PAGE_SIZE - sizeof(evtchn_fifo_control_block_t) + 1);
+                                PAGE_SIZE -
+                                sizeof(*v->evtchn_fifo->control_block) + 1);
 
-    /* Must be 8-bytes aligned. */
-    if ( offset & (8 - 1) )
-        return -EINVAL;
+    /* Must be suitably aligned. */
+    if ( offset & (alignof(*v->evtchn_fifo->control_block) - 1) )
+        return -ENXIO;
 
     spin_lock(&d->event_lock);
 
--- a/xen/include/public/event_channel.h
+++ b/xen/include/public/event_channel.h
@@ -368,6 +368,11 @@ typedef uint32_t event_word_t;
 
 #define EVTCHN_FIFO_NR_CHANNELS (1 << EVTCHN_FIFO_LINK_BITS)
 
+/*
+ * While this structure only requires 4-byte alignment, Xen versions 4.14 and
+ * earlier reject offset values (in struct evtchn_init_control) that aren't a
+ * multiple of 8.
+ */
 struct evtchn_fifo_control_block {
     uint32_t ready;
     uint32_t _rsvd;
--- a/xen/include/xlat.lst
+++ b/xen/include/xlat.lst
@@ -67,6 +67,7 @@
 ?	evtchn_bind_virq		event_channel.h
 ?	evtchn_close			event_channel.h
 ?	evtchn_expand_array		event_channel.h
+?	evtchn_fifo_control_block	event_channel.h
 ?	evtchn_init_control		event_channel.h
 ?	evtchn_op			event_channel.h
 ?	evtchn_reset			event_channel.h