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Re: [PATCH 1/8] xen/guest_access: Harden copy_to_guest_offset to prevent const dest operand

Hi Jan,

On 01/04/2020 07:48, Jan Beulich wrote:
On 31.03.2020 21:13, Julien Grall wrote:
On 31/03/2020 08:26, Jan Beulich wrote:
On 30.03.2020 21:21, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

At the moment, copy_to_guest_offset() will allow the hypervisor to copy
data to guest handle marked const.

Thankfully, no users of the helper will do that. Rather than hoping this
can be caught during review, harden copy_to_guest_offset() so the build
will fail if such users are introduced.

But there are other implications you break:

--- a/xen/include/asm-arm/guest_access.h
+++ b/xen/include/asm-arm/guest_access.h
@@ -126,7 +126,7 @@ int access_guest_memory_by_ipa(struct domain *d, paddr_t 
ipa, void *buf,
     #define __copy_to_guest_offset(hnd, off, ptr, nr) ({    \
       const typeof(*(ptr)) *_s = (ptr);                   \
-    char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
+    typeof(*((hnd).p)) *_d = (hnd).p;                   \
       ((void)((hnd).p == (ptr)));                         \
       __raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));\

Until this change, it is "ptr" which all sizes get derived from,
i.e. it is the internally used type rather than the handle type
which controls this. I'm sure we use this in a few places, to
copy to e.g. a handle derived from "void". Compatibility of types
(disallowing other than void) is checked by the comparison on the
line immediately after the line you change. Yes "_d+(off)" right
above here then changes its result. I consider it pretty likely
you'd notice this issue once you go beyond just build testing.

I missed that part. To be honest, it feels wrong to me to have
"off" != 0 and use a void type for the handle. Would it make
sense to forbid it?

I don't think so - the idea (aiui) has always been for the Xen
internal object's type to control what gets copied, and hence a
void handle is to be fine for both copy-in and copy-out.

As a side node, I have updated __copy_to_guest_offset() but
forgot to update copy_to_guest_offset(). I will look to apply
the modifications we agree on both side.

Ah, sure.

To address this, I guess we need to find an expression along the
lines of that comparison, which does not cause any code to be
generated, but which verifies the properties we care about. The
line you change should be left alone, from all I can tell right

I am not aware of any way before C11 to check if a variable is
const or not. If we wanted to keep allow void type the handle
then a possible approach would be:

#define copy_to_guest_offset(hnd, off, ptr, nr) ({              \
     const typeof(*(ptr)) *_s = (ptr);                           \
     typeof(*((hnd).p)) *_d = (hnd).p;                           \
     size_t mul = (sizeof(*(hnd).p) > 1) ? 1 : sizeof (*_s);     \
     ((void)((hnd).p == (ptr)));                                 \
     raw_copy_to_guest(_d + (off) * mul, _s, sizeof(*_s)*(nr));  \

I don't particularly like it but I could not come up with better so far.

Not very nice indeed, and the conditional expression - at the
first glance being the wrong way round - seems outright
confusing to me.

It is the correct way. If the handle is a void (sizeof(*(hnd).p) == 1), then we need to know the size of the Xen buffer so we can compute the offset correctly. Otherwise, as we have the correct type, we can apply the offset directly and let the compiler do the math for us.

I'll try to find time to experiment some with
this as well, since unless we can find a reasonably neat
solution here, I'm inclined to suggest to leave this as it is

It is difficult to catch the hypervisor who errorneously write to "const" handler. So I would not like a statu-quo.

A slightly uglier version (with more comments) is going to be better than what we currently have.


Julien Grall



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