[xen staging] guest_access: harden *copy_to_guest_offset() to prevent const dest operand

[patchbot@xxxxxxx]

commit 4bdf6b5a7fec876e9bbd70ebe605828ad0fb12a4
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Mon Apr 27 09:28:21 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 27 09:28:21 2020 +0200

    guest_access: harden *copy_to_guest_offset() to prevent const dest operand
    
    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.
    
    There is no easy way to check whether a const is NULL in C99. The
    approach used is to introduce an unused variable that is non-const and
    assign the handle. If the handle were const, this would fail at build
    because without an explicit cast, it is not possible to assign a const
    variable to a non-const variable.
    
    Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/include/asm-arm/guest_access.h | 4 ++++
 xen/include/asm-x86/guest_access.h | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/xen/include/asm-arm/guest_access.h 
b/xen/include/asm-arm/guest_access.h
index 8997a1cbfe..3b8d4ec322 100644
--- a/xen/include/asm-arm/guest_access.h
+++ b/xen/include/asm-arm/guest_access.h
@@ -78,6 +78,8 @@ 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;          \
+    /* Check that the handle is not for a const type */ \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));  \
 })
@@ -127,6 +129,8 @@ 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;          \
+    /* Check that the handle is not for a const type */ \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     __raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));\
 })
diff --git a/xen/include/asm-x86/guest_access.h 
b/xen/include/asm-x86/guest_access.h
index ca700c959a..a22745ceb4 100644
--- a/xen/include/asm-x86/guest_access.h
+++ b/xen/include/asm-x86/guest_access.h
@@ -87,6 +87,8 @@
 #define copy_to_guest_offset(hnd, off, ptr, nr) ({      \
     const typeof(*(ptr)) *_s = (ptr);                   \
     char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
+    /* Check that the handle is not for a const type */ \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));  \
 })
@@ -137,6 +139,8 @@
 #define __copy_to_guest_offset(hnd, off, ptr, nr) ({    \
     const typeof(*(ptr)) *_s = (ptr);                   \
     char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
+    /* Check that the handle is not for a const type */ \
+    void *__maybe_unused _t = (hnd).p;                  \
     ((void)((hnd).p == (ptr)));                         \
     __raw_copy_to_guest(_d+(off), _s, sizeof(*_s)*(nr));\
 })
--
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