[Xen-devel] [PATCH RFC] xen/common: Do not tolerate xmalloc(0, ...)

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Currently, _xmalloc() supports zero-sized allocations by returning a sentinel
poisoned pointer.

I posit that there are no legitimate situation for any code in the hypervisor
to make a zero sized allocation.

Furthermore, the sentinel value will pass a NULL pointer check, and
introduces an unnecessary security risk if it is accidentally used.

Instead, turn a zero-sized allocation into a clean fatal error.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>

---

This is RFC to gauge opinion, and because the amount of testing it has had is
"works on my box", which I admit is not much for a change of this nature.
---
 xen/common/xmalloc_tlsf.c    |   12 ++----------
 xen/include/asm-x86/config.h |    3 ---
 2 files changed, 2 insertions(+), 13 deletions(-)

diff --git a/xen/common/xmalloc_tlsf.c b/xen/common/xmalloc_tlsf.c
index b13317e..519dfbb 100644
--- a/xen/common/xmalloc_tlsf.c
+++ b/xen/common/xmalloc_tlsf.c
@@ -563,21 +563,13 @@ static void tlsf_init(void)
 /*
  * xmalloc()
  */
-
-#ifndef ZERO_BLOCK_PTR
-/* Return value for zero-size allocation, distinguished from NULL. */
-#define ZERO_BLOCK_PTR ((void *)-1L)
-#endif
-
 void *_xmalloc(unsigned long size, unsigned long align)
 {
     void *p = NULL;
     u32 pad;
 
     ASSERT(!in_irq());
-
-    if ( !size )
-        return ZERO_BLOCK_PTR;
+    BUG_ON(size == 0);
 
     ASSERT((align & (align - 1)) == 0);
     if ( align < MEM_ALIGN )
@@ -617,7 +609,7 @@ void xfree(void *p)
 {
     struct bhdr *b;
 
-    if ( p == NULL || p == ZERO_BLOCK_PTR )
+    if ( p == NULL )
         return;
 
     ASSERT(!in_irq());
diff --git a/xen/include/asm-x86/config.h b/xen/include/asm-x86/config.h
index 3569753..591f20b 100644
--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -94,9 +94,6 @@
 /* Primary stack is restricted to 8kB by guard pages. */
 #define PRIMARY_STACK_SIZE 8192
 
-/* Return value for zero-size _xmalloc(), distinguished from NULL. */
-#define ZERO_BLOCK_PTR ((void *)0xBAD0BAD0BAD0BAD0UL)
-
 /* Override include/xen/list.h to make these non-canonical addresses. */
 #define LIST_POISON1  ((void *)0x0100100100100100UL)
 #define LIST_POISON2  ((void *)0x0200200200200200UL)
-- 
1.7.10.4


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