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[Qemu-devel] [PATCH for-1.1] user-exec.c: Don't assert on segfaults for

From: Peter Maydell
Subject: [Qemu-devel] [PATCH for-1.1] user-exec.c: Don't assert on segfaults for non-valid addresses
Date: Thu, 3 May 2012 19:32:15 +0100

h2g() will assert if passed an address that's not a valid guest address,
so handle_cpu_signal() needs to check before passing "data address
which caused a segfault" to it, since for a misbehaving guest
that could be anything. If the address isn't a valid guest address
then we can simply skip the attempt to unprotect a guest page
which was made read-only to catch self-modifying code.

This assertion probably fires more readily now than it used to
do because of recent changes to default to reserving guest address

Signed-off-by: Peter Maydell <address@hidden>
I've tentatively marked this as for-1.1 as it's pretty safe, although
it doesn't buy you a great deal: misbehaving guest binaries will
die cleanly with a segfault rather than qemu asserting and then
locking up (assert() in qemu's linux-user code doesn't really behave
very nicely...)

 user-exec.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/user-exec.c b/user-exec.c
index be6bc4f..d8c2ad9 100644
--- a/user-exec.c
+++ b/user-exec.c
@@ -97,7 +97,8 @@ static inline int handle_cpu_signal(uintptr_t pc, unsigned 
long address,
                 pc, address, is_write, *(unsigned long *)old_set);
     /* XXX: locking issue */
-    if (is_write && page_unprotect(h2g(address), pc, puc)) {
+    if (is_write && h2g_valid(address)
+        && page_unprotect(h2g(address), pc, puc)) {
         return 1;

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