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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH for-1.1] user-exec.c: Don't assert on segfaults for non-valid addresses
Date: Tue, 08 May 2012 12:29:42 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 05/03/2012 01:32 PM, Peter Maydell wrote:
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>

Applied.  Thanks.


Anthony Liguori

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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