[Qemu-devel] [PATCH v2] translate-all: Bugfix for user-mode self-modifying code in 2 page long TB

[Stanislav Shmarov <snarpix@gmail.com>]

In user-mode emulation Translation Block can consist of 2 guest pages.
In that case QEMU also mprotects 2 host pages that are dedicated for
guest memory, containing instructions. QEMU detects self-modifying code
with SEGFAULT signal processing.

In case if instruction in 1st page is modifying memory of 2nd
page (or vice versa) QEMU will mark 2nd page with PAGE_WRITE,
invalidate TB, generate new TB contatining 1 guest instruction and
exit to CPU loop. QEMU won't call mprotect, and new TB will cause
same SEGFAULT. Page will have both PAGE_WRITE_ORG and PAGE_WRITE
flags, so QEMU will handle the signal as guest binary problem,
and exit with guest SEGFAULT.

Solution is retranslate TB before marking pages as PAGE_WRITE,
and remove protection with mprotect on second SEGFAULT.

Signed-off-by: Stanislav Shmarov <snarpix@gmail.com>
---
  v2: Moved setting PAGE_WRITE flag to separte loop, to cover cases,
      pointed by Sergey Fedorov.

 translate-all.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/translate-all.c b/translate-all.c
index eaa95e4..fb3743f 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -2020,13 +2020,8 @@ int page_unprotect(target_ulong address, uintptr_t pc)
         host_start = address & qemu_host_page_mask;
         host_end = host_start + qemu_host_page_size;
 
-        prot = 0;
         for (addr = host_start ; addr < host_end ; addr += TARGET_PAGE_SIZE) {
-            p = page_find(addr >> TARGET_PAGE_BITS);
-            p->flags |= PAGE_WRITE;
-            prot |= p->flags;
-
-            /* and since the content will be modified, we must invalidate
+            /* Since the content will be modified, we must invalidate
                the corresponding translated code. */
             if (tb_invalidate_phys_page(addr, pc)) {
                 mmap_unlock();
@@ -2036,6 +2031,16 @@ int page_unprotect(target_ulong address, uintptr_t pc)
             tb_invalidate_check(addr);
 #endif
         }
+
+        /* If we got here, current TB have been retranslated (in case of
+         * self-modifying code), now it's safe to remove page protection.
+         */
+        prot = 0;
+        for (addr = host_start ; addr < host_end ; addr += TARGET_PAGE_SIZE) {
+            p = page_find(addr >> TARGET_PAGE_BITS);
+            p->flags |= PAGE_WRITE;
+            prot |= p->flags;
+        }
         mprotect((void *)g2h(host_start), qemu_host_page_size,
                  prot & PAGE_BITS);
 
-- 
1.9.3