[qemu-xen staging] linux-user: Ensure mmap_min_addr is non-zero

[patchbot@xxxxxxx]

commit c9f8066697e0d3e77b97f6df423e9d6540b693be
Author:     Richard Henderson <richard.henderson@xxxxxxxxxx>
AuthorDate: Fri Jul 24 14:23:14 2020 -0700
Commit:     Laurent Vivier <laurent@xxxxxxxxx>
CommitDate: Mon Jul 27 22:02:40 2020 +0200

    linux-user: Ensure mmap_min_addr is non-zero
    
    When the chroot does not have /proc mounted, we can read neither
    /proc/sys/vm/mmap_min_addr nor /proc/sys/maps.
    
    The enforcement of mmap_min_addr in the host kernel is done by
    the security module, and so does not apply to processes owned
    by root.  Which leads pgd_find_hole_fallback to succeed in probing
    a reservation at address 0.  Which confuses pgb_reserved_va to
    believe that guest_base has not actually been initialized.
    
    We don't actually want NULL addresses to become accessible, so
    make sure that mmap_min_addr is initialized with a non-zero value.
    
    Buglink: https://bugs.launchpad.net/qemu/+bug/1888728
    Reported-by: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Richard Henderson <richard.henderson@xxxxxxxxxx>
    Tested-by: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx>
    Acked-by: Laurent Vivier <laurent@xxxxxxxxx>
    Message-Id: <20200724212314.545877-1-richard.henderson@xxxxxxxxxx>
    Signed-off-by: Laurent Vivier <laurent@xxxxxxxxx>
---
 linux-user/main.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/linux-user/main.c b/linux-user/main.c
index 3597e99bb1..75c9785157 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -758,14 +758,26 @@ int main(int argc, char **argv, char **envp)
 
         if ((fp = fopen("/proc/sys/vm/mmap_min_addr", "r")) != NULL) {
             unsigned long tmp;
-            if (fscanf(fp, "%lu", &tmp) == 1) {
+            if (fscanf(fp, "%lu", &tmp) == 1 && tmp != 0) {
                 mmap_min_addr = tmp;
-                qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n", 
mmap_min_addr);
+                qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n",
+                              mmap_min_addr);
             }
             fclose(fp);
         }
     }
 
+    /*
+     * We prefer to not make NULL pointers accessible to QEMU.
+     * If we're in a chroot with no /proc, fall back to 1 page.
+     */
+    if (mmap_min_addr == 0) {
+        mmap_min_addr = qemu_host_page_size;
+        qemu_log_mask(CPU_LOG_PAGE,
+                      "host mmap_min_addr=0x%lx (fallback)\n",
+                      mmap_min_addr);
+    }
+
     /*
      * Prepare copy of argv vector for target.
      */
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#staging