[PATCH] linux-user: Ensure mmap_min_addr is non-zero

[Richard Henderson <richard.henderson@linaro.org>]

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@physik.fu-berlin.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 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.
      */
-- 
2.25.1