linux-user: detect overflow of MAP_FIXED mmap

Relaxing the restrictions on 64 bit guests leads to the user being
able to attempt to map right at the edge of addressable memory. This
in turn lead to address overflow tripping the assert in page_set_flags
when the end address wrapped around.

Detect the wrap earlier and correctly -ENOMEM the guest (in the
reported case LTP mmap15).

Fixes: 7d8cbba
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200605154929.26910-15-alex.bennee@linaro.org>

linux-user/mmap.c

@@ -467,7 +467,7 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
          * It can fail only on 64-bit host with 32-bit target.
          * On any other target/host host mmap() handles this error correctly.
          */
-        if (!guest_range_valid(start, len)) {
+        if (end < start || !guest_range_valid(start, len)) {
             errno = ENOMEM;
             goto fail;
         }