linux-user: Emulate /proc/self/maps under mmap_lock

If one thread modifies the mappings and another thread prints them,
a situation may occur that the printer thread sees a guest mapping
without a corresponding host mapping, leading to a crash in
open_self_maps_2().

Cc: qemu-stable@nongnu.org
Fixes: 7b7a3366e142 ("linux-user: Use walk_memory_regions for open_self_maps")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20241014203441.387560-1-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit bbd5630a75e70a0f1bcf04de74c94aa94a145628)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/syscall.c

@@ -8122,17 +8122,19 @@ static int open_self_maps_1(CPUArchState *env, int fd, bool smaps)
 {
     struct open_self_maps_data d = {
         .ts = get_task_state(env_cpu(env)),
-        .host_maps = read_self_maps(),
         .fd = fd,
         .smaps = smaps
     };
 
+    mmap_lock();
+    d.host_maps = read_self_maps();
     if (d.host_maps) {
         walk_memory_regions(&d, open_self_maps_2);
         free_self_maps(d.host_maps);
     } else {
         walk_memory_regions(&d, open_self_maps_3);
     }
+    mmap_unlock();
     return 0;
 }