linux-user: Prohibit brk() to to shrink below initial heap address

Since commit 86f0473 ("linux-user: Fix brk() to release pages") it's
possible for userspace applications to reduce their memory footprint by
calling brk() with a lower address and free up memory. Before that commit
guest heap memory was never unmapped.

But the Linux kernel prohibits to reduce brk() below the initial memory
address which is set at startup by the set_brk() function in binfmt_elf.c.
Such a range check was missed in commit 86f0473.

This patch adds the missing check by storing the initial brk value in
initial_target_brk and verify any new brk addresses against that value.

Tested with the i386 upx binary from
https://github.com/upx/upx/releases/download/v4.0.2/upx-4.0.2-i386_linux.tar.xz

Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
Fixes: 86f0473 ("linux-user: Fix brk() to release pages")
Cc: qemu-stable@nongnu.org
Buglink: upx/upx#683

linux-user/syscall.c

@@ -801,12 +801,13 @@ static inline int host_to_target_sock_type(int host_type)
     return target_type;
 }
 
-static abi_ulong target_brk;
+static abi_ulong target_brk, initial_target_brk;
 static abi_ulong brk_page;
 
 void target_set_brk(abi_ulong new_brk)
 {
     target_brk = TARGET_PAGE_ALIGN(new_brk);
+    initial_target_brk = target_brk;
     brk_page = HOST_PAGE_ALIGN(target_brk);
 }
 
@@ -824,6 +825,11 @@ abi_long do_brk(abi_ulong brk_val)
         return target_brk;
     }
 
+    /* do not allow to shrink below initial brk value */
+    if (brk_val < initial_target_brk) {
+        brk_val = initial_target_brk;
+    }
+
     new_brk = TARGET_PAGE_ALIGN(brk_val);
     new_host_brk_page = HOST_PAGE_ALIGN(brk_val);