diff mbox series

[Stable-8.0.4,20/31] linux-user: Prohibit brk() to to shrink below initial heap address

Message ID 20230725134517.3370706-20-mjt@tls.msk.ru
State New
Headers show
Series Patch Round-up for stable 8.0.4, freeze on 2023-08-05 | expand

Commit Message

Michael Tokarev July 25, 2023, 1:45 p.m. UTC 
From: Helge Deller <deller@gmx.de>

Since commit 86f04735ac ("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 86f04735ac.

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: 86f04735ac ("linux-user: Fix brk() to release pages")
Cc: qemu-stable@nongnu.org
Buglink: https://github.com/upx/upx/issues/683
(cherry picked from commit dfe49864afb06e7e452a4366051697bc4fcfc1a5)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff mbox series

Patch

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 450487af57..e106633837 100644
--- a/linux-user/syscall.c
+++ b/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);