linux-user: Fix qemu brk() to not zero bytes on current page

The qemu brk() implementation is too aggressive and cleans remaining bytes
on the current page above the last brk address.

But some existing applications are buggy and read/write bytes above their
current heap address. On a phyiscal machine this does not trigger a
runtime error as long as the access happens on the same page. Additionally
the Linux kernel allocates only full pages and does no zeroing on already
allocated pages, even if the brk address is lowered.

Fix qemu to behave the same way as the kernel does. Do not touch already
allocated pages, and - when running with different page sizes of guest and
host - zero out only those memory areas where the host page size is bigger
than the guest page size.

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

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index c99ef9c01ef8a52791353138962abc07cca8b63d..ee54eed33b12c0faeee466701a250b6672926c81 100644 (file)
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -829,10 +829,8 @@ abi_long do_brk(abi_ulong brk_val)
 
     /* brk_val and old target_brk might be on the same page */
     if (new_brk == TARGET_PAGE_ALIGN(target_brk)) {
-        if (brk_val > target_brk) {
-            /* empty remaining bytes in (possibly larger) host page */
-            memset(g2h_untagged(target_brk), 0, new_host_brk_page - target_brk);
-        }
+        /* empty remaining bytes in (possibly larger) host page */
+        memset(g2h_untagged(new_brk), 0, new_host_brk_page - new_brk);
         target_brk = brk_val;
         return target_brk;
     }
@@ -840,7 +838,7 @@ abi_long do_brk(abi_ulong brk_val)
     /* Release heap if necesary */
     if (new_brk < target_brk) {
         /* empty remaining bytes in (possibly larger) host page */
-        memset(g2h_untagged(brk_val), 0, new_host_brk_page - brk_val);
+        memset(g2h_untagged(new_brk), 0, new_host_brk_page - new_brk);
 
         /* free unused host pages and set new brk_page */
         target_munmap(new_host_brk_page, brk_page - new_host_brk_page);
@@ -873,7 +871,7 @@ abi_long do_brk(abi_ulong brk_val)
          * come from the remaining part of the previous page: it may
          * contains garbage data due to a previous heap usage (grown
          * then shrunken).  */
-        memset(g2h_untagged(target_brk), 0, brk_page - target_brk);
+        memset(g2h_untagged(brk_page), 0, HOST_PAGE_ALIGN(brk_page) - brk_page);
 
         target_brk = brk_val;
         brk_page = new_host_brk_page;