[PULL,05/19] linux-user/mmap.c: fix integer underflow in target_mremap

Commit Message

Laurent Vivier June 5, 2020, 11:46 a.m. UTC

From: Jonathan Marler <[e-mail]>

Fixes: https://bugs.launchpad.net/bugs/1876373

This code path in mmap occurs when a page size is decreased with mremap.  When a section of pages is shrunk, qemu calls mmap_reserve on the pages that were released.  However, it has the diff operation reversed, subtracting the larger old_size from the smaller new_size.  Instead, it should be subtracting the smaller new_size from the larger old_size.  You can also see in the previous line of the change that this mmap_reserve call only occurs when old_size > new_size.

Bug: https://bugs.launchpad.net/qemu/+bug/1876373
Signed-off-by: Jonathan Marler <[e-mail]>
Reviewded-by: Laurent Vivier <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Laurent Vivier <[e-mail]>
---
 linux-user/mmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index e37803379747..caab62909eb1 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -708,7 +708,7 @@  abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
         if (prot == 0) {
             host_addr = mremap(g2h(old_addr), old_size, new_size, flags);
             if (host_addr != MAP_FAILED && reserved_va && old_size > new_size) {
-                mmap_reserve(old_addr + old_size, new_size - old_size);
+                mmap_reserve(old_addr + old_size, old_size - new_size);
             }
         } else {
             errno = ENOMEM;