linux-kernel - Change in functionality of futex() system call.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 6 Jun 2011 09:28:35 -0500
From:	David Oliver <david@...advisors.com>
To:	linux-kernel@...r.kernel.org
Cc:	Shawn Bohrer <sbohrer@...advisors.com>,
	Zachary Vonler <zvonler@...advisors.com>
Subject: Change in functionality of futex() system call.

Hello,

The functionality of the futex() system call appears to have changed
between versions 2.6.18 and 2.6.32.28.

Specifically, performing a FUTEX_WAIT on a read-only mapped location
results in an EFAULT. Although other operations, such as FUTEX_WAKE,
are only meaningful for writable locations, FUTEX_WAIT is useful for
processes with read-only access to a memory-mapped file.

The code below illustrates the changed behavior (each of the EXPECT
operations succeed on the older kernel, the ASSERTs pass in each
case), assuming the file /tmp/futex_test exists and contains int(42).

With the older kernel, the syscall() suspends until another process
changes the file and issues a FUTEX_WAKE, whereas the new behavior is
for an EFAULT error, independent of the file contents.

Let me know if you need further clarification.

Cheers!

David Oliver.

#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
typedef uint32_t u32;   // for futex.h
#include <linux/futex.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
#include "gtest/gtest.h" // test framework to illustrate issue.

TEST(Futex, futex_in_read_only_file_is_ok) {
  int fd = open("/tmp/futex_test", O_RDONLY);
  ASSERT_GE(fd, 0);
  int* futex = static_cast<int *>(mmap(0, sizeof(int), PROT_READ,
MAP_SHARED, fd, 0));
  ASSERT_NE((int *)(0), futex);

  int rc = syscall(SYS_futex, futex, FUTEX_WAIT, 42, 0, 0, 0);

  EXPECT_NE(-1, rc);              // fails.
  if (rc == -1) {
      EXPECT_NE(errno, EFAULT);   // fails.
  }
}

---------------------------------------------------------------
This email, along with any attachments, is confidential. If you 
believe you received this message in error, please contact the 
sender immediately and delete all copies of the message.  
Thank you.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/