CVE-2017-5669: Insufficient Information (NVD-CWE-noinfo)
value: The do_shmat function in ipc/shm.c in the Linux kernel through 4.9.12 does not restrict the address calculated by a certain rounding operation, which allows local users to map page zero, and consequently bypass a protection mechanism that exists for the mmap system call, by making crafted shmget and shmat system calls in a privileged context.
CWE description: There is insufficient information about the issue to classify it; details are unkown or unspecified.
CVSS v3 base score: 7.8
Published: 2017-02-24T15:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: ipc/shm.c
Commit date: 2017-02-27 18:43:46 -0800
Commit message: ipc/shm: Fix shmat mmap nil-page protection

The issue is described here, with a nice testcase:

    https://bugzilla.kernel.org/show_bug.cgi?id=192931

The problem is that shmat() calls do_mmap_pgoff() with MAP_FIXED, and
the address rounded down to 0.  For the regular mmap case, the
protection mentioned above is that the kernel gets to generate the
address -- arch_get_unmapped_area() will always check for MAP_FIXED and
return that address.  So by the time we do security_mmap_addr(0) things
get funky for shmat().

The testcase itself shows that while a regular user crashes, root will
not have a problem attaching a nil-page.  There are two possible fixes
to this.  The first, and which this patch does, is to simply allow root
to crash as well -- this is also regular mmap behavior, ie when hacking
up the testcase and adding mmap(...  |MAP_FIXED).  While this approach
is the safer option, the second alternative is to ignore SHM_RND if the
rounded address is 0, thus only having MAP_SHARED flags.  This makes the
behavior of shmat() identical to the mmap() case.  The downside of this
is obviously user visible, but does make sense in that it maintains
semantics after the round-down wrt 0 address and mmap.

Passes shm related ltp tests.

Link: http://lkml.kernel.org/r/1486050195-18629-1-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Gareth Evans <gareth.evans@contextis.co.uk>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diff: diff --git a/ipc/shm.c b/ipc/shm.c
index d7805acb44fd..06ea9ef7f54a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1089,12 +1089,12 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
  *
  * NOTE! Despite the name, this is NOT a direct system call entrypoint. The
  * "raddr" thing points to kernel space, and there has to be a wrapper around
  * this.
  */
-long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
-	      unsigned long shmlba)
+long do_shmat(int shmid, char __user *shmaddr, int shmflg,
+	      ulong *raddr, unsigned long shmlba)
 {
 	struct shmid_kernel *shp;
 	unsigned long addr;
 	unsigned long size;
 	struct file *file;
@@ -1111,12 +1111,17 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 	err = -EINVAL;
 	if (shmid < 0)
 		goto out;
 	else if ((addr = (ulong)shmaddr)) {
 		if (addr & (shmlba - 1)) {
-			if (shmflg & SHM_RND)
-				addr &= ~(shmlba - 1);	   /* round down */
+			/*
+			 * Round down to the nearest multiple of shmlba.
+			 * For sane do_mmap_pgoff() parameters, avoid
+			 * round downs that trigger nil-page and MAP_FIXED.
+			 */
+			if ((shmflg & SHM_RND) && addr >= shmlba)
+				addr &= ~(shmlba - 1);
 			else
 #ifndef __ARCH_FORCE_SHMLBA
 				if (addr & ~PAGE_MASK)
 #endif
 					goto out;

Vulnerable code: long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
	      unsigned long shmlba)
			if (shmflg & SHM_RND)
				addr &= ~(shmlba - 1);	   /* round down */
Fixed code: long do_shmat(int shmid, char __user *shmaddr, int shmflg,
	      ulong *raddr, unsigned long shmlba)
			/*
			 * Round down to the nearest multiple of shmlba.
			 * For sane do_mmap_pgoff() parameters, avoid
			 * round downs that trigger nil-page and MAP_FIXED.
			 */
			if ((shmflg & SHM_RND) && addr >= shmlba)
				addr &= ~(shmlba - 1);