[SRU][Trusty][PATCH 1/1] ipc/shm: Fix shmat mmap nil-page protection

Kleber Sacilotto de Souza kleber.souza at canonical.com
Thu Nov 30 17:43:15 UTC 2017

From: Davidlohr Bueso <dave at stgolabs.net>

The issue is described here, with a nice testcase:


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 at suse.de>
Reported-by: Gareth Evans <gareth.evans at contextis.co.uk>
Cc: Manfred Spraul <manfred at colorfullife.com>
Cc: Michael Kerrisk <mtk.manpages at googlemail.com>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(cherry picked from commit 95e91b831f87ac8e1f8ed50c14d709089b4e01b8)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
 ipc/shm.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index d7805acb44fd..06ea9ef7f54a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1091,8 +1091,8 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
  * "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;
@@ -1113,8 +1113,13 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 		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);
 				if (addr & ~PAGE_MASK)

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