[patch 04/16] Revert "ipc/shm: Fix shmat mmap nil-page protection"

[akpm@linux-foundation.org]

From: Davidlohr Bueso <dave@stgolabs.net>
Subject: Revert "ipc/shm: Fix shmat mmap nil-page protection"

Patch series "ipc/shm: shmat() fixes around nil-page".

These patches fix two issues reported[1] a while back by Joe and Andrea
around how shmat(2) behaves with nil-page.

The first reverts a commit that it was incorrectly thought that mapping
nil-page (address=0) was a no no with MAP_FIXED.  This is not the case,
with the exception of SHM_REMAP; which is address in the second patch.

I chose two patches because it is easier to backport and it explicitly
reverts bogus behaviour.  Both patches ought to be in -stable and ltp
testcases need updated (the added testcase around the cve can be modified
to just test for SHM_RND|SHM_REMAP).

[1] lkml.kernel.org/r/20180430172152.nfa564pvgpk3ut7p@linux-n805



This patch (of 2):

95e91b831f87 ("ipc/shm: Fix shmat mmap nil-page protection") worked on the
idea that we should not be mapping as root addr=0 and MAP_FIXED.  However,
it was reported that this scenario is in fact valid, thus making the patch
both bogus and breaks userspace as well.  For example X11's libint10.so
relies on shmat(1, SHM_RND) for lowmem initialization[1].

[1] https://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/os-support/linux/int10/linux.c#n347
Link: http://lkml.kernel.org/r/20180503203243.15045-2-dave@stgolabs.net
Fixes: 95e91b831f87 ("ipc/shm: Fix shmat mmap nil-page protection")
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 ipc/shm.c |    9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff -puN ipc/shm.c~revert-ipc-shm-fix-shmat-mmap-nil-page-protection ipc/shm.c
--- a/ipc/shm.c~revert-ipc-shm-fix-shmat-mmap-nil-page-protection
+++ a/ipc/shm.c
@@ -1363,13 +1363,8 @@ long do_shmat(int shmid, char __user *sh
 
 	if (addr) {
 		if (addr & (shmlba - 1)) {
-			/*
-			 * 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 (shmflg & SHM_RND)
+				addr &= ~(shmlba - 1);  /* round down */
 			else
 #ifndef __ARCH_FORCE_SHMLBA
 				if (addr & ~PAGE_MASK)
_