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/i... 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) _