[PATCH 1/2] Revert "ipc/shm: Fix shmat mmap nil-page protection"

From: Davidlohr Bueso
Date: Thu May 03 2018 - 16:47:45 EST


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

Reported-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
Reported-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
ipc/shm.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 0075990338f4..b81d53c8f459 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1371,13 +1371,8 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,

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)
--
2.13.6