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.lawre...@redhat.com> Reported-by: Andrea Arcangeli <aarca...@redhat.com> Signed-off-by: Davidlohr Bueso <dbu...@suse.de> --- 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
