From b567a01552e29c6452107928ea3c63fb1c9bcbaf Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 25 May 2018 14:47:30 -0700 Subject: [PATCH] ipc/shm: fix shmat() nil address after round-down when remapping commit 8f89c007b6dec16a1793cb88de88fcc02117bbbc upstream. shmat()'s SHM_REMAP option forbids passing a nil address for; this is in fact the very first thing we check for. Andrea reported that for SHM_RND|SHM_REMAP cases we can end up bypassing the initial addr check, but we need to check again if the address was rounded down to nil. As of this patch, such cases will return -EINVAL. Link: http://lkml.kernel.org/r/20180503204934.kk63josdu6u53fbd@linux-n805 Signed-off-by: Davidlohr Bueso Reported-by: Andrea Arcangeli Cc: Joe Lawrence Cc: Manfred Spraul Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Ben Hutchings --- ipc/shm.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/ipc/shm.c b/ipc/shm.c index 0acce7993648..a19f9fa4e9b4 100644 --- a/ipc/shm.c +++ b/ipc/shm.c @@ -1063,9 +1063,17 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, goto out; else if ((addr = (ulong)shmaddr)) { if (addr & (shmlba - 1)) { - if (shmflg & SHM_RND) + if (shmflg & SHM_RND) { addr &= ~(shmlba - 1); /* round down */ - else + + /* + * Ensure that the round-down is non-nil + * when remapping. This can happen for + * cases when addr < shmlba. + */ + if (!addr && (shmflg & SHM_REMAP)) + goto out; + } else #ifndef __ARCH_FORCE_SHMLBA if (addr & ~PAGE_MASK) #endif