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From: Davidlohr Bueso <dave@stgolabs.net>
To: akpm@linux-foundation.org, aarcange@redhat.com
Cc: joe.lawrence@redhat.com, gareth.evans@contextis.co.uk,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	dave@stgolabs.net, stable@kernel.org,
	Davidlohr Bueso <dbueso@suse.de>
Subject: [PATCH 1/2] Revert "ipc/shm: Fix shmat mmap nil-page protection"
Date: Thu,  3 May 2018 13:32:42 -0700
Message-ID: <20180503203243.15045-2-dave@stgolabs.net> (raw)
In-Reply-To: <20180503203243.15045-1-dave@stgolabs.net>

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@redhat.com>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Davidlohr Bueso <dbueso@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

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-03 20:32 [PATCH -next 0/2] ipc/shm: shmat() fixes around nil-page Davidlohr Bueso
2018-05-03 20:32 ` Davidlohr Bueso [this message]
2018-05-03 20:49 ` [PATCH 2/2] ipc/shm: fix shmat() nil address after round-down when remapping Davidlohr Bueso
2018-05-10 18:17 ` [PATCH -next 0/2] ipc/shm: shmat() fixes around nil-page Vlastimil Babka
2018-05-14 16:19   ` Davidlohr Bueso

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