path: root/ipc
diff options
authorDavidlohr Bueso <dave@stgolabs.net>2017-02-27 14:28:24 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2017-02-27 18:43:46 -0800
commit95e91b831f87ac8e1f8ed50c14d709089b4e01b8 (patch)
tree0b2173bf698666cd562b7349e200b6f552ce07e1 /ipc
parenteac0b1c3681676f83d154915723f2853c38d5d1b (diff)
ipc/shm: Fix shmat mmap nil-page protection
The issue is described here, with a nice testcase: https://bugzilla.kernel.org/show_bug.cgi?id=192931 The problem is that shmat() calls do_mmap_pgoff() with MAP_FIXED, and the address rounded down to 0. For the regular mmap case, the protection mentioned above is that the kernel gets to generate the address -- arch_get_unmapped_area() will always check for MAP_FIXED and return that address. So by the time we do security_mmap_addr(0) things get funky for shmat(). The testcase itself shows that while a regular user crashes, root will not have a problem attaching a nil-page. There are two possible fixes to this. The first, and which this patch does, is to simply allow root to crash as well -- this is also regular mmap behavior, ie when hacking up the testcase and adding mmap(... |MAP_FIXED). While this approach is the safer option, the second alternative is to ignore SHM_RND if the rounded address is 0, thus only having MAP_SHARED flags. This makes the behavior of shmat() identical to the mmap() case. The downside of this is obviously user visible, but does make sense in that it maintains semantics after the round-down wrt 0 address and mmap. Passes shm related ltp tests. Link: http://lkml.kernel.org/r/1486050195-18629-1-git-send-email-dave@stgolabs.net Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Reported-by: Gareth Evans <gareth.evans@contextis.co.uk> Cc: Manfred Spraul <manfred@colorfullife.com> Cc: Michael Kerrisk <mtk.manpages@googlemail.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc')
1 files changed, 9 insertions, 4 deletions
diff --git a/ipc/shm.c b/ipc/shm.c
index d7805acb44fd..06ea9ef7f54a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1091,8 +1091,8 @@ out_unlock1:
* "raddr" thing points to kernel space, and there has to be a wrapper around
* this.
-long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
- unsigned long shmlba)
+long do_shmat(int shmid, char __user *shmaddr, int shmflg,
+ ulong *raddr, unsigned long shmlba)
struct shmid_kernel *shp;
unsigned long addr;
@@ -1113,8 +1113,13 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
goto out;
else if ((addr = (ulong)shmaddr)) {
if (addr & (shmlba - 1)) {
- if (shmflg & SHM_RND)
- addr &= ~(shmlba - 1); /* round down */
+ /*
+ * 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 (addr & ~PAGE_MASK)

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