CVE-2015-7613: Race Condition

A data race that can trick the kernel into using initialized memory was found. This vulnerability can at least give access to arbitrary SysV shared memory. It is almost certain that this vulnerability can be used to gain arbitrary code execution in the kernel.

While working on KTSAN, Dmitry Vyukov got a report that says that ipcaddid() installs a not-completely initialized object into the shared object table. In particular, uid/gid are not initialized. ipcobtainobjectcheck() in turn obtains the object and verifies uid/gid for permission purposes. Since the fields are not initialized, the check can falsely succeed.

Race report:

ThreadSanitizer: data-race in ipcobtainobjectcheck

Read at 0xffff88047f810f68 of size 8 by thread 2749 on CPU 5: [<ffffffff8147d84d>] ipcobtainobjectcheck+0x7d/0xd0 ipc/util.c:621 [< inline >] msqobtainobjectcheck ipc/msg.c:90 [<ffffffff8147e708>] msgctlnolock.constprop.9+0x208/0x430 ipc/msg.c:480 [< inline >] SYSCmsgctl ipc/msg.c:538 [<ffffffff8147f061>] SySmsgctl+0xa1/0xb0 ipc/msg.c:522 [<ffffffff81ee3e11>] entrySYSCALL64fastpath+0x31/0x95 arch/x86/entry/entry64.S:188

Previous write at 0xffff88047f810f68 of size 8 by thread 2755 on CPU 4: [<ffffffff8147cf97>] ipcaddid+0x217/0x260 ipc/util.c:257 [<ffffffff8147eb4c>] newque+0xac/0x240 ipc/msg.c:141 [< inline >] ipcgetpublic ipc/util.c:355 [<ffffffff8147daa2>] ipcget+0x202/0x280 ipc/util.c:646 [< inline >] SYSCmsgget ipc/msg.c:255 [<ffffffff8147efaa>] SySmsgget+0x7a/0x90 ipc/msg.c:241 [<ffffffff81ee3e11>] entrySYSCALL64fastpath+0x31/0x95 arch/x86/entry/entry64.S:188

Mutexes locked by thread 2755: Mutex 445417 is locked here: [<ffffffff81ee0d45>] downwrite+0x65/0x80 kernel/locking/rwsem.c:62 [< inline >] ipcgetpublic ipc/util.c:348 [<ffffffff8147d90c>] ipcget+0x6c/0x280 ipc/util.c:646 [< inline >] SYSCmsgget ipc/msg.c:255 [<ffffffff8147efaa>] SySmsgget+0x7a/0x90 ipc/msg.c:241 [<ffffffff81ee3e11>] entrySYSCALL64fastpath+0x31/0x95 arch/x86/entry/entry64.S:188

Mutex 453634 is locked here: [< inline >] rawspinlock include/linux/spinlockapismp.h:158 [<ffffffff81ee37d0>] rawspinlock+0x50/0x70 kernel/locking/spinlock.c:151 [< inline >] spinlock include/linux/spinlock.h:312 [<ffffffff8147ce0e>] ipcaddid+0x8e/0x260 ipc/util.c:238 [<ffffffff8147eb4c>] newque+0xac/0x240 ipc/msg.c:141 [< inline >] ipcgetpublic ipc/util.c:355 [<ffffffff8147daa2>] ipcget+0x202/0x280 ipc/util.c:646 [< inline >] SYSCmsgget ipc/msg.c:255 [<ffffffff8147efaa>] SySmsgget+0x7a/0x90 ipc/msg.c:241 [<ffffffff81ee3e11>] entrySYSCALL64fastpath+0x31/0x95 arch/x86/entry/entry64.S:188

ipcaddid installs new ipc object with idralloc, from this point on it is accessible to other threads. At this point the object contains unitialized garbage. Then it fills in uid, etc:

new->cuid = new->uid = euid; new->gid = new->cgid = egid; new->seq = ids->seq++;

While this happens another thread can get access to the object and do uid check on the unitialized garbage, which can give falsely give accesses to the shared object to a process that should not have access to the object.

Upstream patch:

https://github.com/torvalds/linux/commit/b9a532277938

CVE assignment:

http://seclists.org/oss-sec/2015/q4/7