From a399b29dfbaaaf91162b2dc5a5875dd51bbfa2a1 Mon Sep 17 00:00:00 2001 From: Greg Thelen Date: Thu, 21 Nov 2013 14:32:00 -0800 Subject: [PATCH] ipc,shm: fix shm_file deletion races When IPC_RMID races with other shm operations there's potential for use-after-free of the shm object's associated file (shm_file). Here's the race before this patch: TASK 1 TASK 2 ------ ------ shm_rmid() ipc_lock_object() shmctl() shp = shm_obtain_object_check() shm_destroy() shum_unlock() fput(shp->shm_file) ipc_lock_object() shmem_lock(shp->shm_file) The oops is caused because shm_destroy() calls fput() after dropping the ipc_lock. fput() clears the file's f_inode, f_path.dentry, and f_path.mnt, which causes various NULL pointer references in task 2. I reliably see the oops in task 2 if with shmlock, shmu This patch fixes the races by: 1) set shm_file=NULL in shm_destroy() while holding ipc_object_lock(). 2) modify at risk operations to check shm_file while holding ipc_object_lock(). Example workloads, which each trigger oops... Workload 1: while true; do id=$(shmget 1 4096) shm_rmid $id & shmlock $id & wait done The oops stack shows accessing NULL f_inode due to racing fput: _raw_spin_lock shmem_lock SyS_shmctl Workload 2: while true; do id=$(shmget 1 4096) shmat $id 4096 & shm_rmid $id & wait done The oops stack is similar to workload 1 due to NULL f_inode: touch_atime shmem_mmap shm_mmap mmap_region do_mmap_pgoff do_shmat SyS_shmat Workload 3: while true; do id=$(shmget 1 4096) shmlock $id shm_rmid $id & shmunlock $id & wait done The oops stack shows second fput tripping on an NULL f_inode. The first fput() completed via from shm_destroy(), but a racing thread did a get_file() and queued this fput(): locks_remove_flock __fput ____fput task_work_run do_notify_resume int_signal Fixes: c2c737a0461e ("ipc,shm: shorten critical region for shmat") Fixes: 2caacaa82a51 ("ipc,shm: shorten critical region for shmctl") Signed-off-by: Greg Thelen Cc: Davidlohr Bueso Cc: Rik van Riel Cc: Manfred Spraul Cc: # 3.10.17+ 3.11.6+ Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/shm.c | 28 +++++++++++++++++++++++----- 1 file changed, 23 insertions(+), 5 deletions(-) diff --git a/ipc/shm.c b/ipc/shm.c index d69739610fd4..0bdf21c6814e 100644 --- a/ipc/shm.c +++ b/ipc/shm.c @@ -208,15 +208,18 @@ static void shm_open(struct vm_area_struct *vma) */ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp) { + struct file *shm_file; + + shm_file = shp->shm_file; + shp->shm_file = NULL; ns->shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT; shm_rmid(ns, shp); shm_unlock(shp); - if (!is_file_hugepages(shp->shm_file)) - shmem_lock(shp->shm_file, 0, shp->mlock_user); + if (!is_file_hugepages(shm_file)) + shmem_lock(shm_file, 0, shp->mlock_user); else if (shp->mlock_user) - user_shm_unlock(file_inode(shp->shm_file)->i_size, - shp->mlock_user); - fput (shp->shm_file); + user_shm_unlock(file_inode(shm_file)->i_size, shp->mlock_user); + fput(shm_file); ipc_rcu_putref(shp, shm_rcu_free); } @@ -983,6 +986,13 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf) } shm_file = shp->shm_file; + + /* check if shm_destroy() is tearing down shp */ + if (shm_file == NULL) { + err = -EIDRM; + goto out_unlock0; + } + if (is_file_hugepages(shm_file)) goto out_unlock0; @@ -1101,6 +1111,14 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr, goto out_unlock; ipc_lock_object(&shp->shm_perm); + + /* check if shm_destroy() is tearing down shp */ + if (shp->shm_file == NULL) { + ipc_unlock_object(&shp->shm_perm); + err = -EIDRM; + goto out_unlock; + } + path = shp->shm_file->f_path; path_get(&path); shp->shm_nattch++; -- 2.30.2