#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
From e7d73362ed378499795f27e2da6184d0b242b89c Mon Sep 17 00:00:00 2001 From: Joel Fernandes <joe...@google.com> Date: Wed, 24 Jan 2018 09:47:23 -0800 Subject: [PATCH] ashmem: Fix lockdep issue during llseek
ashmem_mutex create a chain of dependencies like so: (1) mmap syscall -> mmap_sem -> (acquired) ashmem_mmap ashmem_mutex (try to acquire) (block) (2) llseek syscall -> ashmem_llseek -> ashmem_mutex -> (acquired) inode_lock -> inode->i_rwsem (try to acquire) (block) (3) getdents -> iterate_dir -> inode_lock -> inode->i_rwsem (acquired) copy_to_user -> mmap_sem (try to acquire) There is a lock ordering created between mmap_sem and inode->i_rwsem causing a lockdep splat [2] during a syzcaller test, this patch fixes the issue by removing the use of the mutex. The mutex isn't needed as llseeks are synchronized with other llseeks and reads due to fdget_pos as Al mentioned [1]. Further asma->file and asma->size don't change once they're setup, and there's a unique asma->file per file. [1] https://patchwork.kernel.org/patch/10185031/ [2] https://lkml.org/lkml/2018/1/10/48 Cc: Todd Kjos <tk...@google.com> Cc: Arve Hjonnevag <a...@android.com> Cc: Al Viro <v...@zeniv.linux.org.uk> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Reported-by: syzbot+8ec30bb7bf1a981a2...@syzkaller.appspotmail.com Signed-off-by: Joel Fernandes <joe...@google.com> --- drivers/staging/android/ashmem.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c index 0f695df14c9d..8c9c69a93fa4 100644 --- a/drivers/staging/android/ashmem.c +++ b/drivers/staging/android/ashmem.c @@ -331,8 +331,6 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin) struct ashmem_area *asma = file->private_data; int ret; - mutex_lock(&ashmem_mutex); - if (asma->size == 0) { ret = -EINVAL; goto out; @@ -351,7 +349,6 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin) file->f_pos = asma->file->f_pos; out: - mutex_unlock(&ashmem_mutex); return ret; } -- 2.16.0.rc1.238.g530d649a79-goog
