[PATCH] ashmem: Fix lockdep issue during llseek

From: Joel Fernandes
Date: Thu Jan 25 2018 - 21:47:08 EST


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
during a syzcaller test, this patch fixes the issue by releasing the
ashmem_mutex before the call to vfs_llseek, and reacquiring it after.

Cc: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Arve Hjonnevag <arve@xxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot+8ec30bb7bf1a981a2012@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
---
drivers/staging/android/ashmem.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 0f695df14c9d..248983cf2db1 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -343,7 +343,9 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin)
goto out;
}

+ mutex_unlock(&ashmem_mutex);
ret = vfs_llseek(asma->file, offset, origin);
+ mutex_lock(&ashmem_mutex);
if (ret < 0)
goto out;

--
2.16.0.rc1.238.g530d649a79-goog