Problem:
There exists a path in ashmem driver that could lead to acquistion
of mm->mmap_sem, ashmem_mutex in reverse order. This could lead
to deadlock in the system.
For Example, assume that mmap is called on a ashmem region
in the context of a thread say T1.
sys_mmap_pgoff (1. acquires mm->mmap_sem)
|
--> mmap_region
|
----> ashmem_mmap (2. acquires asmem_mutex)
Now if there is a context switch after 1 and before 2,
and if another thread T2 (that shares the mm struct) invokes an
ioctl say ASHMEM_GET_NAME, this can lead to the following path
ashmem_ioctl
|
-->get_name (3. acquires ashmem_mutex)
|
---> copy_to_user (4. acquires the mm->mmap_sem)
Note that the copy_to_user could lead to a valid fault if no
physical page is allocated yet for the user address passed.
Now T1 has mmap_sem and is waiting for ashmem_mutex.
and T2 has the ashmem_mutex and is waiting for mmap_sem
Thus leading to deadlock.
Solution:
Do not call copy_to_user or copy_from_user while holding the
ahsmem_mutex. Instead copy this to a local buffer that lives
in the stack while holding this lock. This will maintain data
integrity as well never reverse the lock order.
Testing:
Created a unit test case to reproduce the problem.
Used the same to test this fix on kernel version 3.4.0
Ported the same patch to 3.8
Signed-off-by: Shankar Brahadeeswaran <shankoo77@xxxxxxxxx>
Reviewed-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
drivers/staging/android/ashmem.c | 44 ++++++++++++++++++++++++++++---------
1 files changed, 33 insertions(+), 11 deletions(-)
diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 634b9ae..9c30ead 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -414,8 +414,7 @@ out:
static int set_name(struct ashmem_area *asma, void __user *name)
{
int ret = 0;
-
- mutex_lock(&ashmem_mutex);
+ char local_name[ASHMEM_NAME_LEN];
/* cannot change an existing mapping's name */
if (unlikely(asma->file)) {
@@ -423,9 +422,22 @@ static int set_name(struct ashmem_area *asma, void __user *name)
goto out;
}
- if (unlikely(copy_from_user(asma->name + ASHMEM_NAME_PREFIX_LEN,
- name, ASHMEM_NAME_LEN)))
+ /*
+ * Holding the ashmem_mutex while doing a copy_from_user might cause
+ * an data abourt which would try to access mmap_sem. If another
+ * thread has invoked ashmem_mmap then it will be holding the
+ * semaphore and will be waiting for ashmem_mutex, there by leading to
+ * deadlock. We'll release the mutex and take the name to a local
+ * variable that does not need protection and later copy the local
+ * variable to the structure member with lock held.
+ */
+ if (unlikely(copy_from_user(local_name, name, ASHMEM_NAME_LEN))) {
ret = -EFAULT;
+ return ret;
+ }
+ mutex_lock(&ashmem_mutex);
+ memcpy(asma->name + ASHMEM_NAME_PREFIX_LEN,
+ local_name, ASHMEM_NAME_LEN);
asma->name[ASHMEM_FULL_NAME_LEN-1] = '\0';
out:
@@ -437,26 +449,36 @@ out:
static int get_name(struct ashmem_area *asma, void __user *name)
{
int ret = 0;
+ size_t len;
+ /*
+ * Have a local variable to which we'll copy the content
+ * from asma with the lock held. Later we can copy this to the user
+ * space safely without holding any locks. So even if we proceed to
+ * wait for mmap_sem, it won't lead to deadlock.
+ */
+ char local_name[ASHMEM_NAME_LEN];
mutex_lock(&ashmem_mutex);
if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0') {
- size_t len;
/*
* Copying only `len', instead of ASHMEM_NAME_LEN, bytes
* prevents us from revealing one user's stack to another.
*/
len = strlen(asma->name + ASHMEM_NAME_PREFIX_LEN) + 1;
- if (unlikely(copy_to_user(name,
- asma->name + ASHMEM_NAME_PREFIX_LEN, len)))
- ret = -EFAULT;
+ memcpy(local_name, asma->name + ASHMEM_NAME_PREFIX_LEN, len);
} else {
- if (unlikely(copy_to_user(name, ASHMEM_NAME_DEF,
- sizeof(ASHMEM_NAME_DEF))))
- ret = -EFAULT;
+ len = sizeof(ASHMEM_NAME_DEF);
+ memcpy(local_name, ASHMEM_NAME_DEF, len);
}
mutex_unlock(&ashmem_mutex);
+ /*
+ * Now we are just copying from the stack variable to userland
+ * No lock held
+ */
+ if (unlikely(copy_to_user(name, local_name, len)))
+ ret = -EFAULT;
return ret;
}
--
1.7.6
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