From 836a896fc18eb27c6aec2b320ef01c17dab7c96d Mon Sep 17 00:00:00 2001 From: Shankar Brahadeeswaran Date: Mon, 18 Feb 2013 10:21:56 +0530 Subject: [PATCH] staging: android: ashmem: get_name,set_name not to hold ashmem_mutex 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. --- drivers/staging/android/ashmem.c | 48 ++++++++++++++++++++++++++++++------- 1 files changed, 39 insertions(+), 9 deletions(-) diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c index 634b9ae..9ddde92 100644 --- a/drivers/staging/android/ashmem.c +++ b/drivers/staging/android/ashmem.c @@ -414,6 +414,11 @@ out: static int set_name(struct ashmem_area *asma, void __user *name) { int ret = 0; + /* + * Local variable to hold the name copied from user space + * This variable is approx of size 270 bytes, not huge + */ + char local_name[ASHMEM_NAME_LEN]; mutex_lock(&ashmem_mutex); @@ -423,9 +428,24 @@ 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. + */ + mutex_unlock(&ashmem_mutex); + if (unlikely(copy_from_user(local_name, name, ASHMEM_NAME_LEN))) { + /* Lock is already dropped, just return */ 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 +457,36 @@ 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; } -- 1.7.0.4