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authorSuren Baghdasaryan <surenb@google.com>2020-07-30 22:26:32 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2020-07-31 15:41:43 +0300
commit3e338d3c95c735dc3265a86016bb4c022ec7cadc (patch)
tree3c1222a64fc7c3a9a9157da7daac1d36b22559c6 /drivers/staging
parent97a6f772f36b7f52bcfa56a581bbd2470cffe23d (diff)
downloadlinux-3e338d3c95c735dc3265a86016bb4c022ec7cadc.tar.xz
staging: android: ashmem: Fix lockdep warning for write operation
syzbot report [1] describes a deadlock when write operation against an ashmem fd executed at the time when ashmem is shrinking its cache results in the following lock sequence: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(fs_reclaim); lock(&sb->s_type->i_mutex_key#13); lock(fs_reclaim); lock(&sb->s_type->i_mutex_key#13); kswapd takes fs_reclaim and then inode_lock while generic_perform_write takes inode_lock and then fs_reclaim. However ashmem does not support writing into backing shmem with a write syscall. The only way to change its content is to mmap it and operate on mapped memory. Therefore the race that lockdep is warning about is not valid. Resolve this by introducing a separate lockdep class for the backing shmem inodes. [1]: https://lkml.kernel.org/lkml/0000000000000b5f9d059aa2037f@google.com/ Reported-by: syzbot+7a0d9d0b26efefe61780@syzkaller.appspotmail.com Signed-off-by: Suren Baghdasaryan <surenb@google.com> Cc: stable <stable@vger.kernel.org> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org> Link: https://lore.kernel.org/r/20200730192632.3088194-1-surenb@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/staging')
-rw-r--r--drivers/staging/android/ashmem.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index c05a214191da..10b4be1f3e78 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -95,6 +95,15 @@ static DEFINE_MUTEX(ashmem_mutex);
static struct kmem_cache *ashmem_area_cachep __read_mostly;
static struct kmem_cache *ashmem_range_cachep __read_mostly;
+/*
+ * A separate lockdep class for the backing shmem inodes to resolve the lockdep
+ * warning about the race between kswapd taking fs_reclaim before inode_lock
+ * and write syscall taking inode_lock and then fs_reclaim.
+ * Note that such race is impossible because ashmem does not support write
+ * syscalls operating on the backing shmem.
+ */
+static struct lock_class_key backing_shmem_inode_class;
+
static inline unsigned long range_size(struct ashmem_range *range)
{
return range->pgend - range->pgstart + 1;
@@ -396,6 +405,7 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
if (!asma->file) {
char *name = ASHMEM_NAME_DEF;
struct file *vmfile;
+ struct inode *inode;
if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0')
name = asma->name;
@@ -407,6 +417,8 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
goto out;
}
vmfile->f_mode |= FMODE_LSEEK;
+ inode = file_inode(vmfile);
+ lockdep_set_class(&inode->i_rwsem, &backing_shmem_inode_class);
asma->file = vmfile;
/*
* override mmap operation of the vmfile so that it can't be