* Carlos Llamas <cmllamas@xxxxxxxxxx> [220829 16:13]: > Syzbot reported a couple issues introduced by commit 44e602b4e52f > ("binder_alloc: add missing mmap_lock calls when using the VMA"), in > which we attempt to acquire the mmap_lock when alloc->vma_vm_mm has not > been initialized yet. > > This can happen if a binder_proc receives a transaction without having > previously called mmap() to setup the binder_proc->alloc space in [1]. > Also, a similar issue occurs via binder_alloc_print_pages() when we try > to dump the debugfs binder stats file in [2]. > > Sample of syzbot's crash report: > ================================================================== > KASAN: null-ptr-deref in range [0x0000000000000128-0x000000000000012f] > CPU: 0 PID: 3755 Comm: syz-executor229 Not tainted 6.0.0-rc1-next-20220819-syzkaller #0 > syz-executor229[3755] cmdline: ./syz-executor2294415195 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022 > RIP: 0010:__lock_acquire+0xd83/0x56d0 kernel/locking/lockdep.c:4923 > [...] > Call Trace: > <TASK> > lock_acquire kernel/locking/lockdep.c:5666 [inline] > lock_acquire+0x1ab/0x570 kernel/locking/lockdep.c:5631 > down_read+0x98/0x450 kernel/locking/rwsem.c:1499 > mmap_read_lock include/linux/mmap_lock.h:117 [inline] > binder_alloc_new_buf_locked drivers/android/binder_alloc.c:405 [inline] > binder_alloc_new_buf+0xa5/0x19e0 drivers/android/binder_alloc.c:593 > binder_transaction+0x242e/0x9a80 drivers/android/binder.c:3199 > binder_thread_write+0x664/0x3220 drivers/android/binder.c:3986 > binder_ioctl_write_read drivers/android/binder.c:5036 [inline] > binder_ioctl+0x3470/0x6d00 drivers/android/binder.c:5323 > vfs_ioctl fs/ioctl.c:51 [inline] > __do_sys_ioctl fs/ioctl.c:870 [inline] > __se_sys_ioctl fs/ioctl.c:856 [inline] > __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x63/0xcd > [...] > ================================================================== > > Fix these issues by setting up alloc->vma_vm_mm pointer during open() > and caching directly from current->mm. This guarantees we have a valid > reference to take the mmap_lock during scenarios described above. > > [1] https://syzkaller.appspot.com/bug?extid=f7dc54e5be28950ac459 > [2] https://syzkaller.appspot.com/bug?extid=a75ebe0452711c9e56d9 > > Fixes: 44e602b4e52f ("binder_alloc: add missing mmap_lock calls when using the VMA") > Reported-by: syzbot+f7dc54e5be28950ac459@xxxxxxxxxxxxxxxxxxxxxxxxx > Reported-by: syzbot+a75ebe0452711c9e56d9@xxxxxxxxxxxxxxxxxxxxxxxxx > Cc: <stable@xxxxxxxxxxxxxxx> # v5.15+ > Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx> > --- > drivers/android/binder_alloc.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c > index 51f4e1c5cd01..9b1778c00610 100644 > --- a/drivers/android/binder_alloc.c > +++ b/drivers/android/binder_alloc.c > @@ -322,7 +322,6 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc, > */ > if (vma) { > vm_start = vma->vm_start; > - alloc->vma_vm_mm = vma->vm_mm; Is this really the null pointer dereference? We check for vma above..? > mmap_assert_write_locked(alloc->vma_vm_mm); > } else { > mmap_assert_locked(alloc->vma_vm_mm); > @@ -795,7 +794,6 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc, > binder_insert_free_buffer(alloc, buffer); > alloc->free_async_space = alloc->buffer_size / 2; > binder_alloc_set_vma(alloc, vma); > - mmgrab(alloc->vma_vm_mm); > > return 0; > > @@ -1091,6 +1089,8 @@ static struct shrinker binder_shrinker = { > void binder_alloc_init(struct binder_alloc *alloc) > { > alloc->pid = current->group_leader->pid; > + alloc->vma_vm_mm = current->mm; > + mmgrab(alloc->vma_vm_mm); > mutex_init(&alloc->mutex); > INIT_LIST_HEAD(&alloc->buffers); > } > -- > 2.37.2.672.g94769d06f0-goog >