This is a note to let you know that I've just added the patch titled

chardev: Avoid potential use-after-free in 'chrdev_open()'

to my char-misc git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git in the char-misc-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 68faa679b8be1a74e6663c21c3a9d25d32f1c079 Mon Sep 17 00:00:00 2001

From: Will Deacon will@kernel.org Date: Thu, 19 Dec 2019 12:02:03 +0000 Subject: chardev: Avoid potential use-after-free in 'chrdev_open()'

'chrdev_open()' calls 'cdev_get()' to obtain a reference to the 'struct cdev *' stashed in the 'i_cdev' field of the target inode structure. If the pointer is NULL, then it is initialised lazily by looking up the kobject in the 'cdev_map' and so the whole procedure is protected by the 'cdev_lock' spinlock to serialise initialisation of the shared pointer.

Unfortunately, it is possible for the initialising thread to fail *after* installing the new pointer, for example if the subsequent '->open()' call on the file fails. In this case, 'cdev_put()' is called, the reference count on the kobject is dropped and, if nobody else has taken a reference, the release function is called which finally clears 'inode->i_cdev' from 'cdev_purge()' before potentially freeing the object. The problem here is that a racing thread can happily take the 'cdev_lock' and see the non-NULL pointer in the inode, which can result in a refcount increment from zero and a warning:

| ------------[ cut here ]------------ | refcount_t: addition on 0; use-after-free. | WARNING: CPU: 2 PID: 6385 at lib/refcount.c:25 refcount_warn_saturate+0x6d/0xf0 | Modules linked in: | CPU: 2 PID: 6385 Comm: repro Not tainted 5.5.0-rc2+ #22 | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 | RIP: 0010:refcount_warn_saturate+0x6d/0xf0 | Code: 05 55 9a 15 01 01 e8 9d aa c8 ff 0f 0b c3 80 3d 45 9a 15 01 00 75 ce 48 c7 c7 00 9c 62 b3 c6 08 | RSP: 0018:ffffb524c1b9bc70 EFLAGS: 00010282 | RAX: 0000000000000000 RBX: ffff9e9da1f71390 RCX: 0000000000000000 | RDX: ffff9e9dbbd27618 RSI: ffff9e9dbbd18798 RDI: ffff9e9dbbd18798 | RBP: 0000000000000000 R08: 000000000000095f R09: 0000000000000039 | R10: 0000000000000000 R11: ffffb524c1b9bb20 R12: ffff9e9da1e8c700 | R13: ffffffffb25ee8b0 R14: 0000000000000000 R15: ffff9e9da1e8c700 | FS: 00007f3b87d26700(0000) GS:ffff9e9dbbd00000(0000) knlGS:0000000000000000 | CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 | CR2: 00007fc16909c000 CR3: 000000012df9c000 CR4: 00000000000006e0 | DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 | DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 | Call Trace: | kobject_get+0x5c/0x60 | cdev_get+0x2b/0x60 | chrdev_open+0x55/0x220 | ? cdev_put.part.3+0x20/0x20 | do_dentry_open+0x13a/0x390 | path_openat+0x2c8/0x1470 | do_filp_open+0x93/0x100 | ? selinux_file_ioctl+0x17f/0x220 | do_sys_open+0x186/0x220 | do_syscall_64+0x48/0x150 | entry_SYSCALL_64_after_hwframe+0x44/0xa9 | RIP: 0033:0x7f3b87efcd0e | Code: 89 54 24 08 e8 a3 f4 ff ff 8b 74 24 0c 48 8b 3c 24 41 89 c0 44 8b 54 24 08 b8 01 01 00 00 89 f4 | RSP: 002b:00007f3b87d259f0 EFLAGS: 00000293 ORIG_RAX: 0000000000000101 | RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3b87efcd0e | RDX: 0000000000000000 RSI: 00007f3b87d25a80 RDI: 00000000ffffff9c | RBP: 00007f3b87d25e90 R08: 0000000000000000 R09: 0000000000000000 | R10: 0000000000000000 R11: 0000000000000293 R12: 00007ffe188f504e | R13: 00007ffe188f504f R14: 00007f3b87d26700 R15: 0000000000000000 | ---[ end trace 24f53ca58db8180a ]---

Since 'cdev_get()' can already fail to obtain a reference, simply move it over to use 'kobject_get_unless_zero()' instead of 'kobject_get()', which will cause the racing thread to return -ENXIO if the initialising thread fails unexpectedly.

Cc: Hillf Danton hdanton@sina.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Al Viro viro@zeniv.linux.org.uk Reported-by: syzbot+82defefbbd8527e1c2cb@syzkaller.appspotmail.com Signed-off-by: Will Deacon will@kernel.org Cc: stable stable@vger.kernel.org Link: https://lore.kernel.org/r/20191219120203.32691-1-will@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/char_dev.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/char_dev.c b/fs/char_dev.c index 00dfe17871ac..c5e6eff5a381 100644 --- a/fs/char_dev.c +++ b/fs/char_dev.c @@ -352,7 +352,7 @@ static struct kobject *cdev_get(struct cdev *p)

if (owner && !try_module_get(owner)) return NULL; - kobj = kobject_get(&p->kobj); + kobj = kobject_get_unless_zero(&p->kobj); if (!kobj) module_put(owner); return kobj;