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Subject[PATCH 4.14 01/39] chardev: Avoid potential use-after-free in chrdev_open()
Date
From: Will Deacon <will@kernel.org>

commit 68faa679b8be1a74e6663c21c3a9d25d32f1c079 upstream.

'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(-)

--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -361,7 +361,7 @@ static struct kobject *cdev_get(struct c

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;

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