On Thu, Dec 19, 2019 at 01:15:07PM +0100, Greg KH wrote: > On Thu, Dec 19, 2019 at 12:02:03PM +0000, Will Deacon wrote: > > '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: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> > > Cc: Hillf Danton <hdanton@xxxxxxxx> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > Reported-by: syzbot+82defefbbd8527e1c2cb@xxxxxxxxxxxxxxxxxxxxxxxxx > > Signed-off-by: Will Deacon <will@xxxxxxxxxx> > > --- > > 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; > > -- > > 2.24.1.735.g03f4e72817-goog > > > > Ugh. > > Due to the location of the lock, this looks like the only viable > solution. > > Al, any objection for me taking this into my tree now to send to Linus? > Will, this did fix the syzbot reproducer, right? Yes, looks like it. It ran overnight without failing (even with my mdelay() hacks applied to make the race more likely). Will
