general protection fault in fuse_ctl_remove_conn

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
0644f186fc9d77bb5bd198369e59fb28927a3692 (Thu Apr 26 23:36:11 2018 +0000)
Merge tag 'for_linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=32c236387d66c4516827

So far this crash happened 2 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6056306666373120
syzkaller reproducer:
https://syzkaller.appspot.com/x/repro.syz?id=6559188280934400
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=4532195645456384
Kernel config:
https://syzkaller.appspot.com/x/.config?id=7043958930931867332
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+32c236...@syzkaller.appspotmail.com
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RBP: 0030656c69662f2e R08: 0000000020000300 R09: 0000000000003833
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe5dea3810
R13: ffffffffffffffff R14: 006c746365737566 R15: 0000000000000044
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
(ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 4504 Comm: syz-executor777 Not tainted 4.17.0-rc2+ #19
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:fuse_ctl_remove_conn+0xc8/0x1b0 fs/fuse/control.c:286
RSP: 0018:ffff8801b0ee7968 EFLAGS: 00010202
RAX: 0000000000000075 RBX: ffff8801ac6dc2c0 RCX: ffffffff82645bb7
RDX: 0000000000000000 RSI: ffffffff82645bda RDI: 00000000000003a8
RBP: ffff8801b0ee7990 R08: ffff8801b1cd2740 R09: ffffed003b5c46c2
R10: ffffed003b5c46c2 R11: ffff8801dae23613 R12: 0000000000000001
R13: ffff8801d0bb5410 R14: dffffc0000000000 R15: 0000000000000000
FS: 00000000026bc880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000001471000 CR3: 00000001b1137000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
fuse_ctl_add_conn+0x261/0x280 fs/fuse/control.c:269
fuse_ctl_fill_super+0xf7/0x160 fs/fuse/control.c:307
mount_single+0xfb/0x170 fs/super.c:1236
fuse_ctl_mount+0x2c/0x40 fs/fuse/control.c:322
mount_fs+0xae/0x328 fs/super.c:1267
vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
vfs_kern_mount fs/namespace.c:1027 [inline]
do_new_mount fs/namespace.c:2518 [inline]
do_mount+0x564/0x3070 fs/namespace.c:2848
ksys_mount+0x12d/0x140 fs/namespace.c:3064
__do_sys_mount fs/namespace.c:3078 [inline]
__se_sys_mount fs/namespace.c:3075 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440579
RSP: 002b:00007ffe5dea3808 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440579
RDX: 00000000200002c0 RSI: 0000000020000280 RDI: 0000000020000240
RBP: 0030656c69662f2e R08: 0000000020000300 R09: 0000000000003833
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe5dea3810
R13: ffffffffffffffff R14: 006c746365737566 R15: 0000000000000044
Code: 8b 5d 00 48 8d 7b 58 48 89 f8 48 c1 e8 03 42 80 3c 30 00 0f 85 cc 00
00 00 4c 8b 7b 58 49 8d bf a8 03 00 00 48 89 f8 48 c1 e8 03 <42> 80 3c 30
00 0f 85 a5 00 00 00 48 89 df 41 83 ec 01 49 83 ed
RIP: fuse_ctl_remove_conn+0xc8/0x1b0 fs/fuse/control.c:286 RSP:
ffff8801b0ee7968
---[ end trace d64f1dab46c839a5 ]---


---
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[Tetsuo Handa]

From 9f41081f8bd6762a6f629e5e23e6d07a62bba69c Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
Date: Sat, 28 Apr 2018 11:24:09 +0900
Subject: [PATCH] fuse: don't keep inode-less dentry at fuse_ctl_add_dentry().

syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
clear d_inode(dentry)->i_private field. Fix this by calling dput() rather
than incrementing fc->ctl_ndents when new_inode() failed.

[1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6

Signed-off-by: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+32c236...@syzkaller.appspotmail.com>
Fixes: bafa96541b250a70 ("fuse: add control filesystem")
Cc: Miklos Szeredi <mik...@szeredi.hu>
---
fs/fuse/control.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/control.c b/fs/fuse/control.c
index b9ea99c..a651f8e 100644
--- a/fs/fuse/control.c
+++ b/fs/fuse/control.c
@@ -211,10 +211,12 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
if (!dentry)
return NULL;

- fc->ctl_dentry[fc->ctl_ndents++] = dentry;
inode = new_inode(fuse_control_sb);
- if (!inode)
+ if (!inode) {
+ dput(dentry);
return NULL;
+ }
+ fc->ctl_dentry[fc->ctl_ndents++] = dentry;

inode->i_ino = get_next_ino();
inode->i_mode = mode;
--
1.8.3.1

[Tetsuo Handa]

[Al Viro]

On Wed, May 09, 2018 at 07:58:58PM +0900, Tetsuo Handa wrote:
> >From 9f41081f8bd6762a6f629e5e23e6d07a62bba69c Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
> Date: Sat, 28 Apr 2018 11:24:09 +0900
> Subject: [PATCH] fuse: don't keep inode-less dentry at fuse_ctl_add_dentry().
>
> syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
> Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
> failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
> clear d_inode(dentry)->i_private field. Fix this by calling dput() rather
> than incrementing fc->ctl_ndents when new_inode() failed.

That looks bloody awful. Sure, everything that accesses fc->ctl_dentry is
synchronous with this, but it would've been much easier to follow if
shoving dentry into that array happened only after it's been fully set
up.

Incidentally, there's a nasty headache waiting to happen in that code -
consider a twit mounting something on that. And think what happens when
connection gets shut down...

[Miklos Szeredi]

Need to call d_invalidate() instead of d_drop() in there. Is that
what you are referring to?

Thanks,
Miklos

[Tetsuo Handa]

I couldn't catch what Al is worrying about. I assumed that dput() is fine
because proc_setup_thread_self() in fs/proc/thread_self.c is doing dput()
when new_inode_pseudo() failed after d_alloc_name().

[Al Viro]

Yes, and do that once on the entire subdirectory...

[Miklos Szeredi]

On Sat, Apr 28, 2018 at 4:29 AM, Tetsuo Handa
<penguin...@i-love.sakura.ne.jp> wrote:
> From 9f41081f8bd6762a6f629e5e23e6d07a62bba69c Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
> Date: Sat, 28 Apr 2018 11:24:09 +0900
> Subject: [PATCH] fuse: don't keep inode-less dentry at fuse_ctl_add_dentry().
>
> syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
> Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
> failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
> clear d_inode(dentry)->i_private field. Fix this by calling dput() rather
> than incrementing fc->ctl_ndents when new_inode() failed.
>
> [1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6

Slightly different fix pushed to:

git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git for-next

Thanks,
Miklos