general protection fault in kernfs_kill_sb (2)

general protection fault in kernfs_kill_sb (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    f0ab773f5c96 Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=140ce817800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=fcce42b221691ff9
dashboard link: https://syzkaller.appspot.com/bug?extid=481ad819c717f6b78df9
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=15078e97800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12bf3697800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+481ad819c717f6b78df9@syzkaller.appspotmail.com

RBP: 00007ffc558bda60 R08: 0000000000000000 R09: 0000000300000000
R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffffff
R13: 0000000000000004 R14: 0000000000001380 R15: 00007ffc558bd2f8
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: 1 PID: 4540 Comm: syz-executor884 Not tainted 4.17.0-rc4+ #43
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:__list_del_entry_valid+0x84/0xf3 lib/list_debug.c:51
RSP: 0018:ffff8801aca6f860 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801ac528498 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8801ac5284a0
RBP: ffff8801aca6f878 R08: fffffbfff11cc855 R09: fffffbfff11cc854
R10: ffff8801aca6f878 R11: ffffffff88e642a7 R12: 0000000000000000
R13: 0000000000000000 R14: ffff8801aca6f908 R15: ffff8801ac528498
FS:  0000000000f86880(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004b6f3c CR3: 00000001d9730000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  __list_del_entry include/linux/list.h:117 [inline]
  list_del include/linux/list.h:125 [inline]
  kernfs_kill_sb+0xa0/0x350 fs/kernfs/mount.c:361
  sysfs_kill_sb+0x22/0x40 fs/sysfs/mount.c:50
  deactivate_locked_super+0x97/0x100 fs/super.c:316
  kernfs_mount_ns+0x753/0x8e0 fs/kernfs/mount.c:335
  sysfs_mount+0xdf/0x200 fs/sysfs/mount.c:36
  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: 0033:0x4426a9
RSP: 002b:00007ffc558bd1b8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004426a9
RDX: 00000000200000c0 RSI: 0000000020000080 RDI: 0000000020000040
RBP: 00007ffc558bda60 R08: 0000000000000000 R09: 0000000300000000
R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffffff
R13: 0000000000000004 R14: 0000000000001380 R15: 00007ffc558bd2f8
Code: c5 0f 84 cc 00 00 00 48 b8 00 02 00 00 00 00 ad de 49 39 c4 0f 84 a5  
00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00  
75 5f 49 8b 14 24 48 39 da 0f 85 ba 00 00 00 48 b8
RIP: __list_del_entry_valid+0x84/0xf3 lib/list_debug.c:51 RSP:  
ffff8801aca6f860
---[ end trace d148f307a34e229f ]---


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Re: general protection fault in kernfs_kill_sb (2)

[Tetsuo Handa]

On 2018/05/13 2:01, syzbot wrote:
> Call Trace:
>  __list_del_entry include/linux/list.h:117 [inline]
>  list_del include/linux/list.h:125 [inline]
>  kernfs_kill_sb+0xa0/0x350 fs/kernfs/mount.c:361
>  sysfs_kill_sb+0x22/0x40 fs/sysfs/mount.c:50
>  deactivate_locked_super+0x97/0x100 fs/super.c:316
>  kernfs_mount_ns+0x753/0x8e0 fs/kernfs/mount.c:335
>  sysfs_mount+0xdf/0x200 fs/sysfs/mount.c:36
>  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: 0033:0x4426a9
> RSP: 002b:00007ffc558bd1b8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004426a9
> RDX: 00000000200000c0 RSI: 0000000020000080 RDI: 0000000020000040
> RBP: 00007ffc558bda60 R08: 0000000000000000 R09: 0000000300000000
> R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffffff
> R13: 0000000000000004 R14: 0000000000001380 R15: 00007ffc558bd2f8
> Code: c5 0f 84 cc 00 00 00 48 b8 00 02 00 00 00 00 ad de 49 39 c4 0f 84 a5 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 75 5f 49 8b 14 24 48 39 da 0f 85 ba 00 00 00 48 b8
> RIP: __list_del_entry_valid+0x84/0xf3 lib/list_debug.c:51 RSP: ffff8801aca6f860
> ---[ end trace d148f307a34e229f ]---

This is what I reported at
https://groups.google.com/d/msg/syzkaller-bugs/ISOJlV2I2QM/qHslGMi3AwAJ .

We are currently waiting for comments from Al Viro.

Re: general protection fault in kernfs_kill_sb (2)

[Al Viro]

On Sun, May 13, 2018 at 11:19:46AM +0900, Tetsuo Handa wrote:

> This is what I reported at
> https://groups.google.com/d/msg/syzkaller-bugs/ISOJlV2I2QM/qHslGMi3AwAJ .
> 
> We are currently waiting for comments from Al Viro.

1) the damn thing is unusable without javashit.  Which gets about
the same reaction as sending something.doc in attachment.  Please,
find a less obnoxious way to archive the thing (or to generate
URLs that would work without that garbage).

2) deactivate_locked_super() *WILL* be called when fill_super() fails.
Live with it; it allows to simplify a whole lot of cleanup logics
in various filesystems.  Again, we are not going for a model where
->kill_sb() is not called for something returned by sget().
Rationale: rarely exercised paths tend to rot, so anything that increases
the duplication of bits and pieces of normal teardown into failure exits
of foo_fill_super() is a bloody bad idea.  If anything, we want to take
a lot of stuff out of ->put_super() instances directly into ->kill_sb()
ones, precisely because ->put_super() is only called for fully set up
filesystems.

3) kernfs needs to be fixed.  The rest of the dropped commits were
made redundant by 8e04944f0ea8; this one wasn't.  Mea culpa.

Re: general protection fault in kernfs_kill_sb (2)

[Al Viro]

On Mon, May 14, 2018 at 12:20:16PM +0900, Tetsuo Handa wrote:

> But there remains a refcount bug because deactivate_locked_super() from
> kernfs_mount_ns() triggers kobj_ns_drop() from sysfs_kill_sb() via
> sb->kill_sb() when kobj_ns_drop() is always called by sysfs_mount()
> if kernfs_mount_ns() returned an error.

Trivial:

unfuck sysfs_mount()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index b428d317ae92..92682fcc41f6 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -25,7 +25,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 {
 	struct dentry *root;
 	void *ns;
-	bool new_sb;
+	bool new_sb = false;
 
 	if (!(flags & SB_KERNMOUNT)) {
 		if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
@@ -35,9 +35,9 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 	ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
 	root = kernfs_mount_ns(fs_type, flags, sysfs_root,
 				SYSFS_MAGIC, &new_sb, ns);
-	if (IS_ERR(root) || !new_sb)
+	if (!new_sb)
 		kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
-	else if (new_sb)
+	else if (!IS_ERR(root))
 		root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
 
 	return root;

Re: general protection fault in kernfs_kill_sb (2)

[Al Viro]

On Mon, May 14, 2018 at 05:04:15AM +0100, Al Viro wrote:
> diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
> index b428d317ae92..92682fcc41f6 100644
> --- a/fs/sysfs/mount.c
> +++ b/fs/sysfs/mount.c
> @@ -25,7 +25,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
>  {
>  	struct dentry *root;
>  	void *ns;
> -	bool new_sb;
> +	bool new_sb = false;
>  
>  	if (!(flags & SB_KERNMOUNT)) {
>  		if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
> @@ -35,9 +35,9 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
>  	ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
>  	root = kernfs_mount_ns(fs_type, flags, sysfs_root,
>  				SYSFS_MAGIC, &new_sb, ns);
> -	if (IS_ERR(root) || !new_sb)
> +	if (!new_sb)
>  		kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
> -	else if (new_sb)
> +	else if (!IS_ERR(root))
>  		root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
>  
>  	return root;

What we want for that kobj_ns_drop() is "no fs instances created" (== no
->kill_sb(), be it now or later, to drop that kobj reference); for setting
->s_iflags - "new instance successfully set up".

That's it; all we need is new_sb that would be accurate on its own.
The problem is with kludging over the cases when it's left uninitialized
(early exits from kernfs_mount_ns()) with IS_ERR(root), which happens to
grab the cases when new_sb *was* set to true.  So the fix is to initialize
new_sb properly and get rid of that kludge.  Which turns the whole thing
into
	if (!new_sb)
		...
	if (!IS_ERR(root) && new_sb)
		...
i.e.
	if (!new_sb)
		...
	else if (!IS_ERR(root))
		...

Re: general protection fault in kernfs_kill_sb (2)

[Stephen Rothwell]

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Hi Al,

On Mon, 14 May 2018 05:04:15 +0100 Al Viro <viro@ZenIV.linux.org.uk> wrote:
>
> On Mon, May 14, 2018 at 12:20:16PM +0900, Tetsuo Handa wrote:
> 
> > But there remains a refcount bug because deactivate_locked_super() from
> > kernfs_mount_ns() triggers kobj_ns_drop() from sysfs_kill_sb() via
> > sb->kill_sb() when kobj_ns_drop() is always called by sysfs_mount()
> > if kernfs_mount_ns() returned an error.  
> 
> Trivial:
> 
> unfuck sysfs_mount()
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

I noticed this turn up in linux-next today.  Can I ask that you please
add an actual commit message to it before sending it on to Linus.
-- 
Cheers,
Stephen Rothwell

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