linux-kernel - Re: [PATCH] fs: add check for symlink corrupted

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Message-ID: <20241116010207.1484956-1-lizhi.xu@windriver.com>
Date: Sat, 16 Nov 2024 09:02:07 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <jack@...e.cz>
CC: <almaz.alexandrovich@...agon-software.com>, <brauner@...nel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <lizhi.xu@...driver.com>, <ntfs3@...ts.linux.dev>,
        <syzbot+73d8fc29ec7cba8286fa@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>, <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] fs: add check for symlink corrupted

On Fri, 15 Nov 2024 12:43:06 +0100, Jan Kara wrote:
> On Fri 15-11-24 17:49:08, Lizhi Xu wrote:
> > syzbot reported a null-ptr-deref in pick_link. [1]
> > When symlink's inode is corrupted, the value of the i_link is 2 in this case,
> > it will trigger null pointer deref when accessing *res in pick_link().
> >
> > To avoid this issue, add a check for inode mode, return -EINVAL when it's
> > not symlink.
> >
> > [1]
> > KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> > CPU: 0 UID: 0 PID: 5310 Comm: syz-executor255 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> > RIP: 0010:pick_link+0x51c/0xd50 fs/namei.c:1864
> 
> Hum, based on line number is:
> 
>         if (*res == '/') { <<<< HERE
>                 error = nd_jump_root(nd);
>                 if (unlikely(error))
> 
> So res would be non-zero but a small number.
> 
> > Code: c1 e8 03 42 80 3c 38 00 74 08 48 89 df e8 fc 00 e9 ff 48 8b 2b 48 85 ed 0f 84 92 00 00 00 e8 7b 36 7f ff 48 89 e8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 a2 05 00 00 0f b6 5d 00 bf 2f 00 00 00
> > RSP: 0018:ffffc9000d147998 EFLAGS: 00010246
> > RAX: 0000000000000000 RBX: ffff88804558dec8 RCX: ffff88801ec7a440
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 0000000000000002 R08: ffffffff8215a35f R09: 1ffffffff203a13d
> > R10: dffffc0000000000 R11: fffffbfff203a13e R12: 1ffff92001a28f93
> > R13: ffffc9000d147af8 R14: 1ffff92001a28f5f R15: dffffc0000000000
> > FS:  0000555577611380(0000) GS:ffff88801fc00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fcc0a595ed8 CR3: 0000000035760000 CR4: 0000000000352ef0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  <TASK>
> >  step_into+0xca9/0x1080 fs/namei.c:1923
> >  lookup_last fs/namei.c:2556 [inline]
> >  path_lookupat+0x16f/0x450 fs/namei.c:2580
> >  filename_lookup+0x256/0x610 fs/namei.c:2609
> >  user_path_at+0x3a/0x60 fs/namei.c:3016
> >  do_mount fs/namespace.c:3844 [inline]
> >  __do_sys_mount fs/namespace.c:4057 [inline]
> >  __se_sys_mount+0x297/0x3c0 fs/namespace.c:4034
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f4b18ad5b19
> > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 17 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007ffc2e486c48 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> > RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 00007f4b18ad5b19
> > RDX: 0000000000000000 RSI: 00000000200000c0 RDI: 0000000000000000
> > RBP: 00007f4b18b685f0 R08: 0000000000000000 R09: 00005555776124c0
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffc2e486c70
> > R13: 00007ffc2e486e98 R14: 431bde82d7b634db R15: 00007f4b18b1e03b
> >  </TASK>
> >
> > Reported-and-tested-by: syzbot+73d8fc29ec7cba8286fa@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=73d8fc29ec7cba8286fa
> > Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> > ---
> >  fs/namei.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 4a4a22a08ac2..f5dbccb3aafc 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1844,6 +1844,9 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
> >  	if (unlikely(error))
> >  		return ERR_PTR(error);
> >
> > +	if (!S_ISLNK(inode->i_mode))
> > +		return ERR_PTR(-EINVAL);
> > +
> 
> So I don't see how we can get here without inode being a symlink.
> pick_link() is called from step_into() which has among other things:
> 
> if (likely(!d_is_symlink(path.dentry)) || ...)
Our idea is the same. Because d_is_symlink() has confirmed the mode of
symlink in step_into(), I will confirm whether the mode of symlink's inode
has changed when the value of i_link is 2 in pick_link().
> 	do something and return
> 
> so we are checking whether the inode is a symlink before calling
> pick_link(). And yes, the d_is_symlink() is using cached type in
> dentry->d_flags so they could mismatch. But inode is not supposed to change
> its type during its lifetime so if there is a mismatch that is the problem
> that needs to be fixed.
I think syzbot executed the following two syscalls when triggering this problem:

link(&(0x7f0000000200)='./file0\x00', &(0x7f0000000240)='./bus\x00')
mount$overlay(0x0, &(0x7f00000000c0)='./bus\x00', 0x0, 0x0, 0x0)

Obviously, this is to mount a link. Whether the mount operation itself will
change or corrupt the i_link value and mode value of the symlink is not
clear to me yet.

BR,
Lizhi