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From: Al Viro <viro@zeniv.linux.org.uk>
To: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: "Pali Rohár" <pali.rohar@gmail.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	"Namjae Jeon" <linkinjeon@gmail.com>,
	"Gabriel Krisman Bertazi" <krisman@collabora.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>
Subject: oopsably broken case-insensitive support in ext4 and f2fs (Re: vfat: Broken case-insensitive support for UTF-8)
Date: Mon, 20 Jan 2020 08:07:21 +0000	[thread overview]
Message-ID: <20200120080721.GB8904@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200120074558.GA8904@ZenIV.linux.org.uk>

On Mon, Jan 20, 2020 at 07:45:58AM +0000, Al Viro wrote:
> On Mon, Jan 20, 2020 at 07:30:40AM +0000, Al Viro wrote:
> 
> > <checks ext4>
> > Yup, that bug is there as well, all right.  Look:
> > #ifdef CONFIG_UNICODE
> > static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
> >                           const char *str, const struct qstr *name)
> > {
> >         struct qstr qstr = {.name = str, .len = len };
> >         struct inode *inode = dentry->d_parent->d_inode;
> > 
> >         if (!IS_CASEFOLDED(inode) || !EXT4_SB(inode->i_sb)->s_encoding) {
> > 
> > Guess what happens if your (lockless) call of ->d_compare() runs
> > into the following sequence:
> > CPU1:	ext4_d_compare() fetches ->d_parent
> > CPU1:	takes a hardware interrupt
> > CPU2:	dentry gets evicted by memory pressure; so is its parent, since
> > it was the only thing that used to keep it pinned.  Eviction of the parent
> > calls dentry_unlink_inode() on the parent, which zeroes its ->d_inode.
> > CPU1:	comes back
> > CPU1:	fetches parent's ->d_inode and gets NULL
> > CPU1:	oopses on null pointer dereference.
> > 
> > It's not impossible to hit.  Note that e.g. vfat_cmpi() is not vulnerable
> > to that problem - ->d_sb is stable and both the superblock and ->nls_io
> > freeing is RCU-delayed.
> > 
> > I hadn't checked ->d_compare() instances for a while; somebody needs to
> > do that again, by the look of it.  The above definitely is broken;
> > no idea how many other instaces had grown such bugs...
> 
> f2fs one also has the same bug.  Anyway, I'm going down right now, will
> check the rest tomorrow morning...

We _probably_ can get away with just checking that inode for NULL and
buggering off if it is (->d_seq mismatch is guaranteed in that case),
but I suspect that we might need READ_ONCE() on both dereferences.
I hate memory barriers...

  reply	other threads:[~2020-01-20  8:07 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-19 22:14 vfat: Broken case-insensitive support for UTF-8 Pali Rohár
2020-01-19 23:08 ` Al Viro
2020-01-19 23:33   ` Pali Rohár
2020-01-20  0:09     ` Al Viro
2020-01-20 11:19       ` Pali Rohár
2020-01-20  4:04 ` OGAWA Hirofumi
2020-01-20  7:30   ` Al Viro
2020-01-20  7:45     ` Al Viro
2020-01-20  8:07       ` Al Viro [this message]
2020-01-20 19:35         ` oopsably broken case-insensitive support in ext4 and f2fs (Re: vfat: Broken case-insensitive support for UTF-8) Al Viro
2020-01-24  4:29           ` Eric Biggers
2020-01-24 17:47             ` Linus Torvalds
2020-01-24 18:03               ` Jaegeuk Kim
2020-01-24 18:45                 ` Eric Biggers
2020-01-20 11:04   ` vfat: Broken case-insensitive support for UTF-8 Pali Rohár
2020-01-20 12:07     ` OGAWA Hirofumi
2020-01-20 21:40       ` Pali Rohár
2020-01-20 22:46         ` Al Viro
2020-01-20 23:57           ` Pali Rohár
2020-01-21  0:07             ` Al Viro
2020-01-21 20:34               ` Pali Rohár
2020-01-21 21:36                 ` Al Viro
2020-01-21 22:14                   ` Al Viro
2020-01-21 22:46                     ` Pali Rohár
2020-01-26 23:08                 ` Pali Rohár
2020-01-21 12:43             ` David Laight
2020-01-22  0:25         ` Gabriel Krisman Bertazi
2020-01-20 15:07     ` David Laight
2020-01-20 15:20       ` Pali Rohár
2020-01-20 15:47         ` David Laight
2020-01-20 16:12           ` Al Viro
2020-01-20 16:51             ` David Laight
2020-01-20 16:27           ` Pali Rohár
2020-01-20 16:43             ` David Laight
2020-01-20 16:56               ` Pali Rohár
2020-01-20 17:37       ` Theodore Y. Ts'o
2020-01-20 17:32   ` Theodore Y. Ts'o
2020-01-20 17:56     ` Pali Rohár
2020-01-21  3:52     ` OGAWA Hirofumi
2020-01-21 11:00       ` Pali Rohár
2020-01-21 12:26         ` OGAWA Hirofumi

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