Linux-FSCrypt Archive on lore.kernel.org
 help / color / Atom feed
From: Eric Biggers <ebiggers@kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Daniel Rosenberg <drosen@google.com>,
	Theodore Ts'o <tytso@mit.edu>,
	linux-ext4@vger.kernel.org, Jaegeuk Kim <jaegeuk@kernel.org>,
	Chao Yu <chao@kernel.org>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-fscrypt@vger.kernel.org,
	Richard Weinberger <richard@nod.at>,
	linux-mtd@lists.infradead.org,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	Gabriel Krisman Bertazi <krisman@collabora.com>,
	kernel-team@android.com
Subject: Re: [PATCH v7 2/8] fs: Add standard casefolding support
Date: Tue, 11 Feb 2020 22:57:34 -0800
Message-ID: <20200212065734.GA157327@sol.localdomain> (raw)
In-Reply-To: <20200212063440.GL870@sol.localdomain>

On Tue, Feb 11, 2020 at 10:34:40PM -0800, Eric Biggers wrote:
> On Mon, Feb 10, 2020 at 11:42:07PM +0000, Al Viro wrote:
> > On Mon, Feb 10, 2020 at 03:11:13PM -0800, Daniel Rosenberg wrote:
> > > On Fri, Feb 7, 2020 at 6:12 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > >
> > > > On Fri, Feb 07, 2020 at 05:35:46PM -0800, Daniel Rosenberg wrote:
> > > >
> > > >
> > > > Again, is that safe in case when the contents of the string str points to
> > > > keeps changing under you?
> > > 
> > > I'm not sure what you mean. I thought it was safe to use the str and
> > > len passed into d_compare. Even if it gets changed under RCU
> > > conditions I thought there was some code to ensure that the name/len
> > > pair passed in is consistent, and any other inconsistencies would get
> > > caught by d_seq later. Are there unsafe code paths that can follow?
> > 
> > If you ever fetch the same byte twice, you might see different values.
> > You need a fairly careful use of READ_ONCE() or equivalents to make
> > sure that you don't get screwed over by that.
> > 
> > Sure, ->d_seq mismatch will throw the result out, but you need to make
> > sure you won't oops/step on uninitialized memory/etc. in process.
> > 
> > It's not impossible to get right, but it's not trivial and you need all
> > code working with that much more careful than normal for string handling.
> 
> It looks like this is a real problem, not just a "theoretical" data race.
> For example, see:
> 
> utf8ncursor():
>         /* The first byte of s may not be an utf8 continuation. */
>         if (len > 0 && (*s & 0xC0) == 0x80)
>                 return -1;
> 
> and then utf8byte():
>                 } else if ((*u8c->s & 0xC0) == 0x80) {
>                         /* This is a continuation of the current character. */
>                         if (!u8c->p)
>                                 u8c->len--;
>                         return (unsigned char)*u8c->s++;
> 
> The first byte of the string is checked in two different functions, so it's very
> likely to be loaded twice.  In between, it could change from a non-continuation
> byte to a continuation byte.  That would cause the string length to be
> decremented from 0 to UINT_MAX.  Then utf8_strncasecmp() would run beyond the
> bounds of the string until something happened to mismatch.
> 
> That's just an example that I found right away; there are probably more.
> 
> IMO, this needs to be fixed before anyone can actually use the ext4 and f2fs
> casefolding stuff.
> 
> I don't know the best solution.  One option is to fix fs/unicode/ to handle
> concurrently modified strings.  Another could be to see what it would take to
> serialize lookups and renames for casefolded directories...
> 

Or (just throwing another idea out there) the dentry's name could be copied to a
temporary buffer in ->d_compare().  The simplest version would be:

	u8 _name[NAME_MAX];

	memcpy(_name, name, len);
	name = _name;

Though, 255 bytes is a bit large for a stack buffer (so for long names it may
need kmalloc with GFP_ATOMIC), and technically it would need a special version
of memcpy() to be guaranteed safe from compiler optimizations (though I expect
this would work in practice).

Alternatively, take_dentry_name_snapshot() kind of does this already, except
that it takes a dentry and not a (name, len) pair.

- Eric

  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-08  1:35 [PATCH v7 0/8] Support fof Casefolding and Encryption Daniel Rosenberg
2020-02-08  1:35 ` [PATCH v7 1/8] unicode: Add utf8_casefold_iter Daniel Rosenberg
2020-02-12  3:38   ` Eric Biggers
2020-02-14 21:47     ` Daniel Rosenberg
2020-02-17 19:02       ` Gabriel Krisman Bertazi
2020-02-08  1:35 ` [PATCH v7 2/8] fs: Add standard casefolding support Daniel Rosenberg
2020-02-08  2:12   ` Al Viro
2020-02-10 23:11     ` Daniel Rosenberg
2020-02-10 23:42       ` Al Viro
2020-02-12  6:34         ` Eric Biggers
2020-02-12  6:57           ` Eric Biggers [this message]
2020-02-20  2:27             ` Daniel Rosenberg
2020-02-12  3:55   ` Eric Biggers
2020-02-08  1:35 ` [PATCH v7 3/8] f2fs: Use generic " Daniel Rosenberg
2020-02-12  4:05   ` Eric Biggers
2020-02-08  1:35 ` [PATCH v7 4/8] ext4: " Daniel Rosenberg
2020-02-08  1:35 ` [PATCH v7 5/8] fscrypt: Have filesystems handle their d_ops Daniel Rosenberg
2020-02-12  4:33   ` Eric Biggers
2020-02-08  1:35 ` [PATCH v7 6/8] f2fs: Handle casefolding with Encryption Daniel Rosenberg
2020-02-12  5:10   ` Eric Biggers
2020-02-12  5:55     ` Al Viro
2020-02-12  6:06       ` Eric Biggers
2020-02-12  5:47   ` Eric Biggers
2020-02-08  1:35 ` [PATCH v7 7/8] ext4: Hande casefolding with encryption Daniel Rosenberg
2020-02-12  5:59   ` Eric Biggers
2020-02-08  1:35 ` [PATCH v7 8/8] ext4: Optimize match for casefolded encrypted dirs Daniel Rosenberg
2020-02-12  6:12 ` [PATCH v7 0/8] Support fof Casefolding and Encryption Eric Biggers
2020-02-13  0:01   ` Daniel Rosenberg

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200212065734.GA157327@sol.localdomain \
    --to=ebiggers@kernel.org \
    --cc=adilger.kernel@dilger.ca \
    --cc=chao@kernel.org \
    --cc=corbet@lwn.net \
    --cc=drosen@google.com \
    --cc=jaegeuk@kernel.org \
    --cc=kernel-team@android.com \
    --cc=krisman@collabora.com \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-ext4@vger.kernel.org \
    --cc=linux-f2fs-devel@lists.sourceforge.net \
    --cc=linux-fscrypt@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mtd@lists.infradead.org \
    --cc=richard@nod.at \
    --cc=tytso@mit.edu \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-FSCrypt Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fscrypt/0 linux-fscrypt/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fscrypt linux-fscrypt/ https://lore.kernel.org/linux-fscrypt \
		linux-fscrypt@vger.kernel.org
	public-inbox-index linux-fscrypt

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fscrypt


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git