linux-ext4 - Re: [PATCH v7 2/8] fs: Add standard casefolding support

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Date:   Tue, 11 Feb 2020 22:34:40 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Daniel Rosenberg <drosen@...gle.com>,
        Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fscrypt@...r.kernel.org, Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 2/8] fs: Add standard casefolding support

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@...iv.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...

- Eric