Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name

From: Gabriel Krisman Bertazi <krisman@collabora.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, stable@vger.kernel.org,
	Al Viro <viro@zeniv.linux.org.uk>,
	Daniel Rosenberg <drosen@google.com>,
	Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Subject: Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name
Date: Sat, 30 May 2020 02:17:02 -0400	[thread overview]
Message-ID: <85d06mkkv5.fsf@collabora.com> (raw)
In-Reply-To: <20200530060216.221456-1-ebiggers@kernel.org> (Eric Biggers's message of "Fri, 29 May 2020 23:02:16 -0700")

Eric Biggers <ebiggers@kernel.org> writes:

> From: Eric Biggers <ebiggers@google.com>
>
> If the dentry name passed to ->d_compare() fits in dentry::d_iname, then
> it may be concurrently modified by a rename.  This can cause undefined
> behavior (possibly out-of-bounds memory accesses or crashes) in
> utf8_strncasecmp(), since fs/unicode/ isn't written to handle strings
> that may be concurrently modified.
>
> Fix this by first copying the filename to a stack buffer if needed.
> This way we get a stable snapshot of the filename.
>
> Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
> Cc: <stable@vger.kernel.org> # v5.2+
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Daniel Rosenberg <drosen@google.com>
> Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/ext4/dir.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> index c654205f648dd..19aef8328bb18 100644
> --- a/fs/ext4/dir.c
> +++ b/fs/ext4/dir.c
> @@ -675,6 +675,7 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
>  	struct qstr qstr = {.name = str, .len = len };
>  	const struct dentry *parent = READ_ONCE(dentry->d_parent);
>  	const struct inode *inode = READ_ONCE(parent->d_inode);
> +	char strbuf[DNAME_INLINE_LEN];
>  
>  	if (!inode || !IS_CASEFOLDED(inode) ||
>  	    !EXT4_SB(inode->i_sb)->s_encoding) {
> @@ -683,6 +684,22 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
>  		return memcmp(str, name->name, len);
>  	}
>  
> +	/*
> +	 * If the dentry name is stored in-line, then it may be concurrently
> +	 * modified by a rename.  If this happens, the VFS will eventually retry
> +	 * the lookup, so it doesn't matter what ->d_compare() returns.
> +	 * However, it's unsafe to call utf8_strncasecmp() with an unstable
> +	 * string.  Therefore, we have to copy the name into a temporary buffer.
> +	 */
> +	if (len <= DNAME_INLINE_LEN - 1) {
> +		unsigned int i;
> +
> +		for (i = 0; i < len; i++)
> +			strbuf[i] = READ_ONCE(str[i]);
> +		strbuf[len] = 0;
> +		qstr.name = strbuf;
> +	}
> +

Could we avoid this if the casefolded version were cached in the dentry?
Then we could use utf8_strncasecmp_folded which would be safe.  Would
this be acceptable for vfs?

>  	return ext4_ci_compare(inode, name, &qstr, false);
>  }

-- 
Gabriel Krisman Bertazi