Re: [PATCH] f2fs: avoid utf8_strncasecmp() with unstable name

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 05/29, Eric Biggers wrote:
> 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: 2c2eb7a300cd ("f2fs: Support case-insensitive file name lookups")
> Cc: <stable@vger.kernel.org> # v5.4+
> 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>

Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>

> ---
>  fs/f2fs/dir.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
> index 44bfc464df787..5c179b72eb8a8 100644
> --- a/fs/f2fs/dir.c
> +++ b/fs/f2fs/dir.c
> @@ -1083,6 +1083,7 @@ static int f2fs_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)) {
>  		if (len != name->len)
> @@ -1090,6 +1091,22 @@ static int f2fs_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;
> +	}
> +
>  	return f2fs_ci_compare(inode, name, &qstr, false);
>  }
>  
> -- 
> 2.26.2