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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20200530060216.221456-1-ebiggers@kernel.org>
Date:   Fri, 29 May 2020 23:02:16 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-ext4@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Daniel Rosenberg <drosen@...gle.com>,
        Gabriel Krisman Bertazi <krisman@...labora.co.uk>
Subject: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name

From: Eric Biggers <ebiggers@...gle.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@...r.kernel.org> # v5.2+
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Daniel Rosenberg <drosen@...gle.com>
Cc: Gabriel Krisman Bertazi <krisman@...labora.co.uk>
Signed-off-by: Eric Biggers <ebiggers@...gle.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;
+	}
+
 	return ext4_ci_compare(inode, name, &qstr, false);
 }
 
-- 
2.26.2