diff mbox series

[5.4,282/314] ext4: avoid utf8_strncasecmp() with unstable name

Message ID 20200623195352.429875944@linuxfoundation.org
State New
Headers show
Series None | expand

Commit Message

Greg KH June 23, 2020, 7:57 p.m. UTC 
From: Eric Biggers <ebiggers@google.com>

commit 2ce3ee931a097e9720310db3f09c01c825a4580c upstream.

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>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20200601200543.59417-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/ext4/dir.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)
diff mbox series

Patch

--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -677,6 +677,7 @@  static int ext4_d_compare(const struct d
 	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) {
@@ -685,6 +686,21 @@  static int ext4_d_compare(const struct d
 		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) {
+		memcpy(strbuf, str, len);
+		strbuf[len] = 0;
+		qstr.name = strbuf;
+		/* prevent compiler from optimizing out the temporary buffer */
+		barrier();
+	}
+
 	return ext4_ci_compare(inode, name, &qstr, false);
 }