From: Eric Biggers <ebigg...@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: <sta...@vger.kernel.org> # v5.2+
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Daniel Rosenberg <dro...@google.com>
Cc: Gabriel Krisman Bertazi <kris...@collabora.co.uk>
Signed-off-by: Eric Biggers <ebigg...@google.com>
Reviewed-by: Andreas Dilger <adil...@dilger.ca>
Link: https://lore.kernel.org/r/20200601200543.59417-1-ebigg...@kernel.org
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
fs/ext4/dir.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -675,6 +675,7 @@ 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) {
@@ -683,6 +684,21 @@ 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);
}
