ext4: avoid utf8_strncasecmp() with unstable name

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>

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index c654205..1d82336 100644 (file)
--- 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 +684,21 @@ 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) {
+               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);
 }