f2fs: avoid utf8_strncasecmp() with unstable name

[ Upstream commit fc3bb095ab02b9e7d89a069ade2cead15c64c504 ]

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>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 594c9ad..e9af46d 100644 (file)
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1063,11 +1063,27 @@ static int f2fs_d_compare(const struct dentry *dentry, unsigned int len,
        const struct inode *dir = READ_ONCE(parent->d_inode);
        const struct f2fs_sb_info *sbi = F2FS_SB(dentry->d_sb);
        struct qstr entry = QSTR_INIT(str, len);
+       char strbuf[DNAME_INLINE_LEN];
        int res;
 
        if (!dir || !IS_CASEFOLDED(dir))
                goto fallback;
 
+       /*
+        * 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;
+               entry.name = strbuf;
+               /* prevent compiler from optimizing out the temporary buffer */
+               barrier();
+       }
+
        res = utf8_strncasecmp(sbi->s_encoding, name, &entry);
        if (res >= 0)
                return res;