linux-kernel - [PATCH 5.7 440/477] f2fs: avoid utf8

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Date:   Tue, 23 Jun 2020 21:57:17 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Daniel Rosenberg <drosen@...gle.com>,
        Gabriel Krisman Bertazi <krisman@...labora.co.uk>,
        Eric Biggers <ebiggers@...gle.com>,
        Chao Yu <yuchao0@...wei.com>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.7 440/477] f2fs: avoid utf8_strncasecmp() with unstable name

From: Eric Biggers <ebiggers@...gle.com>

[ 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@...r.kernel.org> # v5.4+
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>
Reviewed-by: Chao Yu <yuchao0@...wei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/f2fs/dir.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 44eb12a00cd0e..54e90dbb09e78 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1076,11 +1076,27 @@ 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;
-- 
2.25.1