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From: Eric Biggers <ebiggers@kernel.org>
To: linux-ext4@vger.kernel.org
Cc: Daniel Rosenberg <drosen@google.com>,
	stable@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org,
	Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Subject: [PATCH v2] ext4: avoid utf8_strncasecmp() with unstable name
Date: Mon,  1 Jun 2020 13:05:43 -0700
Message-ID: <20200601200543.59417-1-ebiggers@kernel.org> (raw)

From: Eric Biggers <ebiggers@google.com>

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>
---

v2: use memcpy() + barrier() instead of a byte-by-byte copy.

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

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index c654205f648dd..1d82336b1cd45 100644
--- 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);
 }
 
-- 
2.26.2


             reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-01 20:05 Eric Biggers [this message]
2020-06-01 20:49 ` Andreas Dilger
2020-06-11 15:01   ` Theodore Y. Ts'o
2020-06-09 20:28 ` Eric Biggers

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