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 [thread overview]
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
next reply other threads:[~2020-06-01 20:07 UTC|newest]
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 ` [PATCH v2] ext4: avoid utf8_strncasecmp() with unstable name Andreas Dilger
2020-06-11 15:01 ` Theodore Y. Ts'o
2020-06-09 20:28 ` Eric Biggers
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