From: Eric Biggers <ebiggers@xxxxxxxxxx>
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@xxxxxxxxxxxxxxx> # v5.4+
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Rosenberg <drosen@xxxxxxxxxx>
Cc: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
v2: Use memcpy() + barrier() instead of a byte-by-byte copy.
Also rebased onto f2fs/dev.
fs/f2fs/dir.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 29f70f2295cce..d35976785e8c5 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1114,11 +1114,27 @@ 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;
--
2.26.2
