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); }