Eric Biggers ebiggers@kernel.org writes:
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
fs/ext4/dir.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)
diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c index c654205f648dd..19aef8328bb18 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 +684,22 @@ 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) {
unsigned int i;for (i = 0; i < len; i++)strbuf[i] = READ_ONCE(str[i]);strbuf[len] = 0;qstr.name = strbuf;- }
Could we avoid this if the casefolded version were cached in the dentry? Then we could use utf8_strncasecmp_folded which would be safe. Would this be acceptable for vfs?
return ext4_ci_compare(inode, name, &qstr, false); }