ext4: fix race conditions in ->d_compare() and ->d_hash()

Since ->d_compare() and ->d_hash() can be called in RCU-walk mode,
->d_parent and ->d_inode can be concurrently modified, and in
particular, ->d_inode may be changed to NULL.  For ext4_d_hash() this
resulted in a reproducible NULL dereference if a lookup is done in a
directory being deleted, e.g. with:

	int main()
	{
		if (fork()) {
			for (;;) {
				mkdir("subdir", 0700);
				rmdir("subdir");
			}
		} else {
			for (;;)
				access("subdir/file", 0);
		}
	}

... or by running the 't_encrypted_d_revalidate' program from xfstests.
Both repros work in any directory on a filesystem with the encoding
feature, even if the directory doesn't actually have the casefold flag.

I couldn't reproduce a crash in ext4_d_compare(), but it appears that a
similar crash is possible there.

Fix these bugs by reading ->d_parent and ->d_inode using READ_ONCE() and
falling back to the case sensitive behavior if the inode is NULL.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
Cc: <stable@vger.kernel.org> # v5.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200124041234.159740-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 8964778aabefbd2e9868827e918733dfa99fe0c0..0129d14629881e45c21c89a025be6fa2cec24675 100644 (file)
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -671,9 +671,11 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
                          const char *str, const struct qstr *name)
 {
        struct qstr qstr = {.name = str, .len = len };
-       struct inode *inode = dentry->d_parent->d_inode;
+       const struct dentry *parent = READ_ONCE(dentry->d_parent);
+       const struct inode *inode = READ_ONCE(parent->d_inode);
 
-       if (!IS_CASEFOLDED(inode) || !EXT4_SB(inode->i_sb)->s_encoding) {
+       if (!inode || !IS_CASEFOLDED(inode) ||
+           !EXT4_SB(inode->i_sb)->s_encoding) {
                if (len != name->len)
                        return -1;
                return memcmp(str, name->name, len);
@@ -686,10 +688,11 @@ static int ext4_d_hash(const struct dentry *dentry, struct qstr *str)
 {
        const struct ext4_sb_info *sbi = EXT4_SB(dentry->d_sb);
        const struct unicode_map *um = sbi->s_encoding;
+       const struct inode *inode = READ_ONCE(dentry->d_inode);
        unsigned char *norm;
        int len, ret = 0;
 
-       if (!IS_CASEFOLDED(dentry->d_inode) || !um)
+       if (!inode || !IS_CASEFOLDED(inode) || !um)
                return 0;
 
        norm = kmalloc(PATH_MAX, GFP_ATOMIC);