Re: [PATCH v4 3/7] libfs: Validate negative dentries in case-insensitive directories

[Christian Brauner <brauner@kernel.org>]

On Thu, Jul 27, 2023 at 01:28:39PM -0400, Gabriel Krisman Bertazi wrote:
> From: Gabriel Krisman Bertazi <krisman@collabora.com>
> 
> Introduce a dentry revalidation helper to be used by case-insensitive
> filesystems to check if it is safe to reuse a negative dentry.
> 
> A negative dentry is safe to be reused on a case-insensitive lookup if
> it was created during a case-insensitive lookup and this is not a lookup
> that will instantiate a dentry. If this is a creation lookup, we also
> need to make sure the name matches sensitively the name under lookup in
> order to assure the name preserving semantics.
> 
> dentry->d_name is only checked by the case-insensitive d_revalidate hook
> in the LOOKUP_CREATE/LOOKUP_RENAME_TARGET case since, for these cases,
> d_revalidate is always called with the parent inode read-locked, and
> therefore the name cannot change from under us.
> 
> d_revalidate is only called in 4 places: lookup_dcache, __lookup_slow,
> lookup_open and lookup_fast:
> 
>   - lookup_dcache always calls it with zeroed flags, with the exception
>     of when coming from __lookup_hash, which needs the parent locked
>     already, for instance in the open/creation path, which is locked in
>     open_last_lookups.
> 
>   - In __lookup_slow, either the parent inode is read locked by the
>     caller (lookup_slow), or it is called with no flags (lookup_one*).
>     The read lock suffices to prevent ->d_name modifications, with the
>     exception of one case: __d_unalias, will call __d_move to fix a
>     directory accessible from multiple dentries, which effectively swaps
>     ->d_name while holding only the shared read lock.  This happens
>     through this flow:
> 
>     lookup_slow()  //LOOKUP_CREATE
>       d_lookup()
>         ->d_lookup()
>           d_splice_alias()
>             __d_unalias()
>               __d_move()
> 
>     Nevertheless, this case is not a problem because negative dentries
>     are not allowed to be moved with __d_move.
> 
>   - lookup_open also requires the parent to be locked in the creation
>     case, which is done in open_last_lookups.
> 
>   - lookup_fast will indeed be called with the parent unlocked, but it
>     shouldn't be called with LOOKUP_CREATE.  Either it is called in the
>     link_path_walk, where nd->flags doesn't have LOOKUP_CREATE yet or in
>     open_last_lookups. But, in this case, it also never has LOOKUP_CREATE,
>     because it is only called on the !O_CREAT case, which means op->intent
>     doesn't have LOOKUP_CREAT (set in build_open_flags only if O_CREAT is
>     set).
> 
> Finally, for the LOOKUP_RENAME_TARGET, we are doing a rename, so the
> parents inodes are also locked.
> 
> Reviewed-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
> 
> ---
> Changes since v3:
>   - Add comment regarding creation (Eric)
>   - Reorder checks to clarify !flags meaning (Eric)
>   - Add commit message explanaton of the inode read lock wrt.
>     __d_move. (Eric)
> Changes since v2:
>   - Add comments to all rejection cases (Eric)
>   - safeguard against filesystem creating dentries without LOOKUP flags
> ---
>  fs/libfs.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 5b851315eeed..ed04c4dcc312 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1462,9 +1462,64 @@ static int generic_ci_d_hash(const struct dentry *dentry, struct qstr *str)
>  	return 0;
>  }
>  
> +static inline int generic_ci_d_revalidate(struct dentry *dentry,
> +					  const struct qstr *name,
> +					  unsigned int flags)
> +{
> +	if (d_is_negative(dentry)) {
> +		const struct dentry *parent = READ_ONCE(dentry->d_parent);
> +		const struct inode *dir = READ_ONCE(parent->d_inode);
> +
> +		if (dir && needs_casefold(dir)) {
> +			/*
> +			 * Negative dentries created prior to turning the
> +			 * directory case-insensitive cannot be trusted, since
> +			 * they don't ensure any possible case version of the
> +			 * filename doesn't exist.
> +			 */
> +			if (!d_is_casefold_lookup(dentry))
> +				return 0;
> +
> +			/*
> +			 * Filesystems will call into d_revalidate without
> +			 * setting LOOKUP_ flags even for file creation (see
> +			 * lookup_one* variants).  Reject negative dentries in
> +			 * this case, since we can't know for sure it won't be
> +			 * used for creation.
> +			 */
> +			if (!flags)
> +				return 0;
> +
> +			/*
> +			 * If the lookup is for creation, then a negative dentry
> +			 * can only be reused if it's a case-sensitive match,
> +			 * not just a case-insensitive one.  This is needed to
> +			 * make the new file be created with the name the user
> +			 * specified, preserving case.
> +			 */
> +			if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
> +				/*
> +				 * ->d_name won't change from under us in the
> +				 * creation path only, since d_revalidate during
> +				 * creation and renames is always called with
> +				 * the parent inode locked.  It isn't the case
> +				 * for all lookup callpaths, so ->d_name must
> +				 * not be touched outside
> +				 * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
> +				 */
> +				if (dentry->d_name.len != name->len ||
> +				    memcmp(dentry->d_name.name, name->name, name->len))
> +					return 0;
> +			}
> +		}
> +	}
> +	return 1;
> +}
> +
>  static const struct dentry_operations generic_ci_dentry_ops = {
>  	.d_hash = generic_ci_d_hash,
>  	.d_compare = generic_ci_d_compare,
> +	.d_revalidate_name = generic_ci_d_revalidate,
>  };
>  #endif

Wouldn't it make sense to get rid of all this indentation?

	const struct dentry *parent;
	const struct inode *dir;

	if (!d_is_negative(dentry))
		return 1;

	parent = READ_ONCE(dentry->d_parent);
	dir = READ_ONCE(parent->d_inode);

	if (!dir)
		return 1;

	if (!needs_casefold(dir))
		return 1;

	/*
	 * Negative dentries created prior to turning the
	 * directory case-insensitive cannot be trusted, since
	 * they don't ensure any possible case version of the
	 * filename doesn't exist.
	 */
	if (!d_is_casefold_lookup(dentry))
		return 0;

	/*
	 * Filesystems will call into d_revalidate without
	 * setting LOOKUP_ flags even for file creation (see
	 * lookup_one* variants).  Reject negative dentries in
	 * this case, since we can't know for sure it won't be
	 * used for creation.
	 */
	if (!flags)
		return 0;

	/*
	 * If the lookup is for creation, then a negative dentry
	 * can only be reused if it's a case-sensitive match,
	 * not just a case-insensitive one.  This is needed to
	 * make the new file be created with the name the user
	 * specified, preserving case.
	 */
	if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
		/*
		 * ->d_name won't change from under us in the
		 * creation path only, since d_revalidate during
		 * creation and renames is always called with
		 * the parent inode locked.  It isn't the case
		 * for all lookup callpaths, so ->d_name must
		 * not be touched outside
		 * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
		 */
		if (dentry->d_name.len != name->len ||
		    memcmp(dentry->d_name.name, name->name, name->len))
			return 0;
	}
	return 1;