Re: [RFC PATCH 6/6 6.6] libfs: fix infinite directory reads for offset dir

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2024/11/11 8:52, cel@kernel.org 写道:
> From: yangerkun <yangerkun@huawei.com>
> 
> [ Upstream commit 64a7ce76fb901bf9f9c36cf5d681328fc0fd4b5a ]
> 
> After we switch tmpfs dir operations from simple_dir_operations to
> simple_offset_dir_operations, every rename happened will fill new dentry
> to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
> key starting with octx->newx_offset, and then set newx_offset equals to
> free key + 1. This will lead to infinite readdir combine with rename
> happened at the same time, which fail generic/736 in xfstests(detail show
> as below).
> 
> 1. create 5000 files(1 2 3...) under one dir
> 2. call readdir(man 3 readdir) once, and get one entry
> 3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
> 4. loop 2~3, until readdir return nothing or we loop too many
>     times(tmpfs break test with the second condition)
> 
> We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
> directory reads") to fix it, record the last_index when we open dir, and
> do not emit the entry which index >= last_index. The file->private_data

Please notice this requires last_index should never overflow, otherwise
readdir will be messed up.
> now used in offset dir can use directly to do this, and we also update
> the last_index when we llseek the dir file.
> 
> Fixes: a2e459555c5f ("shmem: stable directory offsets")
> Signed-off-by: yangerkun <yangerkun@huawei.com>
> Link: https://lore.kernel.org/r/20240731043835.1828697-1-yangerkun@huawei.com
> Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
> [brauner: only update last_index after seek when offset is zero like Jan suggested]
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Link: https://nvd.nist.gov/vuln/detail/CVE-2024-46701
> [ cel: adjusted to apply to origin/linux-6.6.y ]
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>   fs/libfs.c | 37 +++++++++++++++++++++++++------------
>   1 file changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index a87005c89534..b59ff0dfea1f 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -449,6 +449,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
>   	xa_destroy(&octx->xa);
>   }
>   
> +static int offset_dir_open(struct inode *inode, struct file *file)
> +{
> +	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> +
> +	file->private_data = (void *)ctx->next_offset;
> +	return 0;
> +}

Looks like xarray is still used.

I'm in the cc list ,so I assume you saw my set, then I don't know why
you're ignoring my concerns.

1) next_offset is 32-bit and can overflow in a long-time running
machine.
2) Once next_offset overflows, readdir will skip the files that offset
is bigger.

Thanks,
Kuai

> +
>   /**
>    * offset_dir_llseek - Advance the read position of a directory descriptor
>    * @file: an open directory whose position is to be updated
> @@ -462,6 +470,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
>    */
>   static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>   {
> +	struct inode *inode = file->f_inode;
> +	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> +
>   	switch (whence) {
>   	case SEEK_CUR:
>   		offset += file->f_pos;
> @@ -475,8 +486,9 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>   	}
>   
>   	/* In this case, ->private_data is protected by f_pos_lock */
> -	file->private_data = NULL;
> -	return vfs_setpos(file, offset, U32_MAX);
> +	if (!offset)
> +		file->private_data = (void *)ctx->next_offset;
> +	return vfs_setpos(file, offset, LONG_MAX);
>   }
>   
>   static struct dentry *offset_find_next(struct xa_state *xas)
> @@ -505,7 +517,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
>   			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>   }
>   
> -static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
>   {
>   	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
>   	XA_STATE(xas, &so_ctx->xa, ctx->pos);
> @@ -514,17 +526,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>   	while (true) {
>   		dentry = offset_find_next(&xas);
>   		if (!dentry)
> -			return ERR_PTR(-ENOENT);
> +			return;
> +
> +		if (dentry2offset(dentry) >= last_index) {
> +			dput(dentry);
> +			return;
> +		}
>   
>   		if (!offset_dir_emit(ctx, dentry)) {
>   			dput(dentry);
> -			break;
> +			return;
>   		}
>   
>   		dput(dentry);
>   		ctx->pos = xas.xa_index + 1;
>   	}
> -	return NULL;
>   }
>   
>   /**
> @@ -551,22 +567,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>   static int offset_readdir(struct file *file, struct dir_context *ctx)
>   {
>   	struct dentry *dir = file->f_path.dentry;
> +	long last_index = (long)file->private_data;
>   
>   	lockdep_assert_held(&d_inode(dir)->i_rwsem);
>   
>   	if (!dir_emit_dots(file, ctx))
>   		return 0;
>   
> -	/* In this case, ->private_data is protected by f_pos_lock */
> -	if (ctx->pos == DIR_OFFSET_MIN)
> -		file->private_data = NULL;
> -	else if (file->private_data == ERR_PTR(-ENOENT))
> -		return 0;
> -	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
> +	offset_iterate_dir(d_inode(dir), ctx, last_index);
>   	return 0;
>   }
>   
>   const struct file_operations simple_offset_dir_operations = {
> +	.open		= offset_dir_open,
>   	.llseek		= offset_dir_llseek,
>   	.iterate_shared	= offset_readdir,
>   	.read		= generic_read_dir,
>