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

[yangerkun]

在 2024/11/11 22:39, Chuck Lever III 写道:
> 
> 
>> On Nov 10, 2024, at 9:36 PM, Yu Kuai <yukuai1 at huaweicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2024/11/11 8:52, cel at kernel.org 写道:
>>> From: yangerkun <yangerkun at 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.
> 
> It would help your cause if you could be more specific
> than "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 at huawei.com>
>>> Link: https://lore.kernel.org/r/20240731043835.1828697-1-yangerkun@huawei.com
>>> Reviewed-by: Chuck Lever <chuck.lever at oracle.com>
>>> [brauner: only update last_index after seek when offset is zero like Jan suggested]
>>> Signed-off-by: Christian Brauner <brauner at 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 at 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.
> 
> That's not going to change, as several folks have already
> explained.
> 
> 
>> 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.
> 

I'm sorry, I'm a little busy these days, so I haven't responded to this
series of emails.

> In that case, that entry won't be visible via getdents(3)
> until the directory is re-opened or the process does an
> lseek(fd, 0, SEEK_SET).

Yes.

> 
> That is the proper and expected behavior. I suspect you
> will see exactly that behavior with ext4 and 32-bit
> directory offsets, for example.

Emm...

For this case like this:

1. mkdir /tmp/dir and touch /tmp/dir/file1 /tmp/dir/file2
2. open /tmp/dir with fd1
3. readdir and get /tmp/dir/file1
4. rm /tmp/dir/file2
5. touch /tmp/dir/file2
4. loop 4~5 for 2^32 times
5. readdir /tmp/dir with fd1

For tmpfs now, we may see no /tmp/dir/file2, since the offset has been 
overflow, for ext4 it is ok... So we think this will be a problem.

> 
> Does that not directly address your concern? Or do you
> mean that Erkun's patch introduces a new issue?

Yes, to be honest, my personal feeling is a problem. But for 64bit, it 
may never been trigger.

> 
> If there is a problem here, please construct a reproducer
> against this patch set and post it.
> 
> 
>> 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,
> 
> 
> --
> Chuck Lever
> 
>