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

[Chuck Lever III]

> On Nov 11, 2024, at 10:20 AM, yangerkun <yangerkun at huaweicloud.com> wrote:
> 
> 
> 
> 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.

Thanks for confirming.

In that case, the preferred way to handle it is to fix
the issue in upstream, and then backport that fix to LTS.
Dependence on 64-bit offsets to avoid a failure case
should be considered a workaround, not a real fix, IMHO.

Do you have a few moments to address it, or if not I
will see to it.

I think reducing the xa_limit in simple_offset_add() to,
say, 2..16 would make the reproducer fire almost
immediately.


>> 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


--
Chuck Lever