Re: [PATCH 5.10 5.4 4.19] net/9p: Initialize the iounit field during fid creation

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Aug 15, 2022 at 08:49:25PM -0500, Tyler Hicks wrote:
> [ Upstream commit aa7aeee169480e98cf41d83c01290a37e569be6d ]
> 
> Ensure that the fid's iounit field is set to zero when a new fid is
> created. Certain 9P operations, such as OPEN and CREATE, allow the
> server to reply with an iounit size which the client code assigns to the
> p9_fid struct shortly after the fid is created by p9_fid_create(). On
> the other hand, an XATTRWALK operation doesn't allow for the server to
> specify an iounit value. The iounit field of the newly allocated p9_fid
> struct remained uninitialized in that case. Depending on allocation
> patterns, the iounit value could have been something reasonable that was
> carried over from previously freed fids or, in the worst case, could
> have been arbitrary values from non-fid related usages of the memory
> location.
> 
> The bug was detected in the Windows Subsystem for Linux 2 (WSL2) kernel
> after the uninitialized iounit field resulted in the typical sequence of
> two getxattr(2) syscalls, one to get the size of an xattr and another
> after allocating a sufficiently sized buffer to fit the xattr value, to
> hit an unexpected ERANGE error in the second call to getxattr(2). An
> uninitialized iounit field would sometimes force rsize to be smaller
> than the xattr value size in p9_client_read_once() and the 9P server in
> WSL refused to chunk up the READ on the attr_fid and, instead, returned
> ERANGE to the client. The virtfs server in QEMU seems happy to chunk up
> the READ and this problem goes undetected there.
> 
> Link: https://lkml.kernel.org/r/20220710141402.803295-1-tyhicks@linux.microsoft.com
> Fixes: ebf46264a004 ("fs/9p: Add support user. xattr")
> Cc: stable@vger.kernel.org
> Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> [tyhicks: Adjusted context due to:
>  - Lack of fid refcounting introduced in v5.11 commit 6636b6dcc3db ("9p:
>    add refcount to p9_fid struct")
>  - Difference in how buffer sizes are specified v5.16 commit
>    6e195b0f7c8e ("9p: fix a bunch of checkpatch warnings")]
> Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> ---
> 
> Tested on top of v5.10.136. Verified to apply to v5.4 and v4.19 and the
> resulting code was read for correctness.
> 

All now queued up, thanks.

greg k-h