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

From: Tyler Hicks <[email-masked]>
Date: Sun, 10 Jul 2022 01:25:57 -0500

On 2022-07-09 15:00:05, Tyler Hicks wrote:
> 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
> fid struct shortly after the fid is created in p9_fid_create(). Other
> operations that follow a call to p9_fid_create(), such as an XATTRWALK,
> don't include an iounit value in the reply message from the server. In
> the latter case, the iounit field remained uninitialized. 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. However, there are
> likely other non-xattr implications of this bug that could cause
> inefficient communication between the client and server.
> 
> Cc: [email-masked]
> Signed-off-by: Tyler Hicks <[email-masked]>
> ---
> 
> Note that I haven't had a chance to identify when this bug was
> introduced so I don't yet have a proper Fixes tag. The history looked a
> little tricky to me but I'll have another look in the coming days. We
> started hitting this bug after trying to move from linux-5.10.y to
> linux-5.15.y but I didn't see any obvious changes between those two
> series. I'm not confident of this theory but perhaps the fid refcounting
> changes impacted the fid allocation patterns enough to uncover the
> latent bug?

From reading the source, I believe that this first showed up in commit
ebf46264a004 ("fs/9p: Add support user. xattr") which landed in v2.6.36.
Before that commit, p9_client_read(), p9_client_write(), and
p9_client_readdir() were always passed a fid that came from a file's
private_data and went through the open/create functions that initialized
iounit. That commit was the first that passed a fid directly from
p9_fid_create() to p9_client_read().

Tyler