Subject: Re: [PATCH v2] net/9p: Initialize the iounit field during fid creation

From: Dominique Martinet <[email-masked]>
Date: Sat, 16 Jul 2022 07:23:09 +0900

Tyler Hicks wrote on Sun, Jul 10, 2022 at 09:14:02AM -0500:
> 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.
> 
> Fixes: ebf46264a004 ("fs/9p: Add support user. xattr")
> Cc: [email-masked]
> Signed-off-by: Tyler Hicks <[email-masked]>

Thanks for the v2, looks good to me and tested quickly so I've queued it
up.
(and thanks all the fixes lately and for the reminder, too many patches
lately I thought I had already taken it... Feel free to send 'pings' on
the list)

Since the next merge window is close (probably starts next week-ish) I
won't bother with a separate PR for 5.19; it's been 12 years it can wait
another week.

--
Dominique