linux-kernel - Re: [PATCH 3/3] floppy: suppress UBSAN warning in setup_rw

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Date:   Wed, 22 Apr 2020 09:17:56 +0200
From:   Willy Tarreau <w@....eu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Denis Efremov <efremov@...ux.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] floppy: suppress UBSAN warning in setup_rw_floppy()

On Wed, Apr 22, 2020 at 12:09:21AM -0700, Christoph Hellwig wrote:
> On Tue, Apr 21, 2020 at 03:57:22PM +0300, Denis Efremov wrote:
> > UBSAN: array-index-out-of-bounds in drivers/block/floppy.c:1521:45
> > index 16 is out of range for type 'unsigned char [16]'
> > Call Trace:
> > ...
> >  setup_rw_floppy+0x5c3/0x7f0
> >  floppy_ready+0x2be/0x13b0
> >  process_one_work+0x2c1/0x5d0
> >  worker_thread+0x56/0x5e0
> >  kthread+0x122/0x170
> >  ret_from_fork+0x35/0x40
> > 
> > >From include/uapi/linux/fd.h:
> > struct floppy_raw_cmd {
> > 	...
> > 	unsigned char cmd_count;
> > 	unsigned char cmd[16];
> > 	unsigned char reply_count;
> > 	unsigned char reply[16];
> > 	...
> > }
> > 
> > This out-of-bounds access is intentional. The command in struct
> > floppy_raw_cmd may take up the space initially intended for the reply
> > and the reply count. It is needed for long 82078 commands such as
> > RESTORE, which takes 17 command bytes. Initial cmd size is not enough
> > and since struct setup_rw_floppy is a part of uapi we check that
> > cmd_count is in [0:16+1+16] in raw_cmd_copyin().
> > 
> > The patch replaces array subscript with pointer arithetic to suppress
> > UBSAN warning.
> 
> Urghh.  I think the better way would be to use a union that creates
> a larger cmd field, or something like:
> 
> struct floppy_raw_cmd {
> 	...
> 	u8 buf[34];
> 
> #define BUF_CMD_COUNT	0
> #define BUF_CMD		1
> #define BUF_REPLY_COUNT	17
> #define BUF_REPLY	18
> 
> and use addressing based on that.

But isn't it a problem if struct floppy_raw_cmd is exposed to uapi ?
That said I remember a discussion with Linus who said that most if not
all of the floppy parts leaking to uapi were more of a side effect of
the include files reordering than a deliberate decision to expose it.
So maybe that could remain the best solution indeed.

I must say I don't feel very comfortable either with replacing p[i]
with *(p+i) given that they are all supposed to be interchangeable and
equivalent (as well as i[p] as strange as it can look). So we're not
really protected against a later mechanical change or cleanup that
reintroduces it :-/

Willy