linux-kernel - Re: [drivers/scsi] Question about `st

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Message-Id: <A452650A-330D-4591-B687-3654DE6CF40F@kolumbus.fi>
Date: Tue, 12 Mar 2024 21:18:26 +0200
From: "Kai Mäkisara (Kolumbus)" <kai.makisara@...umbus.fi>
To: Chenyuan Yang <chenyuan0y@...il.com>
Cc: "James E . J . Bottomley" <jejb@...ux.ibm.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 linux-scsi@...r.kernel.org,
 LKML <linux-kernel@...r.kernel.org>,
 Zijie Zhao <zzjas98@...il.com>
Subject: Re: [drivers/scsi] Question about `st_setup`

On 12. Mar 2024, at 16.43, Chenyuan Yang <chenyuan0y@...il.com> wrote:
> 
> Dear Linux Developers for SCSI Driver,
> 
> We are curious about the functionality of `st_setup`
> (https://elixir.bootlin.com/linux/latest/source/drivers/scsi/st.c#L4102).
> 
> ```
> static int __init st_setup(char *str)
> {
>  int i, len, ints[5];
>  char *stp;
> 
>  stp = get_options(str, ARRAY_SIZE(ints), ints);
> 
>  if (ints[0] > 0) {
>    for (i = 0; i < ints[0] && i < ARRAY_SIZE(parms); i++)
>    if (parms[i].val)
>      *parms[i].val = ints[i + 1];
>  }
>  ...
> }
> ```
> 
> For this function, we are trying to understand how it works but not
> sure whether it would be an out-of-bound read.
> 
> The length of both `ints` and `parms` is 5 (the latterdefined at
> https://elixir.bootlin.com/linux/latest/source/drivers/scsi/st.c#L125).
> Thus, when `ints[0]` is 5, we could assign `ints[5]`
> (out-of-bound-read) to `parms[4].val`. Based on our understanding of
> the `get_options` function
> (https://elixir.bootlin.com/linux/latest/source/lib/cmdline.c#L107),
> it could be possible that `ints[0] == 5`, where the first element of
> `ints` indicates the number of parsed options. Hence, it is possible
> to do
> a out-of-bound read once `debug_flag` is enabled (to pass `if
> (parms[i].val)`).
> 
I think your analysis is correct and there is a bug.

> Please correct us if we miss some key prerequisites for this function
> or the data structure.
> Thanks in advance!
> 
> Based on our understanding, the possible fix could be
> ```
> int i, len, ints[6];
> ```
> which allocates `len(parms) + 1` for `ints`.

Yes, this would fix the bug. However, it might be better to define
size of ints[] as ARRAY_SIZE(parms)+1 to connect the size directly
to the definition of parameters.

(The bug applies to the case where st is compiled into the kernel
and a list of integers is used to define the options. Not a common
case, but a bug should be fixed.)

Thanks,
Kai