linux-kernel - Re: [PATCH 11/20] amifb: get rid of pointless access

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Message-ID: <f6fcfa46-6271-45ea-37c2-62bcf0a607cb@samsung.com>
Date:   Thu, 14 May 2020 16:25:35 +0200
From:   Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 11/20] amifb: get rid of pointless access_ok() calls


On 5/14/20 4:07 PM, Al Viro wrote:
> On Thu, May 14, 2020 at 03:45:09PM +0200, Bartlomiej Zolnierkiewicz wrote:
>>
>> Hi Al,
>>
>> On 5/10/20 1:45 AM, Al Viro wrote:
>>> From: Al Viro <viro@...iv.linux.org.uk>
>>>
>>> addresses passed only to get_user() and put_user()
>>
>> This driver lacks checks for {get,put}_user() return values so it will
>> now return 0 ("success") even if {get,put}_user() fails.
>>
>> Am I missing something?
> 
> "now" is interesting, considering
> /* We let the MMU do all checking */
> static inline int access_ok(const void __user *addr,
>                             unsigned long size)
> {
>         return 1;
> }
> in arch/m68k/include/asm/uaccess_mm.h
> 
> Again, access_ok() is *NOT* about checking if memory is readable/writable/there
> in the first place.  All it does is a static check that address is in
> "userland" range - on architectures that have kernel and userland sharing the
> address space.  On architectures where we have separate ASI or equivalents
> thereof for kernel and for userland the fscker is always true.
> 
> If MMU will prevent access to kernel memory by uaccess insns for given address
> range, access_ok() is fine with it.  It does not do anything else.
> 
> And yes, get_user()/put_user() callers should handle the fact that those can
> fail.  Which they bloody well can _after_ _success_ of access_ok().  And
> without any races whatsoever.
> 
> IOW, the lack of such checks is a bug, but it's quite independent from the
> bogus access_ok() call.  On any architecture.  mmap() something, munmap()
> it and pass the address where it used to be to that ioctl().  Failing
> get_user()/put_user() is guaranteed, so's succeeding access_ok().
> 
> And that code is built only on amiga, so access_ok() always succeeds, anyway.

Thank you for in-detail explanations, for this patch:

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>

Could you also please take care of adding missing checks for {get,put}_user()
failures later?

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics