linux-kernel - Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check

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Date:   Wed, 15 Jan 2020 23:00:17 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
CC:     linux-arch <linux-arch@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Ingo Molnar <mingo@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space
 pointer range check

On 1/15/20 6:42 AM, Andrey Konovalov wrote:
>> -       max_addr = user_addr_max();
>> -       src_addr = (unsigned long)untagged_addr(src);
>
> If you end up changing this code, you need to keep the untagged_addr()
> logic, otherwise this breaks arm64 tagged address ABI [1].

It is moot point now, but fwiw untagged_addr() would not have been needed anymore
as it was only needed to compute the pointer difference which my patch got rid of.

> 
> [1] https://www.kernel.org/doc/html/latest/arm64/tagged-address-abi.html
> 
>> -       if (likely(src_addr < max_addr)) {
>> -               unsigned long max = max_addr - src_addr;
>> +       kasan_check_write(dst, count);
>> +       check_object_size(dst, count, false);
>> +       if (user_access_begin(src, count)) {