Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] objtool: Documentation: document UACCESS warnings
To:     Nick Desaulniers <ndesaulniers@google.com>, jpoimboe@redhat.com,
        peterz@infradead.org
Cc:     Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Wolfram Sang <wsa@the-dreams.de>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Raphael Gault <raphael.gault@arm.com>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
References: <20200323212538.GN2452@worktop.programming.kicks-ass.net>
 <20200324001321.39562-1-ndesaulniers@google.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <f55972ee-8e4e-844f-1678-cde1cdcc1fa9@infradead.org>
Date:   Mon, 23 Mar 2020 18:12:26 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.6.0
MIME-Version: 1.0
In-Reply-To: <20200324001321.39562-1-ndesaulniers@google.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 3/23/20 5:13 PM, Nick Desaulniers wrote:
> Compiling with Clang and CONFIG_KASAN=y was exposing a few warnings:
>   call to memset() with UACCESS enabled
> 
> Document how to fix these for future travelers.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/876
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  .../Documentation/stack-validation.txt        | 20 +++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
> index de094670050b..156fee13ba02 100644
> --- a/tools/objtool/Documentation/stack-validation.txt
> +++ b/tools/objtool/Documentation/stack-validation.txt
> @@ -289,6 +289,26 @@ they mean, and suggestions for how to fix them.
>        might be corrupt due to a gcc bug.  For more details, see:
>        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
>  
> +9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled
> +
> +   This means that an unexpected call to a non-whitelisted function exists
> +   outside of arch-specific guards.
> +   X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end()
> +   ARM: PAN: uaccess_enable()/uaccess_enable()
> +
> +   These functions should called to denote a minimal critical section around

                      should be called

> +   access to __user variables. See also: https://lwn.net/Articles/517475/
> +
> +   The intention of the warning is to prevent calls to funcB() from eventually
> +   calling schedule(), potentially leaking the AC flags state, and not
> +   restoring them correctly.
> +
> +   To fix, either:
> +   1) add the correct guards before and after calls to low level functions like
> +      __get_user_size()/__put_user_size().
> +   2) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
> +      funcB obviously does not call schedule().
> +
>  
>  If the error doesn't seem to make sense, it could be a bug in objtool.
>  Feel free to ask the objtool maintainer for help.
> 


-- 
~Randy