linux-kernel - Re: [PATCH] selftests/lam: Test get

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Message-ID: <36yf4cvahtsavqaa5hzmfg6udde6geqw3cpr3hr45rx44qnfa2@53p2wvysdusg>
Date: Wed, 30 Oct 2024 15:08:07 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Shuah Khan
	<shuah@...nel.org>, <linux-kernel@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH] selftests/lam: Test get_user() LAM pointer handling

On 2024-10-30 at 14:31:51 +0200, Kirill A. Shutemov wrote:
>On Tue, Oct 29, 2024 at 03:14:20PM +0100, Maciej Wieczor-Retman wrote:
>> Recent change in how get_user() handles pointers [1] has a specific case
>> for LAM. It assigns a different bitmask that's later used to check
>> whether a pointer comes from userland in get_user().
>> 
>> While currently commented out (until LASS [2] is merged into the kernel)
>> it's worth making changes to the LAM selftest ahead of time.
>> 
>> Add test case to LAM that utilizes a ioctl (FIOASYNC) syscall which uses
>> get_user() in its implementation. Execute the syscall with differently
>> tagged pointers to verify that valid user pointers are passing through
>> and invalid kernel/non-canonical pointers are not.
>> 
>> Code was tested on a Sierra Forest Xeon machine that's LAM capable. The
>> test was ran without issues with both the LAM lines from [1] untouched
>> and commented out. The test was also ran without issues with LAM_SUP
>> both enabled and disabled.
>> 
>> [1] https://lore.kernel.org/all/20241024013214.129639-1-torvalds@linux-foundation.org/
>> [2] https://lore.kernel.org/all/20240710160655.3402786-1-alexander.shishkin@linux.intel.com/
>> 
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>> ---
>>  tools/testing/selftests/x86/lam.c | 85 +++++++++++++++++++++++++++++++
>>  1 file changed, 85 insertions(+)
>> 
>> diff --git a/tools/testing/selftests/x86/lam.c b/tools/testing/selftests/x86/lam.c
>> index 0ea4f6813930..3c53d4b7aa61 100644
>> --- a/tools/testing/selftests/x86/lam.c
>> +++ b/tools/testing/selftests/x86/lam.c
>> @@ -4,6 +4,7 @@
>>  #include <stdlib.h>
>>  #include <string.h>
>>  #include <sys/syscall.h>
>> +#include <sys/ioctl.h>
>>  #include <time.h>
>>  #include <signal.h>
>>  #include <setjmp.h>
>> @@ -43,10 +44,19 @@
>>  #define FUNC_INHERITE           0x20
>>  #define FUNC_PASID              0x40
>>  
>> +/* get_user() pointer test cases */
>> +#define GET_USER_USER           0
>> +#define GET_USER_KERNEL_TOP     1
>> +#define GET_USER_KERNEL_BOT     2
>> +#define GET_USER_KERNEL         3
>> +
>>  #define TEST_MASK               0x7f
>> +#define L5_SIGN_EXT_MASK        (0xFFUL << 56)
>> +#define L4_SIGN_EXT_MASK        (0x1FFFFUL << 47)
>>  
>>  #define LOW_ADDR                (0x1UL << 30)
>>  #define HIGH_ADDR               (0x3UL << 48)
>> +#define L5_ADDR                 (0x1UL << 48)
>>  
>>  #define MALLOC_LEN              32
>>  
>> @@ -370,6 +380,54 @@ static int handle_syscall(struct testcases *test)
>>  	return ret;
>>  }
>>  
>> +static int get_user_syscall(struct testcases *test)
>> +{
>> +	int ret = 0;
>> +	int ptr_value = 0;
>> +	void *ptr = &ptr_value;
>> +	int fd;
>> +
>> +	uint64_t bitmask = ((uint64_t)ptr & L5_ADDR) ? L5_SIGN_EXT_MASK :
>> +						       L4_SIGN_EXT_MASK;
>
>Emm. Do you expect stack to be above at the very top of address space on
>5-level paging machines? It is not true. We don't allocate any memory
>above 46-bit unless asked explicitly.

Right, I'm not sure why I thought that would work here.

>See tools/testing/selftests/mm/va_high_addr_switch.c

Thanks for the tip, I'll use mmap/munmap to determine the enabled pagetable level.

>
>-- 
>  Kiryl Shutsemau / Kirill A. Shutemov

-- 
Kind regards
Maciej Wieczór-Retman