Re: [PATCH] arm64: Fix 32-bit compatible userspace write size overflow error

[Jinjie Ruan]

On 2023/11/16 22:58, Mark Rutland wrote:
> On Thu, Nov 16, 2023 at 03:47:05PM +0800, Jinjie Ruan wrote:
>> For 32-bit compatible userspace program, write with size = -1 return not
>> -1 but unexpected other values, which is due to the __access_ok() check is
>> not right.
> 
> Can you please explain why you believe that is unexpected?
> 
> e.g. Is that documented somewhere? Do you see a real application depending on
> that somewhow?

I think access_ok() needs to ensure that the address is not out of
bounds, which guarantees that address access should not exceed the
32-bit boundary.

> 
>> The specified "addr + size" is greater than 32-bit limit and
>> should return -EFAULT, but TASK_SIZE_MAX still defined as UL(1) << VA_BITS
>> in U32 mode, which is much greater than "addr + size" and cannot catch the
>> overflow error.
> 
> The check against TASK_SIZE_MAX is not intended to catch 32-bit addr + size
> overflow; it's intended to check that uaccesses never touch kernel memory. The
> kernel's uaccess routines use 64-bit (or 65-bit) arithmetic, so these won't
> wrap and access memory at the start of the user address space.

Thank you! My understanding of TASK_SIZE_MAX is wrong.I seems that
"MAX_RW_COUNT" is designed to catch the 32-bit addr + size overflow.

> 
>> Fix above error by checking 32-bit limit if it is 32-bit compatible
>> userspace program.
>>
>> How to reproduce:
>>
>> The test program is as below:
>>
>> cat test.c
>> 	#include <unistd.h>
>> 	#include <fcntl.h>
>> 	#include <stdio.h>
>> 	#include <stdint.h>
>> 	#include <stdlib.h>
>> 	#include <assert.h>
>>
>> 	#define pinfo(fmt, args...) \
>> 	    fprintf(stderr, "[INFO][%s][%d][%s]:"fmt, \
>> 	    __FILE__,__LINE__,__func__,##args)
>>
>> 	#undef SIZE_MAX
>> 	#define SIZE_MAX -1
>>
>> 	int main()
>> 	{
>> 	    char wbuf[3] = { 'x', 'y', 'z' };
>> 	    char *path = "write.tmp";
>> 	    int ret;
>>
>> 	    int fd = open(path, O_RDWR | O_CREAT);
>> 	    if (fd<0)
>> 	    {
>> 	        pinfo("fd=%d\n", fd);
>> 	        exit(-1);
>> 	    }
>>
>> 	    assert(write(fd, wbuf, 3) == 3);
>>
>> 	    ret = write (fd, wbuf, SIZE_MAX);
>> 	    pinfo("ret=%d\n", ret);
>> 	    pinfo("size_max=%d\n",SIZE_MAX);
>> 	    assert(ret==-1);
>> 	    close(fd);
>> 	    pinfo("INFO: end\n");
>>
>> 	    return 0;
>> 	}
>>
>> aarch64-linux-gnu-gcc --static test.c -o test
>> arm-linux-gnueabi-gcc --static test.c -o test32
>>
>> Before applying this patch, userspace 32-bit program return 1112 if the
>> write size = -1 as below:
>> 	/root # ./test
>> 	[INFO][test.c][32][main]:ret=-1
>> 	[INFO][test.c][33][main]:size_max=-1
>> 	[INFO][test.c][36][main]:INFO: end
>> 	/root # ./test32
>> 	[INFO][test.c][32][main]:ret=1112
>> 	[INFO][test.c][33][main]:size_max=-1
>> 	test32: test.c:34: main: Assertion `ret==-1' failed.
>> 	Aborted
>>
>> After applying this patch, userspace 32-bit program return -1 if the write
>> size = -1 as expected as below:
>> 	/root # ./test
>> 	[INFO][test.c][32][main]:ret=-1
>> 	[INFO][test.c][33][main]:size_max=-1
>> 	[INFO][test.c][36][main]:INFO: end
>> 	/root # ./test32
>> 	[INFO][test.c][32][main]:ret=-1
>> 	[INFO][test.c][33][main]:size_max=-1
>> 	[INFO][test.c][36][main]:INFO: end
>>
>> Fixes: 967747bbc084 ("uaccess: remove CONFIG_SET_FS")
> 
> As above, this is *not* a fix. This is the intended behaviour.
> 
> AFAICT, the behaviour didn't change on arm64 in that commit either; we were
> unconditionally using TASK_SIZE_MAX many commits earlier, e.g. in commit:
> 
>   3d2403fd10a1dbb3 ("arm64: uaccess: remove set_fs()")
> 
> ... so the fixes tag is bogus on both fronts.

Thank you!

> 
>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/processor.h | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
>> index e5bc54522e71..6a087d58a90a 100644
>> --- a/arch/arm64/include/asm/processor.h
>> +++ b/arch/arm64/include/asm/processor.h
>> @@ -52,7 +52,12 @@
>>  
>>  #define DEFAULT_MAP_WINDOW_64	(UL(1) << VA_BITS_MIN)
>>  #define TASK_SIZE_64		(UL(1) << vabits_actual)
>> +#ifdef CONFIG_COMPAT
>> +#define TASK_SIZE_MAX		(test_thread_flag(TIF_32BIT) ? \
>> +				UL(0x100000000) : (UL(1) << VA_BITS))
>> +#else
>>  #define TASK_SIZE_MAX		(UL(1) << VA_BITS)
>> +#endif
> 
> This isn't even the same as on 32-bit. On 32-bit arm, the task size split can
> be 1G/3G, 2G/2G, or 3G/1G depending on configuration, and 4G/4G isn't currently
> an option.
> 
> I don't believe that userspace is actually dependent upon this for functional
> reasons, and I don't believe that there's a security issue here. Even if
> access_ok() allows addr+size to go past 4G, the kernel address calculations are
> 64-bit and won't wrap.
> 
> For all the reasons above, I don't beleive this is correct nor do I believe
> this is necesssary. Given that, NAK to this patch.
> 
> Thanks,
> Mark.
> 
>>  
>>  #ifdef CONFIG_COMPAT
>>  #if defined(CONFIG_ARM64_64K_PAGES) && defined(CONFIG_KUSER_HELPERS)
>> -- 
>> 2.34.1
>>
>