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

Jinjie Ruan
2023-11-18T06:09:07Z

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@huawei.com>
>> ---
>> 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