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

Mark Rutland
2023-11-16T14:59:35Z

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?

> 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.

> 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.

> 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