Re: [PATCH -next] uaccess: fix __access_ok limit setup in compat mode

[chenjiahao (C)]

在 2022/3/18 15:44, Arnd Bergmann 写道:
> On Fri, Mar 18, 2022 at 8:11 AM Chen Jiahao <chenjiahao16@xxxxxxxxxx> wrote:
> > In __access_ok, TASK_SIZE_MAX is used to check if a memory access
> > is in user address space, but some cases may get omitted in compat
> > mode.
> >
> > For example, a 32-bit testcase calling pread64(fd, buf, -1, 1)
> > and running in x86-64 kernel, the obviously illegal size "-1" will
> > get ignored by __access_ok. Since from the kernel point of view,
> > 32-bit userspace 0xffffffff is within the limit of 64-bit
> > TASK_SIZE_MAX.
> >
> > Replacing the limit TASK_SIZE_MAX with TASK_SIZE in __access_ok
> > will fix the problem above.
> I don't see what problem this fixes, the choice of TASK_SIZE_MAX in
> __access_ok() is intentional, as this means we can use a compile-time
> constant as the limit, which produces better code.
>
> Any user pointer between COMPAT_TASK_SIZE and TASK_SIZE_MAX is
> not accessible by a user process but will not let user space access
> any kernel data either, which is the point of the check.
>
> In your example of using '-1' as the pointer, access_ok() returns true,
> so the kernel can go on to perform an unchecked __get_user() on
> __put_user() on 0xffffffffull, which causes page fault that is intercepted
> by the ex_table fixup.
>
> This should not result in any user visible difference, in both cases
> user process will see a -EFAULT return code from its system call.
> Are you able to come up with a test case that shows an observable
> difference in behavior?
>
>        Arnd
>
> .

Actually, this patch do comes from a testcase failure, the code is 
pasted below:

#define TMPFILE "__1234567890"
#define BUF_SIZE    1024

int main()
{
    char buf[BUF_SIZE] = {0};
    int fd;
    int ret;
    int err;

    fd = open(TMPFILE, O_CREAT | O_RDWR);
    if(-1 == fd)
    {
        perror("open");
        return 1;
    }

    ret = pread64(fd, buf, -1, 1);
    if((-1 == ret) && (EFAULT == errno))
    {
        close(fd);
        unlink(TMPFILE);
        printf("PASS\n");
        return 0;
    }
    err = errno;
    perror("pread64");
    printf("err = %d\n", err);
    close(fd);
    unlink(TMPFILE);
    printf("FAIL\n");

    return 1;
 }

The expected result is:

PASS

but the result of 32-bit testcase running in x86-64 kernel with compat 
mode is:

pread64: Success
err = 0
FAIL


In my explanation, pread64 is called with count '0xffffffffull' and 
offset '1', which might still not trigger

page fault in 64-bit kernel.


This patch uses TASK_SIZE as the addr_limit to performance a stricter 
address check and intercepts

the illegal pointer address from 32-bit userspace at a very early time. 
Which is roughly the same

address limit check as __access_ok in arch/ia64.


This is why this fixes my testcase failure above, or have I missed 
anything else?


Jiahao