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arm64 test_user_copy crash on copy_from_user(uptr, kptr, size)

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  arm64 test_user_copy crash on copy_from_user(uptr, kptr, size) Arnd Bergmann <arnd@arndb.de> - 2017-05-26 17:30 +0200
    Re: arm64 test_user_copy crash on copy_from_user(uptr, kptr, size) Kees Cook <keescook@chromium.org> - 2017-05-26 17:50 +0200
      Re: arm64 test_user_copy crash on copy_from_user(uptr, kptr, size) Arnd Bergmann <arnd@arndb.de> - 2017-05-27 03:30 +0200
    Re: arm64 test_user_copy crash on copy_from_user(uptr, kptr, size) Arnd Bergmann <arnd@arndb.de> - 2017-05-27 03:30 +0200

#1651428 — arm64 test_user_copy crash on copy_from_user(uptr, kptr, size)

A kselftest run on arm64 on an older 4.4.y stable kernel ran into an
unexpectedly trapping user space access:

[ 1277.857738] Internal error: Accessing user space memory outside
uaccess.h routines: 96000045 [#1] PREEMPT SMP

Apparently the same thing happens on x86 as well, and it still happens on
the latest kernels, see https://bugs.linaro.org/show_bug.cgi?id=3011

The problem here is this test

       ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
                                   PAGE_SIZE),
                   "illegal reversed copy_from_user passed");

where the destination kernel pointer intentionally points into user space
memory, while copy_from_user checks the second argument for being
a valid user space, which it also is not.:

static inline unsigned long __must_check copy_from_user(void *to,
const void __user *from, unsigned long n)
{
        unsigned long res = n;
        kasan_check_write(to, n);

        if (access_ok(VERIFY_READ, from, n)) {
                check_object_size(to, n, false);
                res = __arch_copy_from_user(to, from, n);
        }
        if (unlikely(res))
                memset(to + (n - res), 0, res);
        return res;
}

The memset here will now try to clear user space data, and the
architecture notices that the fault did not come from a proper
uaccess function.

I think this will only happen when CONFIG_ARM64_PAN,
 X86_SMAP or an equivalent feature on another architecture is
enabled, otherwise we just do the access anyway. I don't have
a good idea for avoiding the problem though, other than
removing the specific test that causes it.

       Arnd

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#1651442

On Fri, May 26, 2017 at 8:24 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> A kselftest run on arm64 on an older 4.4.y stable kernel ran into an
> unexpectedly trapping user space access:
>
> [ 1277.857738] Internal error: Accessing user space memory outside
> uaccess.h routines: 96000045 [#1] PREEMPT SMP
>
> Apparently the same thing happens on x86 as well, and it still happens on
> the latest kernels, see https://bugs.linaro.org/show_bug.cgi?id=3011
>
> The problem here is this test
>
>        ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
>                                    PAGE_SIZE),
>                    "illegal reversed copy_from_user passed");

Hi! Yes, I removed that test from the current code:

#if 0
        /*
         * When running with SMAP/PAN/etc, this will Oops the kernel
         * due to the zeroing of userspace memory on failure. This needs
         * to be tested in LKDTM instead, since this test module does not
         * expect to explode.
         */
        ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
                                    PAGE_SIZE),
                    "illegal reversed copy_from_user passed");
#endif

We can send a patch to -stable?

-Kees

>
> where the destination kernel pointer intentionally points into user space
> memory, while copy_from_user checks the second argument for being
> a valid user space, which it also is not.:
>
> static inline unsigned long __must_check copy_from_user(void *to,
> const void __user *from, unsigned long n)
> {
>         unsigned long res = n;
>         kasan_check_write(to, n);
>
>         if (access_ok(VERIFY_READ, from, n)) {
>                 check_object_size(to, n, false);
>                 res = __arch_copy_from_user(to, from, n);
>         }
>         if (unlikely(res))
>                 memset(to + (n - res), 0, res);
>         return res;
> }
>
> The memset here will now try to clear user space data, and the
> architecture notices that the fault did not come from a proper
> uaccess function.
>
> I think this will only happen when CONFIG_ARM64_PAN,
>  X86_SMAP or an equivalent feature on another architecture is
> enabled, otherwise we just do the access anyway. I don't have
> a good idea for avoiding the problem though, other than
> removing the specific test that causes it.
>
>        Arnd



-- 
Kees Cook
Pixel Security

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#1651621

On Fri, May 26, 2017 at 5:41 PM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, May 26, 2017 at 8:24 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>> A kselftest run on arm64 on an older 4.4.y stable kernel ran into an
>> unexpectedly trapping user space access:
>>
>> [ 1277.857738] Internal error: Accessing user space memory outside
>> uaccess.h routines: 96000045 [#1] PREEMPT SMP
>>
>> Apparently the same thing happens on x86 as well, and it still happens on
>> the latest kernels, see https://bugs.linaro.org/show_bug.cgi?id=3011
>>
>> The problem here is this test
>>
>>        ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
>>                                    PAGE_SIZE),
>>                    "illegal reversed copy_from_user passed");
>
> Hi! Yes, I removed that test from the current code:
>
> #if 0
>         /*
>          * When running with SMAP/PAN/etc, this will Oops the kernel
>          * due to the zeroing of userspace memory on failure. This needs
>          * to be tested in LKDTM instead, since this test module does not
>          * expect to explode.
>          */
>         ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
>                                     PAGE_SIZE),
>                     "illegal reversed copy_from_user passed");
> #endif
>
> We can send a patch to -stable?

Ok, good. I've prepared a backport since it doesn't apply cleanly otherwise.

        Arnd

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#1651679

On Fri, May 26, 2017 at 5:40 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Fri, May 26, 2017 at 05:24:47PM +0200, Arnd Bergmann wrote:

> AFAICT, that test was disabled in commit:
>
>   f5f893c57e37ca73 ("usercopy: Adjust tests to deal with SMAP/PAN")
>
> ... or have I misunderstood?

I think you are right, my mistake for not checking the latest kernels
and only relying on the bug report that said it also happens on "mainline".

     Arnd

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