Re: usercopy_warn in __copy_to_user

On Wed, Dec 19, 2018 at 11:51 AM [off-list ref] wrote:

In the 4.19 kernel, we are seeing a USERCOPY_WARN in __copy_to_user
during bootup.
The code-flow is something like this:

(arch/arm64/kernel/signal.c)
struct sigset_t *set;
__copy_to_user(&sf->uc.uc_sigmask, set, sizeof(*set))

(include/linux/uaccess.h)
__copy_to_user(void __user *to, const void *from, unsigned long n)
{
                 might_fault();
                 kasan_check_read(from, n);
                 check_object_size(from, n, true);
                 return raw_copy_to_user(to, from, n);
}

(include/linux/thread_info.h)
static __always_inline void check_object_size(const void *ptr,
                                  unsigned long n, bool to_user)
{
                 if (!__builtin_constant_p(n))
                                 __check_object_size(ptr, n, to_user);
}

Since sizeof(*set) is constant, __builtin_constant_p(n) should return
true.
But we are seeing that its returning the value as false. Because of
which
the code goes on to __check_object_size and generates a USERCOPY_WARN
("usercopy: WARN() on slab cache usercopy
region violations").

We are using LLVM clang version 6.0 to compile the kernel and not gcc.

This is fixed in the latest LLVM and Clang:

https://github.com/ClangBuiltLinux/linux/issues/7

-Kees

In clang, __builtin_constant_p is evaluated immediately, before inlining
or other optimizations run, gcc evaluates it later.
We believe that maybe causing __builtin_constant_p(n) to return false.
There’s upstream work to change LLVM, so __builtin_constant_p works more
like gcc when optimization is enabled, but its still in progress.


For this scenario is there a way to avoid the warning? Should the code
be
written in a different to avoid dependency on compiler?

Thanks,
Rishabh

-- 
Kees Cook

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