Re: usercopy_warn in __copy_to_user
From: Kees Cook
Date: Wed Dec 19 2018 - 16:51:36 EST
On Wed, Dec 19, 2018 at 11:51 AM <rishabhb@xxxxxxxxxxxxxx> 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:
> struct sigset_t *set;
> __copy_to_user(&sf->uc.uc_sigmask, set, sizeof(*set))
> __copy_to_user(void __user *to, const void *from, unsigned long n)
> kasan_check_read(from, n);
> check_object_size(from, n, true);
> return raw_copy_to_user(to, from, n);
> 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
> But we are seeing that its returning the value as false. Because of
> 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:
> 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.
> Thereâs 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
> written in a different to avoid dependency on compiler?