[tip:x86/asm] x86/asm: Annotate copy_user_handle_tail() pointers with __user

[tip-bot for Ben Dooks <tipbot@zytor.com>]

Commit-ID:  b5dbb6799e3e5b8ebdce33b52b2d4ec9c66e15fe
Gitweb:     https://git.kernel.org/tip/b5dbb6799e3e5b8ebdce33b52b2d4ec9c66e15fe
Author:     Ben Dooks <ben.dooks@codethink.co.uk>
AuthorDate: Sat, 30 Mar 2019 11:56:24 +0000
Committer:  Borislav Petkov <bp@suse.de>
CommitDate: Tue, 2 Apr 2019 10:38:51 +0200

x86/asm: Annotate copy_user_handle_tail() pointers with __user

copy_user_handle_tail() clearly uses both @from and @to as pointers to
user-space memory.

Currently, it triggers sparse warning on using the calls to get and put
to user-space which can be fixed easily by changing the call to take
__user annotated pointers:

  arch/x86/lib/usercopy_64.c:68:21: warning: incorrect type in argument 1 (different address spaces)
  arch/x86/lib/usercopy_64.c:68:21:    expected void const volatile [noderef] <asn:1>*<noident>
  arch/x86/lib/usercopy_64.c:68:21:    got char *
  arch/x86/lib/usercopy_64.c:70:21: warning: incorrect type in argument 1 (different address spaces)
  arch/x86/lib/usercopy_64.c:70:21:    expected void const volatile [noderef] <asn:1>*<noident>
  arch/x86/lib/usercopy_64.c:70:21:    got char *to

Linus further explains the reasoning why it was done this way:

On Thu, Mar 28, 2019 at 12:24 AM Borislav Petkov <bp@alien8.de> wrote:
> Well, but copy_user_generic() (which ends up calling the
> copy_user_handle_tail() eventually) casts those __user pointers to
> (__force void *). Converting them back to __user looks strange to me.
>
> Linus?

Well, it does that because the x86 version of copy_user_generic() can
work in either direction, so it works when either the source or
destination (or both) are user pointers, but they don't _have_ to be.

So the "userness" of a pointer in that context is a bit ambiguous, and
so we've picked the pointers to be just plain "void *".

That said, arguably we should have gone the other way and just made
them both "__user" pointers, and do the cast the other way around.

But there's no absolutely right answer here, and nobody should ever
use copy_user_generic() directly (ie it is very much meant to be only
used as a internal helper for the cases that get the pointer
annotations right).

 [ bp: massage. ]

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: mingo@redhat.co
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190330115624.4000-1-ben.dooks@codethink.co.uk
---
 arch/x86/include/asm/uaccess_64.h | 2 +-
 arch/x86/lib/usercopy_64.c        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index a9d637bc301d..cbca2cb28939 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -208,7 +208,7 @@ __copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
 }
 
 unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len);
+copy_user_handle_tail(char __user *to, char __user *from, unsigned len);
 
 unsigned long
 mcsafe_handle_tail(char *to, char *from, unsigned len);
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index ee42bb0cbeb3..aa180424e77a 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -60,7 +60,7 @@ EXPORT_SYMBOL(clear_user);
  * it is not necessary to optimize tail handling.
  */
 __visible unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len)
+copy_user_handle_tail(char __user *to, char __user *from, unsigned len)
 {
 	for (; len; --len, to++) {
 		char c;