x86: clean up/fix 'copy_in_user()' tail zeroing

The rule for 'copy_from_user()' is that it zeroes the remaining kernel
buffer even when the copy fails halfway, just to make sure that we don't
leave uninitialized kernel memory around.  Because even if we check for
errors, some kernel buffers stay around after thge copy (think page
cache).

However, the x86-64 logic for user copies uses a copy_user_generic()
function for all the cases, that set the "zerorest" flag for any fault
on the source buffer.  Which meant that it didn't just try to clear the
kernel buffer after a failure in copy_from_user(), it also tried to
clear the destination user buffer for the "copy_in_user()" case.

Not only is that pointless, it also means that the clearing code has to
worry about the tail clearing taking page faults for the user buffer
case.  Which is just stupid, since that case shouldn't happen in the
first place.

Get rid of the whole "zerorest" thing entirely, and instead just check
if the destination is in kernel space or not.  And then just use
memset() to clear the tail of the kernel buffer if necessary.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 12a26b979bf1..f2f9b39b274a 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -231,6 +231,6 @@ __copy_from_user_inatomic_nocache(void *dst, const void __user *src,
 }
 
 unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len, unsigned zerorest);
+copy_user_handle_tail(char *to, char *from, unsigned len);
 
 #endif /* _ASM_X86_UACCESS_64_H */