Revert "x86/fault: BUG() when uaccess helpers fault on kernel addresses"

This reverts commit 9da3f2b74054406f87dff7101a569217ffceb29b.

It was well-intentioned, but wrong.  Overriding the exception tables for
instructions for random reasons is just wrong, and that is what the new
code did.

It caused problems for tracing, and it caused problems for strncpy_from_user(),
because the new checks made perfectly valid use cases break, rather than
catch things that did bad things.

Unchecked user space accesses are a problem, but that's not a reason to
add invalid checks that then people have to work around with silly flags
(in this case, that 'kernel_uaccess_faults_ok' flag, which is just an
odd way to say "this commit was wrong" and was sprinked into random
places to hide the wrongness).

The real fix to unchecked user space accesses is to get rid of the
special "let's not check __get_user() and __put_user() at all" logic.
Make __{get|put}_user() be just aliases to the regular {get|put}_user()
functions, and make it impossible to access user space without having
the proper checks in places.

The raison d'être of the special double-underscore versions used to be
that the range check was expensive, and if you did multiple user
accesses, you'd do the range check up front (like the signal frame
handling code, for example).  But SMAP (on x86) and PAN (on ARM) have
made that optimization pointless, because the _real_ expense is the "set
CPU flag to allow user space access".

Do let's not break the valid cases to catch invalid cases that shouldn't
even exist.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Tobin C. Harding <tobin@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 6 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bba3afb4e9bf..f9b43c989577 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -739,12 +739,6 @@ struct task_struct {
 	unsigned			use_memdelay:1;
 #endif
 
-	/*
-	 * May usercopy functions fault on kernel addresses?
-	 * This is not just a single bit because this can potentially nest.
-	 */
-	unsigned int			kernel_uaccess_faults_ok;
-
 	unsigned long			atomic_flags; /* Flags requiring atomic access. */
 
 	struct restart_block		restart_block;