[PATCH RFC] uaccess: user_access_begin_after_access_ok()

From: Michael S. Tsirkin
Date: Tue Jun 02 2020 - 04:45:15 EST

So vhost needs to poke at userspace *a lot* in a quick succession. It
is thus benefitial to enable userspace access, do our thing, then
disable. Except access_ok has already been pre-validated with all the
relevant nospec checks, so we don't need that. Add an API to allow
userspace access after access_ok and barrier_nospec are done.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Jason, so I've been thinking using something along these lines,
then switching vhost to use unsafe_copy_to_user and friends would
solve lots of problems you observed with SMAP.

What do you think? Do we need any other APIs to make it practical?

arch/x86/include/asm/uaccess.h | 1 +
include/linux/uaccess.h | 1 +
2 files changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index d8f283b9a569..fa5afb3a54fe 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -483,6 +483,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
return 1;
#define user_access_begin(a,b) user_access_begin(a,b)
+#define user_access_begin_after_access_ok() __uaccess_begin()
#define user_access_end() __uaccess_end()

#define user_access_save() smap_save()
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 67f016010aad..4c0a959ad639 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -370,6 +370,7 @@ extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);

#ifndef user_access_begin
#define user_access_begin(ptr,len) access_ok(ptr, len)
+#define user_access_begin_after_access_ok() do { } while (0)
#define user_access_end() do { } while (0)
#define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
#define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user(x,p),e)