[PATCH 2/2] powerpc/uaccess: Move might_fault() into user_access_begin()

[Michael Ellerman <mpe@ellerman.id.au>]

We have a might_fault() check in __unsafe_put_user_goto(), but that is
dangerous as it potentially calls lots of code while user access is
enabled.

It also triggers the check Alexey added to the irq restore path to
catch cases like that:

  WARNING: CPU: 30 PID: 1 at arch/powerpc/include/asm/book3s/64/kup.h:324 arch_local_irq_restore+0x160/0x190
  NIP arch_local_irq_restore+0x160/0x190
  LR  lock_is_held_type+0x140/0x200
  Call Trace:
    0xc00000007f392ff8 (unreliable)
    ___might_sleep+0x180/0x320
    __might_fault+0x50/0xe0
    filldir64+0x2d0/0x5d0
    call_filldir+0xc8/0x180
    ext4_readdir+0x948/0xb40
    iterate_dir+0x1ec/0x240
    sys_getdents64+0x80/0x290
    system_call_exception+0x160/0x280
    system_call_common+0xf0/0x27c

So remove the might fault check from unsafe_put_user().

Any call to unsafe_put_user() must be inside a region that's had user
access enabled with user_access_begin(), so move the might_fault() in
there. That also allows us to drop the is_kernel_addr() test, because
there should be no code using user_access_begin() in order to access a
kernel address.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/include/asm/uaccess.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 70347ee34c94..71640eca7341 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -214,8 +214,6 @@ do {								\
 #define __unsafe_put_user_goto(x, ptr, size, label)		\
 do {								\
 	__typeof__(*(ptr)) __user *__pu_addr = (ptr);		\
-	if (!is_kernel_addr((unsigned long)__pu_addr))		\
-		might_fault();					\
 	__chk_user_ptr(ptr);					\
 	__put_user_size_goto((x), __pu_addr, (size), label);	\
 } while (0)
@@ -494,6 +492,8 @@ extern void memcpy_page_flushcache(char *to, struct page *page, size_t offset,
 
 static __must_check inline bool user_access_begin(const void __user *ptr, size_t len)
 {
+	might_fault();
+
 	if (unlikely(!access_ok(ptr, len)))
 		return false;
 	allow_read_write_user((void __user *)ptr, ptr, len);
-- 
2.25.1