riscv: evaluate put_user() arg before enabling user access

The <asm/uaccess.h> header has a problem with put_user(a, ptr) if
the 'a' is not a simple variable, such as a function. This can lead
to the compiler producing code as so:

1:	enable_user_access()
2:	evaluate 'a' into register 'r'
3:	put 'r' to 'ptr'
4:	disable_user_acess()

The issue is that 'a' is now being evaluated with the user memory
protections disabled. So we try and force the evaulation by assigning
'x' to __val at the start, and hoping the compiler barriers in
 enable_user_access() do the job of ordering step 2 before step 1.

This has shown up in a bug where 'a' sleeps and thus schedules out
and loses the SR_SUM flag. This isn't sufficient to fully fix, but
should reduce the window of opportunity. The first instance of this
we found is in scheudle_tail() where the code does:

$ less -N kernel/sched/core.c

4263  if (current->set_child_tid)
4264         put_user(task_pid_vnr(current), current->set_child_tid);

Here, the task_pid_vnr(current) is called within the block that has
enabled the user memory access. This can be made worse with KASAN
which makes task_pid_vnr() a rather large call with plenty of
opportunity to sleep.

Signed-off-by: Ben Dooks <[email protected]>
Reported-by: [email protected]
Suggested-by: Arnd Bergman <[email protected]>
--
Changes since v1:
- fixed formatting and updated the patch description with more info

Changes since v2:
- fixed commenting on __put_user() ([email protected])

Change since v3:
- fixed RFC in patch title. Should be ready to merge.

Signed-off-by: Palmer Dabbelt <[email protected]>

diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 824b2c9da75bd2095274c2ef73933944cc900845..f944062c9d990e7964358f8c12e218084af14a8c 100644 (file)
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -306,7 +306,9 @@ do {                                                                \
  * data types like structures or arrays.
  *
  * @ptr must have pointer-to-simple-variable type, and @x must be assignable
- * to the result of dereferencing @ptr.
+ * to the result of dereferencing @ptr. The value of @x is copied to avoid
+ * re-ordering where @x is evaluated inside the block that enables user-space
+ * access (thus bypassing user space protection if @x is a function).
  *
  * Caller must check the pointer with access_ok() before calling this
  * function.
@@ -316,12 +318,13 @@ do {                                                              \
 #define __put_user(x, ptr)                                     \
 ({                                                             \
        __typeof__(*(ptr)) __user *__gu_ptr = (ptr);            \
+       __typeof__(*__gu_ptr) __val = (x);                      \
        long __pu_err = 0;                                      \
                                                                \
        __chk_user_ptr(__gu_ptr);                               \
                                                                \
        __enable_user_access();                                 \
-       __put_user_nocheck(x, __gu_ptr, __pu_err);              \
+       __put_user_nocheck(__val, __gu_ptr, __pu_err);          \
        __disable_user_access();                                \
                                                                \
        __pu_err;                                               \