arm64: Preventing READ_IMPLIES_EXEC propagation

Like arch/arm/, we inherit the READ_IMPLIES_EXEC personality flag across
fork(). This is undesirable for a number of reasons:

  * ELF files that don't require executable stack can end up with it
    anyway

  * We end up performing un-necessary I-cache maintenance when mapping
    what should be non-executable pages

  * Restricting what is executable is generally desirable when defending
    against overflow attacks

This patch clears the personality flag when setting up the personality for
newly spwaned native tasks. Given that semi-recent AArch64 toolchains emit
a non-executable PT_GNU_STACK header, userspace applications can already
not rely on READ_IMPLIES_EXEC so shouldn't be adversely affected by this
change.

Cc: <stable@vger.kernel.org>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dong Bo <dongbo4@huawei.com>
[will: added comment to compat code, rewrote commit message]
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/elf.h

@@ -142,6 +142,7 @@ typedef struct user_fpsimd_state elf_fpregset_t;
 ({									\
 	clear_bit(TIF_32BIT, &current->mm->context.flags);		\
 	clear_thread_flag(TIF_32BIT);					\
+	current->personality &= ~READ_IMPLIES_EXEC;			\
 })
 
 /* update AT_VECTOR_SIZE_ARCH if the number of NEW_AUX_ENT entries changes */
@@ -187,6 +188,11 @@ typedef compat_elf_greg_t		compat_elf_gregset_t[COMPAT_ELF_NGREG];
 					 ((x)->e_flags & EF_ARM_EABI_MASK))
 
 #define compat_start_thread		compat_start_thread
+/*
+ * Unlike the native SET_PERSONALITY macro, the compat version inherits
+ * READ_IMPLIES_EXEC across a fork() since this is the behaviour on
+ * arch/arm/.
+ */
 #define COMPAT_SET_PERSONALITY(ex)					\
 ({									\
 	set_bit(TIF_32BIT, &current->mm->context.flags);		\