Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

On Wed, Apr 5, 2017 at 10:49 AM, Catalin Marinas
[off-list ref] wrote:

On Wed, Apr 05, 2017 at 07:36:17AM -0700, Thomas Garnier wrote:

quoted

On Wed, Apr 5, 2017 at 7:22 AM, Catalin Marinas [off-list ref] wrote:

quoted

On Tue, Apr 04, 2017 at 10:47:27AM -0700, Thomas Garnier wrote:

quoted

+
      ldr     x1, [tsk, #TSK_TI_FLAGS]
      and     x2, x1, #_TIF_WORK_MASK
      cbnz    x2, work_pending

@@ -779,6 +788,12 @@ finish_ret_to_user:
      kernel_exit 0
 ENDPROC(ret_to_user)

+addr_limit_fail:
+     stp     x0, lr, [sp,#-16]!
+     bl      asm_verify_pre_usermode_state
+     ldp     x0, lr, [sp],#16
+     ret     lr

Where is this supposed to return? What is the value of lr when branching
to addr_limit_fail?

It is not supposed to return. Do you think I should remove stp, ldp,
ret and jut add a brk 0x100 or jmp/call a break/bug function?

Can you not just make addr_limit_fail a C function which never returns
(similar to what we to with bad_mode() on arm64)? Since addr_limit_fail
is only called when the segment is not the right one, I don't really see
why you need another call to asm_verify_pre_usermode_state() to do a
similar check again. Just panic in addr_limit_fail (unless I
misunderstood what you are trying to achieve).

Calling asm_verify_pre_usermode_state has the advantage of having a
clear BUG_ON for the error (versus a panic description).

What do you think about replacing asm_verify_pre_usermode_state by a
"address_limit_fail" function that still calls
verify_pre_usermode_state but panic afterwards (because it should
never return)?

The assembly code would be easier to understand and in case of error
the BUG_ON is clear for the user.

--
Catalin

-- 
Thomas