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Subject[PATCH 5.4 099/109] ARC: entry: fix potential EFA clobber when TIF_SYSCALL_TRACE
Date
From: Vineet Gupta <vgupta@synopsys.com>

commit 00fdec98d9881bf5173af09aebd353ab3b9ac729 upstream.

Trap handler for syscall tracing reads EFA (Exception Fault Address),
in case strace wants PC of trap instruction (EFA is not part of pt_regs
as of current code).

However this EFA read is racy as it happens after dropping to pure
kernel mode (re-enabling interrupts). A taken interrupt could
context-switch, trigger a different task's trap, clobbering EFA for this
execution context.

Fix this by reading EFA early, before re-enabling interrupts. A slight
side benefit is de-duplication of FAKE_RET_FROM_EXCPN in trap handler.
The trap handler is common to both ARCompact and ARCv2 builds too.

This just came out of code rework/review and no real problem was reported
but is clearly a potential problem specially for strace.

Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arc/kernel/entry.S | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)

--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -153,7 +153,6 @@ END(EV_Extension)
tracesys:
; save EFA in case tracer wants the PC of traced task
; using ERET won't work since next-PC has already committed
- lr r12, [efa]
GET_CURR_TASK_FIELD_PTR TASK_THREAD, r11
st r12, [r11, THREAD_FAULT_ADDR] ; thread.fault_address

@@ -196,15 +195,9 @@ tracesys_exit:
; Breakpoint TRAP
; ---------------------------------------------
trap_with_param:
-
- ; stop_pc info by gdb needs this info
- lr r0, [efa]
+ mov r0, r12 ; EFA in case ptracer/gdb wants stop_pc
mov r1, sp

- ; Now that we have read EFA, it is safe to do "fake" rtie
- ; and get out of CPU exception mode
- FAKE_RET_FROM_EXCPN
-
; Save callee regs in case gdb wants to have a look
; SP will grow up by size of CALLEE Reg-File
; NOTE: clobbers r12
@@ -231,6 +224,10 @@ ENTRY(EV_Trap)

EXCEPTION_PROLOGUE

+ lr r12, [efa]
+
+ FAKE_RET_FROM_EXCPN
+
;============ TRAP 1 :breakpoints
; Check ECR for trap with arg (PROLOGUE ensures r10 has ECR)
bmsk.f 0, r10, 7
@@ -238,9 +235,6 @@ ENTRY(EV_Trap)

;============ TRAP (no param): syscall top level

- ; First return from Exception to pure K mode (Exception/IRQs renabled)
- FAKE_RET_FROM_EXCPN
-
; If syscall tracing ongoing, invoke pre-post-hooks
GET_CURR_THR_INFO_FLAGS r10
btst r10, TIF_SYSCALL_TRACE

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