[PATCH v3 7/8] m68k: fix livelock in uaccess

[Michael Schmitz <schmitzmic@gmail.com>]

From: Al Viro <viro@zeniv.linux.org.uk>

commit bd75497a77ccae9a5e1b0f1a3fa8283d67b21f4c upstream.

m68k equivalent of 26178ec11ef3 "x86: mm: consolidate VM_FAULT_RETRY handling"
If e.g. get_user() triggers a page fault and a fatal signal is caught, we might
end up with handle_mm_fault() returning VM_FAULT_RETRY and not doing anything
to page tables.  In such case we must *not* return to the faulting insn -
that would repeat the entire thing without making any progress; what we need
instead is to treat that as failed (user) memory access.

Tested-by: Finn Thain <fthain@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[MSch: v4.4 backport compile errors fixes]
Cc: <cip-dev@lists.cip-project.org> # 4.4
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
---
 arch/m68k/mm/fault.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 6a94cdd0c830..f94df64c3784 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -139,8 +139,11 @@ good_area:
 	fault = handle_mm_fault(mm, vma, address, flags);
 	pr_debug("handle_mm_fault returns %d\n", fault);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return 0;
+	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
-- 
2.17.1