ARM: fix cacheflush with PAN

BugLink: https://bugs.launchpad.net/bugs/2101042

[ Upstream commit ca29cfcc4a21083d671522ad384532e28a43f033 ]

It seems that the cacheflush syscall got broken when PAN for LPAE was
implemented. User access was not enabled around the cache maintenance
instructions, causing them to fault.

Fixes: 7af5b901e847 ("ARM: 9358/2: Implement PAN for LPAE by TTBR0 page table walks disablement")
Reported-by: Michał Pecio <michal.pecio@gmail.com>
Tested-by: Michał Pecio <michal.pecio@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Author: Russell King (Oracle)

arch/arm/kernel/traps.c

@@ -569,6 +569,7 @@ static int bad_syscall(int n, struct pt_regs *regs)
 static inline int
 __do_cache_op(unsigned long start, unsigned long end)
 {
+	unsigned int ua_flags;
 	int ret;
 
 	do {
@@ -577,7 +578,9 @@ __do_cache_op(unsigned long start, unsigned long end)
 		if (fatal_signal_pending(current))
 			return 0;
 
+		ua_flags = uaccess_save_and_enable();
 		ret = flush_icache_user_range(start, start + chunk);
+		uaccess_restore(ua_flags);
 		if (ret)
 			return ret;