arm64: entry: Allow tramp_alias to access symbols after the 4K boundary

commit 6c5bf79b69 upstream.

Systems using kpti enter and exit the kernel through a trampoline mapping
that is always mapped, even when the kernel is not. tramp_valias is a macro
to find the address of a symbol in the trampoline mapping.

Adding extra sets of vectors will expand the size of the entry.tramp.text
section to beyond 4K. tramp_valias will be unable to generate addresses
for symbols beyond 4K as it uses the 12 bit immediate of the add
instruction.

As there are now two registers available when tramp_alias is called,
use the extra register to avoid the 4K limit of the 12 bit immediate.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[ Removed SDEI for backport ]
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: Ibd2b06eb4150994db92502c7042a486b1187313d

arch/arm64/kernel/entry.S

@@ -99,9 +99,12 @@
 .org .Lventry_start\@ + 128	// Did we overflow the ventry slot?
 	.endm
 
-	.macro tramp_alias, dst, sym
+	.macro tramp_alias, dst, sym, tmp
 	mov_q	\dst, TRAMP_VALIAS
-	add	\dst, \dst, #(\sym - .entry.tramp.text)
+	adr_l	\tmp, \sym
+	add	\dst, \dst, \tmp
+	adr_l	\tmp, .entry.tramp.text
+	sub	\dst, \dst, \tmp
 	.endm
 
 	// This macro corrupts x0-x3. It is the caller's duty
@@ -329,11 +332,11 @@ alternative_else_nop_endif
 alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 	bne	4f
-	msr	far_el1, x30
-	tramp_alias	x30, tramp_exit_native
+	msr	far_el1, x29
+	tramp_alias	x30, tramp_exit_native, x29
 	br	x30
 4:
-	tramp_alias	x30, tramp_exit_compat
+	tramp_alias	x30, tramp_exit_compat, x29
 	br	x30
 #endif
 	.else