[PATCH] input: i8042 - fix a double-fetch vulnerability introduced by GCC

[Kunbo Zhang]

We found GCC (at least 9.4.0 and 12.1) introduces a double-fetch of `i8042_ports[I8042_AUX_PORT_NO].serio` at drivers/input/serio/i8042.c:408.

One comparison of the global variable `i8042_ports[I8042_AUX_PORT_NO].serio` has been compiled to three ones,
and thus two extra fetches are introduced.
As in the source code, the global variable is tested (at line 408) before three assignments of irq_bit, disable_bit and port_name.
However, as shown in the following disassembly of i8042_port_close(), 
the variable (0x0(%rip)) is fetched and tested three times for each 
assignment of irq_bit, disable_bit and port_name.

0000000000000e50 <i8042_port_close>:
i8042_port_close():
./drivers/input/serio/i8042.c:408
     e50:	48 39 3d 00 00 00 00    cmp    %rdi,0x0(%rip)        # first load
./drivers/input/serio/i8042.c:403
     e57:	41 54                   push   %r12
./drivers/input/serio/i8042.c:408
     e59:	b8 ef ff ff ff          mov    $0xffffffef,%eax
     e5e:	49 c7 c4 00 00 00 00    mov    $0x0,%r12
./drivers/input/serio/i8042.c:403
     e65:	55                      push   %rbp
./drivers/input/serio/i8042.c:408
     e66:	48 c7 c2 00 00 00 00    mov    $0x0,%rdx
./drivers/input/serio/i8042.c:419
     e6d:	be 60 10 00 00          mov    $0x1060,%esi
./drivers/input/serio/i8042.c:403
     e72:	53                      push   %rbx
./drivers/input/serio/i8042.c:408
     e73:	bb df ff ff ff          mov    $0xffffffdf,%ebx
     e78:	0f 45 d8                cmovne %eax,%ebx
     e7b:	0f 95 c0                setne  %al
     e7e:	83 e8 03                sub    $0x3,%eax
     e81:	48 39 3d 00 00 00 00    cmp    %rdi,0x0(%rip)        # second load
     e88:	40 0f 94 c5             sete   %bpl
     e8c:	83 c5 01                add    $0x1,%ebp
     e8f:	48 39 3d 00 00 00 00    cmp    %rdi,0x0(%rip)        # third load
./drivers/input/serio/i8042.c:419
     e96:	48 c7 c7 00 00 00 00    mov    $0x0,%rdi
./drivers/input/serio/i8042.c:408
     e9d:	4c 0f 45 e2             cmovne %rdx,%r12

We have not found any lock protection for the three fetches of `i8042_ports[I8042_AUX_PORT_NO].serio` yet.
If the value of this global variable is modified concurrently among the three fetches, the corresponding assignment of 
disable_bit or port_name will possibly be incorrect.

This patch fixs this by saving the checked value in advance and using a barrier() to prevent compiler optimizations.
This is inspired by a similar compiler-introduced double fetch situation [1] in driver/xen (?).

[1] GitHub link of commit <8135cf8b092723dbfcc611fe6fdcb3a36c9951c5> ( Save xen_pci_op commands before processing it )
---
 drivers/input/serio/i8042.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/input/serio/i8042.c b/drivers/input/serio/i8042.c
index f9486495baef..554a2340ca84 100644
--- a/drivers/input/serio/i8042.c
+++ b/drivers/input/serio/i8042.c
@@ -405,7 +405,9 @@ static void i8042_port_close(struct serio *serio)
 	int disable_bit;
 	const char *port_name;
 
-	if (serio == i8042_ports[I8042_AUX_PORT_NO].serio) {
+	struct serio *tmp = i8042_ports[I8042_AUX_PORT_NO].serio;
+	barrier();
+	if (serio == tmp) {
 		irq_bit = I8042_CTR_AUXINT;
 		disable_bit = I8042_CTR_AUXDIS;
 		port_name = "AUX";

Signed-off-by: Kunbo Zhang <absoler@xxxxxxxxxxxxxxxx>

-- 
2.25.1