| Message ID | 20221104072347.74314-1-absoler@smail.nju.edu.cn |
|---|---|
| State | New |
| Headers | show |
| Series | input: i8042 - fix a double-fetch vulnerability introduced by GCC | expand |
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";
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(-) Signed-off-by: Kunbo Zhang <absoler@smail.nju.edu.cn>