Messages in this thread | | | Date | Sun, 23 Aug 2020 06:55:59 +0200 | From | Willy Tarreau <> | Subject | Re: Why KASAN doesn't detect this stack oob fault? |
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On Sun, Aug 23, 2020 at 11:04:34AM +0800, richard clark wrote: > Hi guys, > > I ins a kmod with below code in a KASAN enabled kernel ( > 5.7.0, > CONFIG_KASAN=y > CONFIG_KASAN_GENERIC=y > CONFIG_KASAN_OUTLINE=y): > > static int kmod_init(void) > { > int i; > int arr[4]; > > for (i = 0; i < 20; i++) { > arr[i] = i; > printk("arr[%d] = %d\n", i, arr[i]); > } > return 0; > } > > The output is after insmod: > > [ 1511.800683] arr[0] = 0 > [ 1511.800685] arr[1] = 1 > [ 1511.800686] arr[2] = 2 > [ 1511.800687] arr[3] = 3 > [ 1511.800688] arr[4] = 4 > [ 1511.800690] arr[5] = 5 > [ 1511.800691] arr[6] = 6 > [ 1511.800692] arr[7] = 7 > [ 1511.800693] arr[8] = 8 > [ 1511.800694] arr[9] = 9 > [ 1511.800695] arr[10] = 10 > [ 1511.800696] arr[11] = 11 > [ 1511.800697] arr[12] = 12 > [ 1511.800699] arr[13] = 13 > [ 1511.800700] arr[14] = 14 > [ 1511.800701] arr[15] = 15 > [ 1511.800702] arr[16] = 16 > [ 1511.800704] arr[17] = 17 > [ 1511.800705] arr[18] = 18 > [ 1511.800706] arr[19] = 19 > > The kernel is not tainted and the gcc version is 7.5 used to build the kernel. > The question is: > 1. Why the stack out-of-bound can work? > 2. Why the KASAN doesn't detect this?
Have you verified in the output code that the compiler didn't optimize the stack access away since it doesn't need it ?
Just to make sure, do it in two distinct loops so that there are more chances for the stack to be really used:
static int kmod_init(void) { int i; int arr[4]; for (i = 0; i < 20; i++) arr[i] = i;
for (i = 0; i < 20; i++) printk("arr[%d] = %d\n", i, arr[i]);
return 0; }
Willy
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