[Bug 1921948] Re: MTE tags not checked properly for unaligned accesses at EL1

[Alex Bennée]

It gets further without but still spams a lot of failure messages:

The buggy address belongs to the object at ffffff80036a2200
 which belongs to the cache kmalloc-128 of size 128
The buggy address is located 11 bytes to the right of
 128-byte region [ffffff80036a2200, ffffff80036a2280)
The buggy address belongs to the page:
page:0000000046e01872 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 
pfn:0x436a2
flags: 0x3fc0000000000200(slab)
raw: 3fc0000000000200 dead000000000100 dead000000000122 f9ffff8001c01e00
raw: 0000000000000000 0000000080100010 00000001ffffffff f3ffff80036a2401
page dumped because: kasan: bad access detected
pages's memcg:f3ffff80036a2401

Memory state around the buggy address:
 ffffff80036a2000: f6 f6 f6 f6 f6 f6 f6 f6 fe fe fe fe fe fe fe fe
 ffffff80036a2100: fa fa fa fa fe fe fe fe fe fe fe fe fe fe fe fe
>ffffff80036a2200: f9 f9 f9 f9 f9 f9 f9 f9 fe fe fe fe fe fe fe fe
                                           ^
 ffffff80036a2300: fc fc fc fc fe fe fe fe fe fe fe fe fe fe fe fe
 ffffff80036a2400: f3 f3 f3 f3 f3 f3 f3 f3 fe fe fe fe fe fe fe fe
==================================================================
Disabling lock debugging due to kernel taint
    # kmalloc_oob_right: EXPECTATION FAILED at lib/test_kasan.c:86
    Expected fail_data.report_expected == fail_data.report_found, but
        fail_data.report_expected == 1
        fail_data.report_found == 0
    not ok 1 - kmalloc_oob_right
    # kmalloc_oob_left: EXPECTATION FAILED at lib/test_kasan.c:98
    Expected fail_data.report_expected == fail_data.report_found, but
        fail_data.report_expected == 1
        fail_data.report_found == 0
    not ok 2 - kmalloc_oob_left
    # kmalloc_node_oob_right: EXPECTATION FAILED at lib/test_kasan.c:110
    Expected fail_data.report_expected == fail_data.report_found, but
        fail_data.report_expected == 1
        fail_data.report_found == 0
    not ok 3 - kmalloc_node_oob_right
    # kmalloc_pagealloc_oob_right: EXPECTATION FAILED at lib/test_kasan.c:130
    Expected fail_data.report_expected == fail_data.report_found, but
        fail_data.report_expected == 1
        fail_data.report_found == 0
    not ok 4 - kmalloc_pagealloc_oob_right
    # kmalloc_pagealloc_uaf: EXPECTATION FAILED at lib/test_kasan.c:148
    Expected fail_data.report_expected == fail_data.report_found, but
        fail_data.report_expected == 1
        fail_data.report_found == 0
    not ok 5 - kmalloc_pagealloc_uaf

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https://bugs.launchpad.net/bugs/1921948

Title:
  MTE tags not checked properly for unaligned accesses at EL1

Status in QEMU:
  In Progress

Bug description:
  For kernel memory accesses that span across two memory granules,
  QEMU's MTE implementation only checks the tag of the first granule but
  not of the second one.

  To reproduce this, build the Linux kernel with CONFIG_KASAN_HW_TAGS
  enabled, apply the patch below, and boot the kernel:

  diff --git a/sound/last.c b/sound/last.c
  index f0bb98780e70..04745cb30b74 100644
  --- a/sound/last.c
  +++ b/sound/last.c
  @@ -5,12 +5,18 @@
    */
   
   #include <linux/init.h>
  +#include <linux/slab.h>
   #include <sound/core.h>
   
   static int __init alsa_sound_last_init(void)
   {
          struct snd_card *card;
          int idx, ok = 0;
  +
  +       char *ptr = kmalloc(128, GFP_KERNEL);
  +       pr_err("KASAN report should follow:\n");
  +       *(volatile unsigned long *)(ptr + 124);
  +       kfree(ptr);
          
          printk(KERN_INFO "ALSA device list:\n");
          for (idx = 0; idx < SNDRV_CARDS; idx++) {

  KASAN tags the 128 allocated bytes with the same tag as the returned
  pointer. The memory granule that follows the 128 allocated bytes has a
  different tag (with 1/15 probability).

  Expected result: a tag fault is detected and a KASAN report is printed when 
accessing bytes [124, 130).
  Observed result: no tag fault is detected and no KASAN report is printed.

  Here are the flags that I use to run QEMU if they matter:

  qemu-system-aarch64 -s -machine virt,mte=on -cpu max -m 2G -smp 2 -net
  user,host=10.0.2.10,hostfwd=tcp:127.0.0.1:10021-:22 -net nic
  -nographic -kernel ./Image -append "console=ttyAMA0 root=/dev/vda
  earlyprintk=serial" -drive file=./fs.img,format=raw,if=virtio -no-
  shutdown -no-reboot

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