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

[Peter Collingbourne]

It looks like there's still a bug here: I'm seeing false positive MTE
faults for unaligned accesses that touch multiple pages. This userspace
assembly program demonstrates the problem, but for some reason it only
reproduces some of the time for me:

.arch_extension memtag

.globl _start
_start:
mov x0, #0x37 // PR_SET_TAGGED_ADDR_CTRL
mov x1, #0x3 // PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_ASYNC
mov x2, #0
mov x3, #0
mov x4, #0
mov x8, #0xa7 // prctl
svc #0

mov x0, xzr
mov w1, #0x2000
mov w2, #0x23 // PROT_READ|PROT_WRITE|PROT_MTE
mov w3, #0x22 // MAP_PRIVATE|MAP_ANONYMOUS
mov w4, #0xffffffff
mov x5, xzr
mov x8, #0xde // mmap
svc #0

mov x1, #(1 << 56)
add x0, x0, x1
add x0, x0, #0xff0
stg x0, [x0]
stg x0, [x0, #16]
str x1, [x0, #12]

mov x0, #0
mov x8, #0x5d // exit
svc #0

** Changed in: qemu
       Status: Fix Committed => Confirmed

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

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

Status in QEMU:
  Confirmed

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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