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

From: Richard Henderson
Subject: [Bug 1921948] Re: MTE tags not checked properly for unaligned accesses at EL1
Date: Wed, 07 Apr 2021 21:19:11 -0000

Re comments #8 and #10, I don't replicate that.
I get full pass on KASAN_UNIT_TEST with
and without virtualization enabled.

Re comment #9, if there are bugs suspected in qemu, they
need to be reported, or we'll never hear about them.

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  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=,hostfwd=tcp: -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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