From:   Haibo Li <haibo.li@mediatek.com>
To:     Marco Elver <elver@google.com>
CC:     Dmitry Vyukov <dvyukov@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        <kasan-dev@googlegroups.com>, <linux-kernel@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-mediatek@lists.infradead.org>, <xiaoming.yu@mediatek.com>,
        <haibo.li@mediatek.com>
Subject: [PATCH] kcsan:fix alignment_fault when read unaligned instrumented memory
Date:   Wed, 8 Mar 2023 10:20:57 +0800
Message-ID: <20230308022057.151078-1-haibo.li@mediatek.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain
Precedence: bulk

After enable kcsan on arm64+linux-5.15,it reports alignment_fault
when access unaligned address.
Here is the oops log:
"
Trying to unpack rootfs image as initramfs.....
Unable to handle kernel paging request at virtual address
  ffffff802a0d8d7171
Mem abort info:o:
  ESR = 0x9600002121
  EC = 0x25: DABT (current EL), IL = 32 bitsts
  SET = 0, FnV = 0 0
  EA = 0, S1PTW = 0 0
  FSC = 0x21: alignment fault
Data abort info:o:
  ISV = 0, ISS = 0x0000002121
  CM = 0, WnR = 0 0
swapper pgtable: 4k pages, 39-bit VAs, pgdp=000000002835200000
[ffffff802a0d8d71] pgd=180000005fbf9003, p4d=180000005fbf9003,
pud=180000005fbf9003, pmd=180000005fbe8003, pte=006800002a0d8707
Internal error: Oops: 96000021 [#1] PREEMPT SMP
Modules linked in:
CPU: 2 PID: 45 Comm: kworker/u8:2 Not tainted
  5.15.78-android13-8-g63561175bbda-dirty #1
...
pc : kcsan_setup_watchpoint+0x26c/0x6bc
lr : kcsan_setup_watchpoint+0x88/0x6bc
sp : ffffffc00ab4b7f0
x29: ffffffc00ab4b800 x28: ffffff80294fe588 x27: 0000000000000001
x26: 0000000000000019 x25: 0000000000000001 x24: ffffff80294fdb80
x23: 0000000000000000 x22: ffffffc00a70fb68 x21: ffffff802a0d8d71
x20: 0000000000000002 x19: 0000000000000000 x18: ffffffc00a9bd060
x17: 0000000000000001 x16: 0000000000000000 x15: ffffffc00a59f000
x14: 0000000000000001 x13: 0000000000000000 x12: ffffffc00a70faa0
x11: 00000000aaaaaaab x10: 0000000000000054 x9 : ffffffc00839adf8
x8 : ffffffc009b4cf00 x7 : 0000000000000000 x6 : 0000000000000007
x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffffffc00a70fb70
x2 : 0005ff802a0d8d71 x1 : 0000000000000000 x0 : 0000000000000000
Call trace:
 kcsan_setup_watchpoint+0x26c/0x6bc
 __tsan_read2+0x1f0/0x234
 inflate_fast+0x498/0x750
 zlib_inflate+0x1304/0x2384
 __gunzip+0x3a0/0x45c
 gunzip+0x20/0x30
 unpack_to_rootfs+0x2a8/0x3fc
 do_populate_rootfs+0xe8/0x11c
 async_run_entry_fn+0x58/0x1bc
 process_one_work+0x3ec/0x738
 worker_thread+0x4c4/0x838
 kthread+0x20c/0x258
 ret_from_fork+0x10/0x20
Code: b8bfc2a8 2a0803f7 14000007 d503249f (78bfc2a8) )
---[ end trace 613a943cb0a572b6 ]-----
"

After checking linux 6.3-rc1 on QEMU arm64,it still has the possibility
to read unaligned address in read_instrumented_memory(qemu can not
emulate alignment fault)

To fix alignment fault and read the value of instrumented memory
more effective,bypass the unaligned access in read_instrumented_memory.

Signed-off-by: Haibo Li <haibo.li@mediatek.com>
---
 kernel/kcsan/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 54d077e1a2dc..88e75d7d85d2 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -337,6 +337,11 @@ static void delay_access(int type)
  */
 static __always_inline u64 read_instrumented_memory(const volatile void *ptr, size_t size)
 {
+	bool aligned_read = (size == 1) || IS_ALIGNED((unsigned long)ptr, size);
+
+	if (!aligned_read)
+		return 0;
+
 	switch (size) {
 	case 1:  return READ_ONCE(*(const u8 *)ptr);
 	case 2:  return READ_ONCE(*(const u16 *)ptr);
-- 
2.25.1