Re: [PATCH] kcsan:fix alignment_fault when read unaligned instrumented memory

From: Haibo Li
Date: Wed Mar 08 2023 - 04:41:38 EST

> On Wed, 8 Mar 2023 at 03:21, 'Haibo Li' via kasan-dev
> <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> >
> > After enable kcsan on arm64+linux-5.15,it reports alignment_fault when
> > access unaligned address.
>
> Is this KCSAN's fault or the fault of the code being instrumented?
> I.e. if you disable KCSAN, is there still an alignment fault reported?
If disable KCSAN,the alignment fault will not occur.

>
> Because as-is, I don't understand how the instrumentation alone will cause an
> alignment fault, because for every normal memory access there is a
> corresponding instrumented access - therefore, that'd suggest that the real
> access was also unaligned. Note that the compiler inserts instrumentation
> _before_ the actual access, so if there's a problem, that problem will manifest
> inside KCSAN.
>
> Can you provide more information about what's going on (type of access, size
> of access, etc.)?
>
Here is the source code of inflate_fast+0x498(lib/zlib_inflate/inffast.c +258):
if (dist > 2) {
unsigned short *sfrom;

sfrom = (unsigned short *)(from);
loops = len >> 1;
do {
if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
>*sout++ = *sfrom++;
it reads 2 bytes from sfrom and sfrom is unaligned.

> > 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
>
> ^^ is it possible that an access in "inflate_fast" is unaligned?
Here is the instruction for inflate_fast+0x498:
ffffffc008948980 <inflate_fast>:
...
ffffffc008948e10: e0 03 1c aa mov x0, x28
ffffffc008948e14: 06 3a e9 97 bl 0xffffffc00839762c <__tsan_unaligned_read2>
ffffffc008948e18: e0 03 17 aa mov x0, x23
>ffffffc008948e1c: 9a 27 40 78 ldrh w26, [x28], #2

And the instruction for kcsan_setup_watchpoint+0x26c:
ffffffc00839ab90 <kcsan_setup_watchpoint>:
...
>ffffffc00839adfc: a8 fe df 48 ldarh w8, [x21]

The instruction is different.READ_ONCE uses ldarh,which requires the access address is aligned.
As ARM v8 arm said:
"
Load-Acquire, Load-AcquirePC and Store-Release, other than Load-Acquire Exclusive Pair and
Store-Release-Exclusive Pair, access only a single data element. This access is single-copy atomic. The address of the data object must be aligned to the size of the data element being accessed, otherwise the access generates an
Alignment fault."

while ldrh accepts unaligned address.
That's why it is ok while disable KCSAN.

>
> > 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@xxxxxxxxxxxx>
> > ---
> > 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);
>
> (size==1) check is redundant because IS_ALIGNED(.., 1) should always return
> true.
Thanks.I will drop it.
>
> And this will also penalize other architectures which can do unaligned accesses.
> So this check probably wants to be guarded by "IS_ENABLED(CONFIG_ARM64)"
> or something.

Agree.Is it acceptable to use IS_ENABLED(CONFIG_ARM64) here?
>
> > + if (!aligned_read)
> > + return 0;
> > +
> > switch (size) {
> > case 1: return READ_ONCE(*(const u8 *)ptr);
> > case 2: return READ_ONCE(*(const u16 *)ptr);