[syzbot] KASAN: null-ptr-deref Read in hugepage_vma_check

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c6d7e3b385f1 Add linux-next specific files for 20220616
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10d9fb1bf00000
kernel config: https://syzkaller.appspot.com/x/.config?x=9d495405e4a98620
dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4d875b...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
BUG: KASAN: null-ptr-deref in test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
BUG: KASAN: null-ptr-deref in hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
Read of size 8 at addr 00000000000005a8 by task syz-executor.5/21978

CPU: 0 PID: 21978 Comm: syz-executor.5 Not tainted 5.19.0-rc2-next-20220616-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
kasan_report+0xbe/0x1f0 mm/kasan/report.c:495
check_region_inline mm/kasan/generic.c:183 [inline]
kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
instrument_atomic_read include/linux/instrumented.h:71 [inline]
test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
show_smap+0x1c6/0x470 fs/proc/task_mmu.c:866
traverse.part.0+0xcf/0x5f0 fs/seq_file.c:111
traverse fs/seq_file.c:101 [inline]
seq_read_iter+0x90f/0x1280 fs/seq_file.c:195
seq_read+0x337/0x4b0 fs/seq_file.c:162
do_loop_readv_writev fs/read_write.c:763 [inline]
do_loop_readv_writev fs/read_write.c:750 [inline]
do_iter_read+0x4f8/0x750 fs/read_write.c:805
vfs_readv+0xe5/0x150 fs/read_write.c:923
do_preadv fs/read_write.c:1015 [inline]
__do_sys_preadv fs/read_write.c:1065 [inline]
__se_sys_preadv fs/read_write.c:1060 [inline]
__x64_sys_preadv+0x22b/0x310 fs/read_write.c:1060
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f5c1d889109
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f5c1ea69168 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 00007f5c1d99bf60 RCX: 00007f5c1d889109
RDX: 0000000000000001 RSI: 00000000200006c0 RDI: 0000000000000005
RBP: 00007f5c1d8e305d R08: 0000000000000000 R09: 0000000000000000
R10: 00000000fffffffe R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffd8514b43f R14: 00007f5c1ea69300 R15: 0000000000022000
</TASK>
==================================================================


---
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[Andrew Morton]

On Fri, 17 Jun 2022 00:04:23 -0700 syzbot <syzbot+4d875b...@syzkaller.appspotmail.com> wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: c6d7e3b385f1 Add linux-next specific files for 20220616
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=10d9fb1bf00000
> kernel config: https://syzkaller.appspot.com/x/.config?x=9d495405e4a98620
> dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4
> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+4d875b...@syzkaller.appspotmail.com
>
> ==================================================================
> BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
> BUG: KASAN: null-ptr-deref in test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
> BUG: KASAN: null-ptr-deref in hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
> Read of size 8 at addr 00000000000005a8 by task syz-executor.5/21978

vma->vm_mm is NULL in show_smap->hugepage_vma_check().

Perhaps m_start() should be checking the mm after taking
mmap_read_lock() instead of before?

Matthew, you mucked with it last ;) Can you please take a look?

[Matthew Wilcox]

On Fri, Jun 17, 2022 at 04:52:56PM -0700, Andrew Morton wrote:
> On Fri, 17 Jun 2022 00:04:23 -0700 syzbot <syzbot+4d875b...@syzkaller.appspotmail.com> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: c6d7e3b385f1 Add linux-next specific files for 20220616
> > git tree: linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=10d9fb1bf00000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=9d495405e4a98620
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4
> > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+4d875b...@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
> > BUG: KASAN: null-ptr-deref in test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
> > BUG: KASAN: null-ptr-deref in hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
> > Read of size 8 at addr 00000000000005a8 by task syz-executor.5/21978
>
> vma->vm_mm is NULL in show_smap->hugepage_vma_check().
>
> Perhaps m_start() should be checking the mm after taking
> mmap_read_lock() instead of before?
>
> Matthew, you mucked with it last ;) Can you please take a look?

I think this is Yang Shi's fault. The last patch in that series which
touches this area does:

seq_printf(m, "THPeligible: %d\n",
- hugepage_vma_check(vma, vma->vm_flags, true));
+ hugepage_vma_check(vma, vma->vm_flags, true, false));

and hugepage_vma_check() does:

if ((vm_flags & VM_NOHUGEPAGE) ||
test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
return false;

Before those patches, it calls transparent_hugepage_active() which
does not (afaict) touch vma->vm_mm.

I haven't been paying attention to that patch series, so I don't have
a constructive suggestion for how to fix.

[Yang Shi]

No, transparent_hugepage_active() also could touch vma->vm_mm. The
above code was transhuge_vma_enabled() called by
transparent_hugepage_active() before my patch, my patch just
open-coded it.

But hugepage_vma_check() does check vma in different order from
transparent_hugepage_active(), so the old code may not reach it at
all.

And I don't think the mm could go away since it is pinned, but the
returned vma may not belong to this mm, for example, gate vma which
doesn't have a mm. Its size is PAGE_SIZE, so the old code returns
earlier before touching mm.

One possible solution is to move vma size check before touching mm, like:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2e2a8b5bc567..1a9f15a89091 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -73,6 +73,16 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
unsigned long vm_flags,
bool smaps, bool in_pf)
{
+ /*
+ * Check alignment for file vma and size for both file and anon vma.
+ *
+ * Skip the check for page fault. Huge fault does the check in fault
+ * handlers. And this check is not suitable for huge PUD fault.
+ */
+ if (!in_pf &&
+ !transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
+ return false;
+
/*
* Explicitly disabled through madvise or prctl, or some
* architectures may disable THP for some mappings, for
@@ -99,16 +109,6 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
if (vm_flags & VM_NO_KHUGEPAGED)
return false;

- /*
- * Check alignment for file vma and size for both file and anon vma.
- *
- * Skip the check for page fault. Huge fault does the check in fault
- * handlers. And this check is not suitable for huge PUD fault.
- */
- if (!in_pf &&
- !transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
- return false;
-
/*
* Enabled via shmem mount options or sysfs settings.
* Must be done before hugepage flags check since shmem has its

Or we may have to check gate vma explicitly.

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 34d1d36073ea Add linux-next specific files for 20220621
git tree: linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=15a34140080000
kernel config: https://syzkaller.appspot.com/x/.config?x=b24b62d1c051cfc8

dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14097a3ff00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1466c63ff00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4d875b...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
BUG: KASAN: null-ptr-deref in test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
BUG: KASAN: null-ptr-deref in hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82

Read of size 8 at addr 00000000000005a8 by task syz-executor405/3606

CPU: 0 PID: 3606 Comm: syz-executor405 Not tainted 5.19.0-rc3-next-20220621-syzkaller #0

RIP: 0033:0x7fcb44a75239
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffedce269f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fcb44a75239
RDX: 0000000000000001 RSI: 00000000200006c0 RDI: 0000000000000003
RBP: 00007ffedce26a00 R08: 0000000000000000 R09: 65732f636f72702f
R10: 00000000fffffffe R11: 0000000000000246 R12: 00007fcb44a39120
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>
==================================================================

[syzbot]

syzbot has bisected this issue to:

commit 0a44ebaaa3f3ecfacd2744467957882eabd92cec
Author: Yang Shi <shy8...@gmail.com>
Date: Thu Jun 16 17:48:37 2022 +0000

mm: thp: kill transparent_hugepage_active()

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=166ba760080000
start commit: 34d1d36073ea Add linux-next specific files for 20220621
git tree: linux-next
final oops: https://syzkaller.appspot.com/x/report.txt?x=156ba760080000
console output: https://syzkaller.appspot.com/x/log.txt?x=116ba760080000

kernel config: https://syzkaller.appspot.com/x/.config?x=b24b62d1c051cfc8
dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14097a3ff00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1466c63ff00000

Reported-by: syzbot+4d875b...@syzkaller.appspotmail.com
Fixes: 0a44ebaaa3f3 ("mm: thp: kill transparent_hugepage_active()")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[Zach O'Keefe]

I think this is already fixed already with Andrew's fix in mm-unstable:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index df04c0057271..fbe02815c41a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -73,6 +73,9 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
unsigned long vm_flags,
bool smaps)
{
+ if (!vma->vm_mm)
+ return false;
+
if (!transhuge_vma_enabled(vma, vm_flags))
return false;

Which isn't included in the tree testing here. IMHO this is cleaner than messing
with an already subtle ordering.

[Yang Shi]

Yes, we just need to keep one. And I agree Andrew's fix is much simpler.

[syzbot]

Auto-closing this bug as obsolete.
No recent activity, existing reproducers are no longer triggering the issue.