Re: [syzbot] [io-uring?] KASAN: null-ptr-deref Read in io_sqe_buffer_register

[David Hildenbrand <david@redhat.com>]

On 05.09.25 01:20, Jens Axboe wrote:
> On 9/4/25 9:36 AM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    4ac65880ebca Add linux-next specific files for 20250904
>> git tree:       linux-next
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1785fe62580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=fbc16d9faf3a88a4
>> dashboard link: https://syzkaller.appspot.com/bug?extid=1ab243d3eebb2aabf4a4
>> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13f23e62580000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12cb6312580000
>>
>> Downloadable assets:
>> disk image: https://storage.googleapis.com/syzbot-assets/36645a51612c/disk-4ac65880.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/bba80d634bef/vmlinux-4ac65880.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/e58dd70dfd0f/bzImage-4ac65880.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+1ab243d3eebb2aabf4a4@syzkaller.appspotmail.com
>>
>> ==================================================================
>> BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:68 [inline]
>> BUG: KASAN: null-ptr-deref in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
>> BUG: KASAN: null-ptr-deref in PageCompound include/linux/page-flags.h:331 [inline]
>> BUG: KASAN: null-ptr-deref in io_buffer_account_pin io_uring/rsrc.c:668 [inline]
>> BUG: KASAN: null-ptr-deref in io_sqe_buffer_register+0x369/0x20a0 io_uring/rsrc.c:817
>> Read of size 8 at addr 0000000000000000 by task syz.0.17/6020
>>
>> CPU: 0 UID: 0 PID: 6020 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
>> Call Trace:
>>   <TASK>
>>   dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>>   kasan_report+0x118/0x150 mm/kasan/report.c:595
>>   check_region_inline mm/kasan/generic.c:-1 [inline]
>>   kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:200
>>   instrument_atomic_read include/linux/instrumented.h:68 [inline]
>>   _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
>>   PageCompound include/linux/page-flags.h:331 [inline]
>>   io_buffer_account_pin io_uring/rsrc.c:668 [inline]
>>   io_sqe_buffer_register+0x369/0x20a0 io_uring/rsrc.c:817
>>   __io_sqe_buffers_update io_uring/rsrc.c:322 [inline]
>>   __io_register_rsrc_update+0x55e/0x11b0 io_uring/rsrc.c:360
>>   io_register_rsrc_update+0x196/0x1a0 io_uring/rsrc.c:391
>>   __io_uring_register io_uring/register.c:736 [inline]
>>   __do_sys_io_uring_register io_uring/register.c:926 [inline]
>>   __se_sys_io_uring_register+0x795/0x11b0 io_uring/register.c:903
>>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>   do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>> RIP: 0033:0x7f99b1f8ebe9
>> 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 a8 ff ff ff f7 d8 64 89 01 48
>> RSP: 002b:00007f99b2d88038 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
>> RAX: ffffffffffffffda RBX: 00007f99b21c5fa0 RCX: 00007f99b1f8ebe9
>> RDX: 00002000000003c0 RSI: 0000000000000010 RDI: 0000000000000003
>> RBP: 00007f99b2011e19 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000020 R11: 0000000000000246 R12: 0000000000000000
>> R13: 00007f99b21c6038 R14: 00007f99b21c5fa0 R15: 00007ffeadfa5958
>>   </TASK>
>> ==================================================================

#syz test

 From bfd07c995814354f6b66c5b6a72e96a7aa9fb73b Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Fri, 5 Sep 2025 08:38:43 +0200
Subject: [PATCH] fixup: mm/gup: remove record_subpages()

pages is not adjusted by the caller, but idnexed by existing *nr.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
  mm/gup.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 010fe56f6e132..22420f2069ee1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2981,6 +2981,7 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t 
*pmdp, unsigned long addr,
  		return 0;
  	}

+	pages += *nr;
  	*nr += refs;
  	for (; refs; refs--)
  		*(pages++) = page++;
@@ -3024,6 +3025,7 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t 
*pudp, unsigned long addr,
  		return 0;
  	}

+	pages += *nr;
  	*nr += refs;
  	for (; refs; refs--)
  		*(pages++) = page++;
-- 
2.50.1


-- 
Cheers

David / dhildenb