linux-kernel - Re: [syzbot] [fuse?] KASAN: slab-out-of-bounds Write in fuse_dev_do

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Message-ID: <CAJnrk1bSD+HfwLqbFv8gsRsPt0kRsr8JZcEXdqBWuKh2Qnz_yA@mail.gmail.com>
Date: Fri, 22 Aug 2025 17:53:57 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: syzbot <syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	miklos@...redi.hu, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [fuse?] KASAN: slab-out-of-bounds Write in fuse_dev_do_write

On Fri, Aug 22, 2025 at 4:24 PM syzbot
<syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    cf6fc5eefc5b Merge tag 's390-6.17-3' of git://git.kernel.o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15348c42580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
> dashboard link: https://syzkaller.appspot.com/bug?extid=2d215d165f9354b9c4ea
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=147a5062580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=139caa34580000
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-cf6fc5ee.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/a27518272e48/vmlinux-cf6fc5ee.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/cf3f4cc06dfd/bzImage-cf6fc5ee.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2d215d165f9354b9c4ea@...kaller.appspotmail.com
>
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in fuse_retrieve fs/fuse/dev.c:1911 [inline]
> BUG: KASAN: slab-out-of-bounds in fuse_notify_retrieve fs/fuse/dev.c:1959 [inline]
> BUG: KASAN: slab-out-of-bounds in fuse_notify fs/fuse/dev.c:2067 [inline]
> BUG: KASAN: slab-out-of-bounds in fuse_dev_do_write+0x308b/0x3420 fs/fuse/dev.c:2158
> Write of size 4 at addr ffff88803b8fc6dc by task syz.0.17/6135
>
> CPU: 0 UID: 0 PID: 6135 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:94 [inline]
>  dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xcd/0x630 mm/kasan/report.c:482
>  kasan_report+0xe0/0x110 mm/kasan/report.c:595
>  fuse_retrieve fs/fuse/dev.c:1911 [inline]
>  fuse_notify_retrieve fs/fuse/dev.c:1959 [inline]
>  fuse_notify fs/fuse/dev.c:2067 [inline]
>  fuse_dev_do_write+0x308b/0x3420 fs/fuse/dev.c:2158
>  fuse_dev_write+0x155/0x1e0 fs/fuse/dev.c:2242
>  new_sync_write fs/read_write.c:593 [inline]
>  vfs_write+0x7d3/0x11d0 fs/read_write.c:686
>  ksys_write+0x12a/0x250 fs/read_write.c:738
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f440eb8ebe9
> 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:00007f440f9e1038 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 00007f440edb5fa0 RCX: 00007f440eb8ebe9
> RDX: 0000000000000030 RSI: 0000200000000140 RDI: 0000000000000004
> RBP: 00007f440ec11e19 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f440edb6038 R14: 00007f440edb5fa0 R15: 00007ffddcd08da8
>  </TASK>

Thanks for the report. I think the issue arises in cases where the
calculation for num_pages has to get rounded down to fc->max_pages and
a non-zero offset is passed in.

> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.

#syz test: upstream cf6fc5eefc5bbbbff92a085039ff74cdbd065c29

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e80cd8f2c049..e84e05de9cdb 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1893,7 +1893,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

        index = outarg->offset >> PAGE_SHIFT;

-       while (num) {
+       while (num && num_pages) {
                struct folio *folio;
                unsigned int folio_offset;
                unsigned int nr_bytes;
@@ -1914,6 +1914,7 @@ static int fuse_retrieve(struct fuse_mount *fm,
struct inode *inode,

                offset = 0;
                num -= nr_bytes;
+               num_pages -= nr_pages;
                total_len += nr_bytes;
                index += nr_pages;
        }