Re: [PATCH] fuse: check file before running readpage

[Gao Xiang]

On 2024/11/14 10:04, Lizhi Xu wrote:
> syzbot reported a null-ptr-deref in fuse_read_args_fill. [1]
>
> About this case, calltrace is:
> erofs_read_superblock()->
>    erofs_read_metabuf()->
>      erofs_bread()->
>        read_mapping_folio()->
>          do_read_cache_folio()->
>            filemap_read_folio()->
>              fuse_read_folio()->
>                fuse_do_readpage()->
>                  fuse_read_args_fill()
>
> erofs_bread() calls read_mapping_folio() passing NULL file, which causes a
> NULL pointer dereference in fuse_read_args_fill.
> To avoid this issue, need to add a check for file in fuse_read_folio().
>
> [1]
> KASAN: null-ptr-deref in range [0x0000000000000060-0x0000000000000067]
> CPU: 3 UID: 0 PID: 5947 Comm: syz-executor314 Not tainted 6.12.0-rc5-syzkaller-00044-gc1e939a21eb1 #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:fuse_read_args_fill fs/fuse/file.c:631 [inline]
> RIP: 0010:fuse_do_readpage+0x276/0x640 fs/fuse/file.c:880
> Code: e8 9f c7 91 fe 8b 44 24 10 89 44 24 78 41 89 c4 e8 8f c7 91 fe 48 8d 7b 60 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 1d 03 00 00 48 b8 00 00 00 00 00 fc ff df 48 8b
> RSP: 0018:ffffc90006a0f820 EFLAGS: 00010206
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff82fbb4c9
> RDX: 000000000000000c RSI: ffffffff82fbb4f1 RDI: 0000000000000060
> RBP: 0000000000000000 R08: 0000000000000007 R09: 7fffffffffffffff
> R10: 0000000000000fff R11: ffffffff961d4b88 R12: 0000000000001000
> R13: ffff8880382b8000 R14: ffff888025153780 R15: ffffc90006a0f8b8
> FS:  00007f7583f3d6c0(0000) GS:ffff88806a900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000240 CR3: 0000000030d30000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   <TASK>
>   fuse_read_folio+0xb0/0x100 fs/fuse/file.c:905
>   filemap_read_folio+0xc6/0x2a0 mm/filemap.c:2367
>   do_read_cache_folio+0x263/0x5c0 mm/filemap.c:3825
>   read_mapping_folio include/linux/pagemap.h:1011 [inline]
>   erofs_bread+0x34d/0x7e0 fs/erofs/data.c:41
>   erofs_read_superblock fs/erofs/super.c:281 [inline]
>   erofs_fc_fill_super+0x2b9/0x2500 fs/erofs/super.c:625
>   vfs_get_super fs/super.c:1280 [inline]
>   get_tree_nodev+0xda/0x190 fs/super.c:1299
>   erofs_fc_get_tree+0x1fe/0x2e0 fs/erofs/super.c:723
>   vfs_get_tree+0x8f/0x380 fs/super.c:1800
>   do_new_mount fs/namespace.c:3507 [inline]
>   path_mount+0x14e6/0x1f20 fs/namespace.c:3834
>   do_mount fs/namespace.c:3847 [inline]
>   __do_sys_mount fs/namespace.c:4057 [inline]
>   __se_sys_mount fs/namespace.c:4034 [inline]
>   __x64_sys_mount+0x294/0x320 fs/namespace.c:4034
>   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>   do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Reported-and-tested-by: syzbot+0b1279812c46e48bb0c1@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=0b1279812c46e48bb0c1
> Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>

Most filesystems (except for some network fses and FUSE)
won't use `file` at all and as documented in
https://docs.kernel.org/filesystems/vfs.html

But I will pass in `file` for this issue instead in order
to support use cases over FUSE, other than just bail out
like this.

Thanks,
Gao Xiang