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Message-ID: <1dddb237-1460-8122-7caf-f0acd7c91b5c@igalia.com>
Date: Tue, 17 Dec 2024 17:01:17 +0530
From: Bhupesh Sharma <bhsharma@...lia.com>
To: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>,
Bhupesh <bhupesh@...lia.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, kernel-dev@...lia.com,
linux-kernel@...r.kernel.org, revest@...gle.com, adilger.kernel@...ger.ca,
syzbot+b244bda78289b00204ed@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] fs/ext4/xattr: Check for 'xattr_sem' inside
'ext4_xattr_delete_inode'
Hi Cascardo,
Thanks for looking at the patch.
On 12/12/24 11:05 PM, Thadeu Lima de Souza Cascardo wrote:
> On Thu, Dec 12, 2024 at 12:23:31PM +0530, Bhupesh wrote:
>> Once we are inside the 'ext4_xattr_delete_inode' function and trying
>> to delete the inode, the 'xattr_sem' should be unlocked.
>>
>> We need trylock here to avoid false-positive warning from lockdep
>> about reclaim circular dependency.
>>
>> This fixes the following KASAN reported issue:
>>
>> ==================================================================
>> BUG: KASAN: slab-use-after-free in ext4_xattr_inode_dec_ref_all+0xb8c/0xe90
>> Read of size 4 at addr ffff888012c120c4 by task repro/2065
>>
>> CPU: 1 UID: 0 PID: 2065 Comm: repro Not tainted 6.13.0-rc2+ #11
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
>> Call Trace:
>> <TASK>
>> dump_stack_lvl+0x1fd/0x300
>> ? tcp_gro_dev_warn+0x260/0x260
>> ? _printk+0xc0/0x100
>> ? read_lock_is_recursive+0x10/0x10
>> ? irq_work_queue+0x72/0xf0
>> ? __virt_addr_valid+0x17b/0x4b0
>> print_address_description+0x78/0x390
>> print_report+0x107/0x1f0
>> ? __virt_addr_valid+0x17b/0x4b0
>> ? __virt_addr_valid+0x3ff/0x4b0
>> ? __phys_addr+0xb5/0x160
>> ? ext4_xattr_inode_dec_ref_all+0xb8c/0xe90
>> kasan_report+0xcc/0x100
>> ? ext4_xattr_inode_dec_ref_all+0xb8c/0xe90
>> ext4_xattr_inode_dec_ref_all+0xb8c/0xe90
>> ? ext4_xattr_delete_inode+0xd30/0xd30
>> ? __ext4_journal_ensure_credits+0x5f0/0x5f0
>> ? __ext4_journal_ensure_credits+0x2b/0x5f0
>> ? inode_update_timestamps+0x410/0x410
>> ext4_xattr_delete_inode+0xb64/0xd30
>> ? ext4_truncate+0xb70/0xdc0
>> ? ext4_expand_extra_isize_ea+0x1d20/0x1d20
>> ? __ext4_mark_inode_dirty+0x670/0x670
>> ? ext4_journal_check_start+0x16f/0x240
>> ? ext4_inode_is_fast_symlink+0x2f2/0x3a0
>> ext4_evict_inode+0xc8c/0xff0
>> ? ext4_inode_is_fast_symlink+0x3a0/0x3a0
>> ? do_raw_spin_unlock+0x53/0x8a0
>> ? ext4_inode_is_fast_symlink+0x3a0/0x3a0
>> evict+0x4ac/0x950
>> ? proc_nr_inodes+0x310/0x310
>> ? trace_ext4_drop_inode+0xa2/0x220
>> ? _raw_spin_unlock+0x1a/0x30
>> ? iput+0x4cb/0x7e0
>> do_unlinkat+0x495/0x7c0
>> ? try_break_deleg+0x120/0x120
>> ? 0xffffffff81000000
>> ? __check_object_size+0x15a/0x210
>> ? strncpy_from_user+0x13e/0x250
>> ? getname_flags+0x1dc/0x530
>> __x64_sys_unlinkat+0xc8/0xf0
>> do_syscall_64+0x65/0x110
>> entry_SYSCALL_64_after_hwframe+0x67/0x6f
>> RIP: 0033:0x434ffd
>> Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 8
>> RSP: 002b:00007ffc50fa7b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000107
>> RAX: ffffffffffffffda RBX: 00007ffc50fa7e18 RCX: 0000000000434ffd
>> RDX: 0000000000000000 RSI: 0000000020000240 RDI: 0000000000000005
>> RBP: 00007ffc50fa7be0 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
>> R13: 00007ffc50fa7e08 R14: 00000000004bbf30 R15: 0000000000000001
>> </TASK>
>>
>> The buggy address belongs to the object at ffff888012c12000
>> which belongs to the cache filp of size 360
>> The buggy address is located 196 bytes inside of
>> freed 360-byte region [ffff888012c12000, ffff888012c12168)
>>
>> The buggy address belongs to the physical page:
>> page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x12c12
>> head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
>> flags: 0x40(head|node=0|zone=0)
>> page_type: f5(slab)
>> raw: 0000000000000040 ffff888000ad7640 ffffea0000497a00 dead000000000004
>> raw: 0000000000000000 0000000000100010 00000001f5000000 0000000000000000
>> head: 0000000000000040 ffff888000ad7640 ffffea0000497a00 dead000000000004
>> head: 0000000000000000 0000000000100010 00000001f5000000 0000000000000000
>> head: 0000000000000001 ffffea00004b0481 ffffffffffffffff 0000000000000000
>> head: 0000000000000002 0000000000000000 00000000ffffffff 0000000000000000
>> page dumped because: kasan: bad access detected
>>
>> Memory state around the buggy address:
>> ffff888012c11f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> ffff888012c12000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>> ffff888012c12080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> ^
>> ffff888012c12100: fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc
>> ffff888012c12180: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>> ==================================================================
>>
>> Reported-by: syzbot+b244bda78289b00204ed@...kaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=b244bda78289b00204ed
>> Signed-off-by: Bhupesh <bhupesh@...lia.com>
>> ---
>> fs/ext4/xattr.c | 16 +++++++++++++++-
>> 1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
>> index 7647e9f6e190..e1d29aa76165 100644
>> --- a/fs/ext4/xattr.c
>> +++ b/fs/ext4/xattr.c
>> @@ -2926,7 +2926,20 @@ int ext4_xattr_delete_inode(handle_t *handle, struct inode *inode,
>> struct ext4_iloc iloc = { .bh = NULL };
>> struct ext4_xattr_entry *entry;
>> struct inode *ea_inode;
>> - int error;
>> + int error = 0;
>> +
>> + /*
>> + * We are the only ones holding inode reference. The xattr_sem should
>> + * better be unlocked! We could as well just not acquire xattr_sem at
>> + * all but this makes the code more futureproof. OTOH we need trylock
>> + * here to avoid false-positive warning from lockdep about reclaim
>> + * circular dependency.
>> + */
>> + if (WARN_ON_ONCE(!down_write_trylock(&EXT4_I(inode)->xattr_sem)))
>> + return error;
>> +
>> + if (!EXT4_I(inode)->i_file_acl)
>> + goto cleanup;
>>
> This is ignoring the case where attributes are all in the inode, not in the
> separate ACL block.
>
> That's why this apparently fixes the problem. ext4_xattr_inode_dec_ref_all
> is never called anymore.
Ok, I got your point, I will try to use existing inode checking
functions instead, to check for the 'attributes present in inode' case.
However, as we were discussing on irc, I think it would make sense still
to have the 'EXT4_I(inode)->xattr_sem' held while we delete the inode
inside '
ext4_xattr_delete_inode'. Please let me know in case you have other ideas.
I will send two patches then - one fixing the inode attribute issue and the other for holding the xattr_sem while we are inside the critical region deleting the inode.
Thanks,
Bhupesh
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