Re: [PATCH 1/1] fs/ext4/xattr: Check for 'xattr_sem' inside 'ext4_xattr_delete_inode'

[Thadeu Lima de Souza Cascardo <cascardo@igalia.com>]

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@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b244bda78289b00204ed
> Signed-off-by: Bhupesh <bhupesh@igalia.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.

Cascardo.

>  	error = ext4_journal_ensure_credits(handle, extra_credits,
>  			ext4_free_metadata_revoke_credits(inode->i_sb, 1));
> @@ -3015,6 +3028,7 @@ int ext4_xattr_delete_inode(handle_t *handle, struct inode *inode,
>  cleanup:
>  	brelse(iloc.bh);
>  	brelse(bh);
> +	up_write(&EXT4_I(inode)->xattr_sem);
>  	return error;
>  }
>  
> -- 
> 2.38.1
> 
>