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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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