GHSA-M65F-MMQM-4FQ6
Vulnerability from github – Published: 2024-08-22 03:31 – Updated: 2024-09-12 15:32In the Linux kernel, the following vulnerability has been resolved:
btrfs: get rid of warning on transaction commit when using flushoncommit
When using the flushoncommit mount option, during almost every transaction commit we trigger a warning from __writeback_inodes_sb_nr():
$ cat fs/fs-writeback.c: (...) static void __writeback_inodes_sb_nr(struct super_block *sb, ... { (...) WARN_ON(!rwsem_is_locked(&sb->s_umount)); (...) } (...)
The trace produced in dmesg looks like the following:
[947.473890] WARNING: CPU: 5 PID: 930 at fs/fs-writeback.c:2610 __writeback_inodes_sb_nr+0x7e/0xb3 [947.481623] Modules linked in: nfsd nls_cp437 cifs asn1_decoder cifs_arc4 fscache cifs_md4 ipmi_ssif [947.489571] CPU: 5 PID: 930 Comm: btrfs-transacti Not tainted 95.16.3-srb-asrock-00001-g36437ad63879 #186 [947.497969] RIP: 0010:__writeback_inodes_sb_nr+0x7e/0xb3 [947.502097] Code: 24 10 4c 89 44 24 18 c6 (...) [947.519760] RSP: 0018:ffffc90000777e10 EFLAGS: 00010246 [947.523818] RAX: 0000000000000000 RBX: 0000000000963300 RCX: 0000000000000000 [947.529765] RDX: 0000000000000000 RSI: 000000000000fa51 RDI: ffffc90000777e50 [947.535740] RBP: ffff888101628a90 R08: ffff888100955800 R09: ffff888100956000 [947.541701] R10: 0000000000000002 R11: 0000000000000001 R12: ffff888100963488 [947.547645] R13: ffff888100963000 R14: ffff888112fb7200 R15: ffff888100963460 [947.553621] FS: 0000000000000000(0000) GS:ffff88841fd40000(0000) knlGS:0000000000000000 [947.560537] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [947.565122] CR2: 0000000008be50c4 CR3: 000000000220c000 CR4: 00000000001006e0 [947.571072] Call Trace: [947.572354] [947.573266] btrfs_commit_transaction+0x1f1/0x998 [947.576785] ? start_transaction+0x3ab/0x44e [947.579867] ? schedule_timeout+0x8a/0xdd [947.582716] transaction_kthread+0xe9/0x156 [947.585721] ? btrfs_cleanup_transaction.isra.0+0x407/0x407 [947.590104] kthread+0x131/0x139 [947.592168] ? set_kthread_struct+0x32/0x32 [947.595174] ret_from_fork+0x22/0x30 [947.597561] [947.598553] ---[ end trace 644721052755541c ]---
This is because we started using writeback_inodes_sb() to flush delalloc when committing a transaction (when using -o flushoncommit), in order to avoid deadlocks with filesystem freeze operations. This change was made by commit ce8ea7cc6eb313 ("btrfs: don't call btrfs_start_delalloc_roots in flushoncommit"). After that change we started producing that warning, and every now and then a user reports this since the warning happens too often, it spams dmesg/syslog, and a user is unsure if this reflects any problem that might compromise the filesystem's reliability.
We can not just lock the sb->s_umount semaphore before calling writeback_inodes_sb(), because that would at least deadlock with filesystem freezing, since at fs/super.c:freeze_super() sync_filesystem() is called while we are holding that semaphore in write mode, and that can trigger a transaction commit, resulting in a deadlock. It would also trigger the same type of deadlock in the unmount path. Possibly, it could also introduce some other locking dependencies that lockdep would report.
To fix this call try_to_writeback_inodes_sb() instead of writeback_inodes_sb(), because that will try to read lock sb->s_umount and then will only call writeback_inodes_sb() if it was able to lock it. This is fine because the cases where it can't read lock sb->s_umount are during a filesystem unmount or during a filesystem freeze - in those cases sb->s_umount is write locked and sync_filesystem() is called, which calls writeback_inodes_sb(). In other words, in all cases where we can't take a read lock on sb->s_umount, writeback is already being triggered elsewhere.
An alternative would be to call btrfs_start_delalloc_roots() with a number of pages different from LONG_MAX, for example matching the number of delalloc bytes we currently have, in ---truncated---
{
"affected": [],
"aliases": [
"CVE-2022-48920"
],
"database_specific": {
"cwe_ids": [
"CWE-667"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-08-22T02:15:06Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nbtrfs: get rid of warning on transaction commit when using flushoncommit\n\nWhen using the flushoncommit mount option, during almost every transaction\ncommit we trigger a warning from __writeback_inodes_sb_nr():\n\n $ cat fs/fs-writeback.c:\n (...)\n static void __writeback_inodes_sb_nr(struct super_block *sb, ...\n {\n (...)\n WARN_ON(!rwsem_is_locked(\u0026sb-\u003es_umount));\n (...)\n }\n (...)\n\nThe trace produced in dmesg looks like the following:\n\n [947.473890] WARNING: CPU: 5 PID: 930 at fs/fs-writeback.c:2610 __writeback_inodes_sb_nr+0x7e/0xb3\n [947.481623] Modules linked in: nfsd nls_cp437 cifs asn1_decoder cifs_arc4 fscache cifs_md4 ipmi_ssif\n [947.489571] CPU: 5 PID: 930 Comm: btrfs-transacti Not tainted 95.16.3-srb-asrock-00001-g36437ad63879 #186\n [947.497969] RIP: 0010:__writeback_inodes_sb_nr+0x7e/0xb3\n [947.502097] Code: 24 10 4c 89 44 24 18 c6 (...)\n [947.519760] RSP: 0018:ffffc90000777e10 EFLAGS: 00010246\n [947.523818] RAX: 0000000000000000 RBX: 0000000000963300 RCX: 0000000000000000\n [947.529765] RDX: 0000000000000000 RSI: 000000000000fa51 RDI: ffffc90000777e50\n [947.535740] RBP: ffff888101628a90 R08: ffff888100955800 R09: ffff888100956000\n [947.541701] R10: 0000000000000002 R11: 0000000000000001 R12: ffff888100963488\n [947.547645] R13: ffff888100963000 R14: ffff888112fb7200 R15: ffff888100963460\n [947.553621] FS: 0000000000000000(0000) GS:ffff88841fd40000(0000) knlGS:0000000000000000\n [947.560537] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033\n [947.565122] CR2: 0000000008be50c4 CR3: 000000000220c000 CR4: 00000000001006e0\n [947.571072] Call Trace:\n [947.572354] \u003cTASK\u003e\n [947.573266] btrfs_commit_transaction+0x1f1/0x998\n [947.576785] ? start_transaction+0x3ab/0x44e\n [947.579867] ? schedule_timeout+0x8a/0xdd\n [947.582716] transaction_kthread+0xe9/0x156\n [947.585721] ? btrfs_cleanup_transaction.isra.0+0x407/0x407\n [947.590104] kthread+0x131/0x139\n [947.592168] ? set_kthread_struct+0x32/0x32\n [947.595174] ret_from_fork+0x22/0x30\n [947.597561] \u003c/TASK\u003e\n [947.598553] ---[ end trace 644721052755541c ]---\n\nThis is because we started using writeback_inodes_sb() to flush delalloc\nwhen committing a transaction (when using -o flushoncommit), in order to\navoid deadlocks with filesystem freeze operations. This change was made\nby commit ce8ea7cc6eb313 (\"btrfs: don\u0027t call btrfs_start_delalloc_roots\nin flushoncommit\"). After that change we started producing that warning,\nand every now and then a user reports this since the warning happens too\noften, it spams dmesg/syslog, and a user is unsure if this reflects any\nproblem that might compromise the filesystem\u0027s reliability.\n\nWe can not just lock the sb-\u003es_umount semaphore before calling\nwriteback_inodes_sb(), because that would at least deadlock with\nfilesystem freezing, since at fs/super.c:freeze_super() sync_filesystem()\nis called while we are holding that semaphore in write mode, and that can\ntrigger a transaction commit, resulting in a deadlock. It would also\ntrigger the same type of deadlock in the unmount path. Possibly, it could\nalso introduce some other locking dependencies that lockdep would report.\n\nTo fix this call try_to_writeback_inodes_sb() instead of\nwriteback_inodes_sb(), because that will try to read lock sb-\u003es_umount\nand then will only call writeback_inodes_sb() if it was able to lock it.\nThis is fine because the cases where it can\u0027t read lock sb-\u003es_umount\nare during a filesystem unmount or during a filesystem freeze - in those\ncases sb-\u003es_umount is write locked and sync_filesystem() is called, which\ncalls writeback_inodes_sb(). In other words, in all cases where we can\u0027t\ntake a read lock on sb-\u003es_umount, writeback is already being triggered\nelsewhere.\n\nAn alternative would be to call btrfs_start_delalloc_roots() with a\nnumber of pages different from LONG_MAX, for example matching the number\nof delalloc bytes we currently have, in \n---truncated---",
"id": "GHSA-m65f-mmqm-4fq6",
"modified": "2024-09-12T15:32:59Z",
"published": "2024-08-22T03:31:34Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2022-48920"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/850a77c999b81dd2724efd2684068d6f90db8c16"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a0f0cf8341e34e5d2265bfd3a7ad68342da1e2aa"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e4d044dbffcd570351f21c747fc77ff90aed7f2e"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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