On 11/8/19 5:16 AM, syzbot wrote: > Hello, > > syzbot found the following crash on: > > HEAD commit: 05f22368 x86, kcsan: Enable KCSAN for x86 > git tree: https://github.com/google/ktsan.git kcsan > console output: https://syzkaller.appspot.com/x/log.txt?x=10d7fd88e00000 > kernel config: https://syzkaller.appspot.com/x/.config?x=87d111955f40591f > dashboard link: https://syzkaller.appspot.com/bug?extid=3ef049d50587836c0606 > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > Unfortunately, I don't have any reproducer for this crash yet. > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > Reported-by: syzbot+3ef049d50587836c0606@xxxxxxxxxxxxxxxxxxxxxxxxx > > ================================================================== > BUG: KCSAN: data-race in __alloc_file / __alloc_file > > write to 0xffff8880bb157398 of 4 bytes by task 10993 on cpu 0: > get_cred include/linux/cred.h:253 [inline] > __alloc_file+0x74/0x210 fs/file_table.c:105 > alloc_empty_file+0x8f/0x180 fs/file_table.c:151 > alloc_file+0x4e/0x2b0 fs/file_table.c:193 > alloc_file_pseudo+0x11c/0x1b0 fs/file_table.c:232 > anon_inode_getfile fs/anon_inodes.c:91 [inline] > anon_inode_getfile+0x103/0x1d0 fs/anon_inodes.c:74 > __do_sys_perf_event_open+0xd32/0x1ac0 kernel/events/core.c:11100 > __se_sys_perf_event_open kernel/events/core.c:10867 [inline] > __x64_sys_perf_event_open+0x70/0x90 kernel/events/core.c:10867 > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > write to 0xffff8880bb157398 of 4 bytes by task 11004 on cpu 1: > get_cred include/linux/cred.h:253 [inline] > __alloc_file+0x74/0x210 fs/file_table.c:105 > alloc_empty_file+0x8f/0x180 fs/file_table.c:151 > path_openat+0x74/0x36e0 fs/namei.c:3514 > do_filp_open+0x11e/0x1b0 fs/namei.c:3555 > do_sys_open+0x3b3/0x4f0 fs/open.c:1097 > __do_sys_open fs/open.c:1115 [inline] > __se_sys_open fs/open.c:1110 [inline] > __x64_sys_open+0x55/0x70 fs/open.c:1110 > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Reported by Kernel Concurrency Sanitizer on: > CPU: 1 PID: 11004 Comm: syz-executor.5 Not tainted 5.4.0-rc3+ #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 > ================================================================== > > > --- > This bug is generated by a bot. It may contain errors. > See https://goo.gl/tpsmEJ for more information about syzbot. > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx. > > syzbot will keep track of this bug report. See: > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. Linus, what do you think of the following fix ? I also took the opportunity avoiding dirtying a cache line if this was possible. diff --git a/include/linux/cred.h b/include/linux/cred.h index 18639c069263fbe79dfd5a36163c656dca5da220..01b5b7d4e054ddca0df676dc1ceb068e5d71a3f8 100644 --- a/include/linux/cred.h +++ b/include/linux/cred.h @@ -250,7 +250,14 @@ static inline const struct cred *get_cred(const struct cred *cred) if (!cred) return cred; validate_creds(cred); - nonconst_cred->non_rcu = 0; + + /* + * Avoid dirtying one cache line. The WRITE_ONCE() also pairs + * with itself, since we run without protection of a lock. + */ + if (READ_ONCE(nonconst_cred->non_rcu)) + WRITE_ONCE(nonconst_cred->non_rcu, 0); + return get_new_cred(nonconst_cred); } @@ -262,7 +269,14 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred) if (!atomic_inc_not_zero(&nonconst_cred->usage)) return NULL; validate_creds(cred); - nonconst_cred->non_rcu = 0; + + /* + * Avoid dirtying one cache line. The WRITE_ONCE() also pairs + * with itself, since we run without protection of a lock. + */ + if (READ_ONCE(nonconst_cred->non_rcu)) + WRITE_ONCE(nonconst_cred->non_rcu, 0); + return cred; }
