Hi, On Thu, 17 Oct 2019 at 14:36, syzbot <syzbot+e392f8008a294fdf8891@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > Hello, > > syzbot found the following crash on: > > HEAD commit: d724f94f 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=17884db3600000 > kernel config: https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80 > dashboard link: https://syzkaller.appspot.com/bug?extid=e392f8008a294fdf8891 > 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+e392f8008a294fdf8891@xxxxxxxxxxxxxxxxxxxxxxxxx > > ================================================================== > BUG: KCSAN: data-race in task_dump_owner / task_dump_owner > > write to 0xffff8881255bb7fc of 4 bytes by task 7804 on cpu 0: > task_dump_owner+0xd8/0x260 fs/proc/base.c:1742 > pid_update_inode+0x3c/0x70 fs/proc/base.c:1818 > pid_revalidate+0x91/0xd0 fs/proc/base.c:1841 > d_revalidate fs/namei.c:765 [inline] > d_revalidate fs/namei.c:762 [inline] > lookup_fast+0x7cb/0x7e0 fs/namei.c:1613 > walk_component+0x6d/0xe80 fs/namei.c:1804 > link_path_walk.part.0+0x5d3/0xa90 fs/namei.c:2139 > link_path_walk fs/namei.c:2070 [inline] > path_openat+0x14f/0x3530 fs/namei.c:3532 > do_filp_open+0x11e/0x1b0 fs/namei.c:3563 > do_sys_open+0x3b3/0x4f0 fs/open.c:1089 > __do_sys_open fs/open.c:1107 [inline] > __se_sys_open fs/open.c:1102 [inline] > __x64_sys_open+0x55/0x70 fs/open.c:1102 > do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > write to 0xffff8881255bb7fc of 4 bytes by task 7813 on cpu 1: > task_dump_owner+0xd8/0x260 fs/proc/base.c:1742 > pid_update_inode+0x3c/0x70 fs/proc/base.c:1818 > pid_revalidate+0x91/0xd0 fs/proc/base.c:1841 > d_revalidate fs/namei.c:765 [inline] > d_revalidate fs/namei.c:762 [inline] > lookup_fast+0x7cb/0x7e0 fs/namei.c:1613 > walk_component+0x6d/0xe80 fs/namei.c:1804 > lookup_last fs/namei.c:2271 [inline] > path_lookupat.isra.0+0x13a/0x5a0 fs/namei.c:2316 > filename_lookup+0x145/0x2d0 fs/namei.c:2346 > user_path_at_empty+0x4c/0x70 fs/namei.c:2606 > user_path_at include/linux/namei.h:60 [inline] > vfs_statx+0xd9/0x190 fs/stat.c:187 > vfs_stat include/linux/fs.h:3188 [inline] > __do_sys_newstat+0x51/0xb0 fs/stat.c:341 > __se_sys_newstat fs/stat.c:337 [inline] > __x64_sys_newstat+0x3a/0x50 fs/stat.c:337 > do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Reported by Kernel Concurrency Sanitizer on: > CPU: 1 PID: 7813 Comm: ps Not tainted 5.3.0+ #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > Google 01/01/2011 > ================================================================== My understanding is, that for every access to /proc/<pid>, d_revalidate is called, and /proc-fs implementation simply says that pid_revalidate always revalidates by rewriting uid/gid because "owning task may have performed a setuid(), etc." presumably so every access to a /proc/<pid> entry always has the right uid/gid (in effect updating /proc/<pid> lazily via d_revalidate). Is it possible that one of the tasks above could be preempted after doing its writes to *ruid/*rgid, another thread writing some other values (after setuid / seteuid), and then the preempted thread seeing the other values? Assertion here should never fail: === TASK 1 === | seteuid(1000); | seteuid(0); | stat("/proc/<pid-of-task-1>", &fstat); | assert(fstat.st_uid == 0); === TASK 2 === | stat("/proc/<pid-of-task-1>", ...); Best Wishes, -- Marco
