From cf28f3bbfca097d956f9021cb710dfad56adcc62 Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Mon, 17 Aug 2020 10:42:14 -0700 Subject: [PATCH] bpf: Use get_file_rcu() instead of get_file() for task_file iterator With latest `bpftool prog` command, we observed the following kernel panic. BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD dfe894067 P4D dfe894067 PUD deb663067 PMD 0 Oops: 0010 [#1] SMP CPU: 9 PID: 6023 ... RIP: 0010:0x0 Code: Bad RIP value. RSP: 0000:ffffc900002b8f18 EFLAGS: 00010286 RAX: ffff8883a405f400 RBX: ffff888e46a6bf00 RCX: 000000008020000c RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8883a405f400 RBP: ffff888e46a6bf50 R08: 0000000000000000 R09: ffffffff81129600 R10: ffff8883a405f300 R11: 0000160000000000 R12: 0000000000002710 R13: 000000e9494b690c R14: 0000000000000202 R15: 0000000000000009 FS: 00007fd9187fe700(0000) GS:ffff888e46a40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: ffffffffffffffd6 CR3: 0000000de5d33002 CR4: 0000000000360ee0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: rcu_core+0x1a4/0x440 __do_softirq+0xd3/0x2c8 irq_exit+0x9d/0xa0 smp_apic_timer_interrupt+0x68/0x120 apic_timer_interrupt+0xf/0x20 RIP: 0033:0x47ce80 Code: Bad RIP value. RSP: 002b:00007fd9187fba40 EFLAGS: 00000206 ORIG_RAX: ffffffffffffff13 RAX: 0000000000000002 RBX: 00007fd931789160 RCX: 000000000000010c RDX: 00007fd9308cdfb4 RSI: 00007fd9308cdfb4 RDI: 00007ffedd1ea0a8 RBP: 00007fd9187fbab0 R08: 000000000000000e R09: 000000000000002a R10: 0000000000480210 R11: 00007fd9187fc570 R12: 00007fd9316cc400 R13: 0000000000000118 R14: 00007fd9308cdfb4 R15: 00007fd9317a9380 After further analysis, the bug is triggered by Commit eaaacd23910f ("bpf: Add task and task/file iterator targets") which introduced task_file bpf iterator, which traverses all open file descriptors for all tasks in the current namespace. The latest `bpftool prog` calls a task_file bpf program to traverse all files in the system in order to associate processes with progs/maps, etc. When traversing files for a given task, rcu read_lock is taken to access all files in a file_struct. But it used get_file() to grab a file, which is not right. It is possible file->f_count is 0 and get_file() will unconditionally increase it. Later put_file() may cause all kind of issues with the above as one of sympotoms. The failure can be reproduced with the following steps in a few seconds: $ cat t.c #include #include #include #include #include #define N 10000 int fd[N]; int main() { int i; for (i = 0; i < N; i++) { fd[i] = open("./note.txt", 'r'); if (fd[i] < 0) { fprintf(stderr, "failed\n"); return -1; } } for (i = 0; i < N; i++) close(fd[i]); return 0; } $ gcc -O2 t.c $ cat run.sh #/bin/bash for i in {1..100} do while true; do ./a.out; done & done $ ./run.sh $ while true; do bpftool prog >& /dev/null; done This patch used get_file_rcu() which only grabs a file if the file->f_count is not zero. This is to ensure the file pointer is always valid. The above reproducer did not fail for more than 30 minutes. Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets") Suggested-by: Josef Bacik Signed-off-by: Yonghong Song Signed-off-by: Alexei Starovoitov Reviewed-by: Josef Bacik Link: https://lore.kernel.org/bpf/20200817174214.252601-1-yhs@fb.com --- kernel/bpf/task_iter.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c index 232df29793e9..f21b5e1e4540 100644 --- a/kernel/bpf/task_iter.c +++ b/kernel/bpf/task_iter.c @@ -178,10 +178,11 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info, f = fcheck_files(curr_files, curr_fd); if (!f) continue; + if (!get_file_rcu(f)) + continue; /* set info->fd */ info->fd = curr_fd; - get_file(f); rcu_read_unlock(); return f; }