From: Alexey Dobriyan adobriyan@gmail.com Subject: proc: fix coredump vs read /proc/*/stat race
do_task_stat() accesses IP and SP of a task without bumping reference count of a stack (which became an entity with independent lifetime at some point).
Steps to reproduce:
#include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <sys/time.h> #include <sys/resource.h> #include <unistd.h> #include <sys/wait.h>
int main(void) { setrlimit(RLIMIT_CORE, &(struct rlimit){});
while (1) { char buf[64]; char buf2[4096]; pid_t pid; int fd;
pid = fork(); if (pid == 0) { *(volatile int *)0 = 0; }
snprintf(buf, sizeof(buf), "/proc/%u/stat", pid); fd = open(buf, O_RDONLY); read(fd, buf2, sizeof(buf2)); close(fd);
waitpid(pid, NULL, 0); } return 0; }
BUG: unable to handle kernel paging request at 0000000000003fd8 IP: do_task_stat+0x8b4/0xaf0 PGD 800000003d73e067 P4D 800000003d73e067 PUD 3d558067 PMD 0 Oops: 0000 [#1] PREEMPT SMP PTI CPU: 0 PID: 1417 Comm: a.out Not tainted 4.15.0-rc8-dirty #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1.fc27 04/01/2014 RIP: 0010:do_task_stat+0x8b4/0xaf0 RSP: 0018:ffffc90000607cc8 EFLAGS: 00010202 RAX: 0000000000000000 RBX: ffff88003e0e6680 RCX: ffff88003d6bd500 RDX: 0000000000000001 RSI: 000000000000000d RDI: ffff88003c1f8000 RBP: ffff88003c1f8000 R08: 0000000000000001 R09: 0000000000001000 R10: 00007f4d79555460 R11: ffff88003c325800 R12: ffff88003d7643c0 R13: ffffffff81a28f80 R14: 0000000000000001 R15: 0000000000000001 FS: 00007f4d79772700(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000003fd8 CR3: 000000003d9c0000 CR4: 00000000000006b0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: ? flush_tlb_mm_range+0x8b/0x100 proc_single_show+0x43/0x70 seq_read+0xe6/0x3b0 __vfs_read+0x1e/0x120 ? __check_object_size+0xa4/0x180 vfs_read+0x84/0x110 SyS_read+0x3d/0xa0 entry_SYSCALL_64_fastpath+0x13/0x6c RIP: 0033:0x7f4d7928cba0 RSP: 002b:00007ffddb245158 EFLAGS: 00000246 Code: 03 b7 a0 01 00 00 4c 8b 4c 24 70 4c 8b 44 24 78 4c 89 74 24 18 e9 91 f9 ff ff f6 45 4d 02 0f 84 fd f7 ff ff 48 8b 45 40 48 89 ef <48> 8b 80 d8 3f 00 00 48 89 44 24 20 e8 9b 97 eb ff 48 89 44 24 RIP: do_task_stat+0x8b4/0xaf0 RSP: ffffc90000607cc8 CR2: 0000000000003fd8
John Ogness said: for my tests I added an else case to verify that the race is hit and correctly mitigated.
Link: http://lkml.kernel.org/r/20180116175054.GA11513@avx2 Signed-off-by: Alexey Dobriyan adobriyan@gmail.com Reported-by: "Kohli, Gaurav" gkohli@codeaurora.org Tested-by: John Ogness john.ogness@linutronix.de Cc: Peter Zijlstra a.p.zijlstra@chello.nl Cc: Ingo Molnar mingo@elte.hu Cc: Oleg Nesterov oleg@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
fs/proc/array.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff -puN fs/proc/array.c~proc-fix-coredump-vs-read-proc-stat-race fs/proc/array.c --- a/fs/proc/array.c~proc-fix-coredump-vs-read-proc-stat-race +++ a/fs/proc/array.c @@ -430,8 +430,11 @@ static int do_task_stat(struct seq_file * safe because the task has stopped executing permanently. */ if (permitted && (task->flags & PF_DUMPCORE)) { - eip = KSTK_EIP(task); - esp = KSTK_ESP(task); + if (try_get_task_stack(task)) { + eip = KSTK_EIP(task); + esp = KSTK_ESP(task); + put_task_stack(task); + } } }
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