From: Alexey Dobriyan <adobriyan@xxxxxxxxx>
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@xxxxxxxxx>
Reported-by: "Kohli, Gaurav" <gkohli@xxxxxxxxxxxxxx>
Tested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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);
+ }
}
}
_