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Subject[PATCH 4.14 65/89] proc: fix coredump vs read /proc/*/stat race
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Alexey Dobriyan <adobriyan@gmail.com>

commit 8bb2ee192e482c5d500df9f2b1b26a560bd3026f upstream.

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
Call Trace:
proc_single_show+0x43/0x70
seq_read+0xe6/0x3b0
__vfs_read+0x1e/0x120
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/proc/array.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -424,8 +424,11 @@ 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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