4.9-stable review patch. If anyone has any objections, please let me know.
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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 @@ -423,8 +423,11 @@ 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); + } } }