[PATCH 0/1] coredump: Fix null pointer dereference when kernel.core_pattern is "|"

[Matthew Ruffell]

Hello,

A user was setting their kernel.core_pattern to "|" to disable coredumps, and
encountered the following null pointer dereference on a Ubuntu 5.3.0-29-generic
kernel:

Steps to reproduce:
Save the following intentionally broken program, save as socktest.c:

#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>

int main()
{
    int listenfd = 0;
    struct sockaddr_in serv_addr;

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(6000);

    bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));

    listen(listenfd, 10);

    *(int*)33 = 33;

    return 0;
}

$ sudo sysctl kernel.core_pattern="|"
$ gcc -o socktest socktest.c
$ ./socktest
<program will hang and will not be killable>

dmesg output:

BUG: kernel NULL pointer dereference, address: 0000000000000020
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0 
Oops: 0000 [#1] SMP PTI
CPU: 1 PID: 1026 Comm: socktest Not tainted 5.3.0-29-generic #31-Ubuntu
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
RIP: 0010:do_coredump+0x536/0xb30
Code: 00 48 8b bd 18 ff ff ff 48 85 ff 74 05 e8 c2 47 fa ff 65 48 8b 04 25 c0 6b 01 00 48 8b 00 48 8b 7d a0 a8 04 0f 85 65 05 00 00 <48> 8b 57 20 0f b7 02 66 25 00 f0 66 3d 00 80 0f 84 9b 03 00 00 49
RSP: 0000:ffffa784c0887ca8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8c5c743caec0 RCX: 00000000000019ab
RDX: 0000000000000000 RSI: ffffa784c0887c68 RDI: 0000000000000000
RBP: ffffa784c0887dd8 R08: 0000000000000400 R09: ffffa784c0887be0
R10: ffff8c5c79c51850 R11: 0000000000000000 R12: ffff8c5c70b869c0
R13: 0000000000000001 R14: 0000000000000000 R15: ffffffffa4d15920
FS:  00007f5b7288d540(0000) GS:ffff8c5c7bb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000020 CR3: 000000017ab8a006 CR4: 0000000000160ee0
Call Trace:
? signal_wake_up_state+0x2a/0x30
? __send_signal+0x1eb/0x3f0
get_signal+0x159/0x880
do_signal+0x34/0x280
? do_user_addr_fault+0x34f/0x450
exit_to_usermode_loop+0xbf/0x160
prepare_exit_to_usermode+0x77/0xa0
retint_user+0x8/0x8

This happens on kernels 5.3 and above. On kernels 5.2 and prior, the user would
expect to see the following message in dmesg instead:

Core dump to | pipe failed

And the program would terminate on a standard segmentation fault.

Now, do_coredump+0x536 points more or less to the file_start_write() function
in do_coredump():

565 void do_coredump(const kernel_siginfo_t *siginfo)
566 {
...
788     if (!dump_interrupted()) {
789         file_start_write(cprm.file);
...
810 }

But this is not the "real" cause of the fault.

The problem was introduced in the following commit:

commit 315c69261dd3fa12dbc830d4fa00d1fad98d3b03
Author: Paul Wise <pabs3@xxxxxxxxxxxxx>
Date: Fri Aug 2 21:49:05 2019 -0700
Subject: coredump: split pipe command whitespace before expanding template

Here is the actual fault. When we enter format_corename(), cn->corename[0] is
set to '\0' after being allocated on the heap:

191 static int format_corename(struct core_name *cn, struct coredump_params *cprm,
192                size_t **argv, int *argc)
193 {
...
196     int ispipe = (*pat_ptr == '|');
...
202     cn->corename = NULL;
203     if (expand_corename(cn, core_name_size))
204         return -ENOMEM;
205     cn->corename[0] = '\0';
...
}

ispipe is also 1, since the first character of the core_pattern is "|".

In the next if statement:

207     if (ispipe) {
208         int argvs = sizeof(core_pattern) / 2;
209         (*argv) = kmalloc_array(argvs, sizeof(**argv), GFP_KERNEL);
210         if (!(*argv))
211             return -ENOMEM;
212         (*argv)[(*argc)++] = 0;
213         ++pat_ptr;
214     }
215 
216     /* Repeat as long as we have more pattern to process and more output
217        space */
218     while (*pat_ptr) {

argv[0] is set to 0, and after, we do not enter the while(*pat_ptr) loop because
we have already reached the end of the core_pattern string.

Back in do_coredump():

676         for (argi = 0; argi < argc; argi++)
677             helper_argv[argi] = cn.corename + argv[argi];
678         helper_argv[argi] = NULL;

helper_argv[0] is set to cn.corename, which still has '\0' at index 0, and
argv[0] = 0, so helper_argv[0] == cn.corename.

When the call to call_usermodehelper_setup() is issued:

680         retval = -ENOMEM;
681         sub_info = call_usermodehelper_setup(helper_argv[0],
682                         helper_argv, NULL, GFP_KERNEL,
683                         umh_pipe_setup, NULL, &cprm);
684         if (sub_info)
685             retval = call_usermodehelper_exec(sub_info,
686                               UMH_WAIT_EXEC);

sub_info->path is set to helper_argv[0], and in call_usermodehelper_exec():

548 int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
549 {
550     DECLARE_COMPLETION_ONSTACK(done);
551     int retval = 0;
552 
553     if (!sub_info->path) {
554         call_usermodehelper_freeinfo(sub_info);
555         return -EINVAL;
556     } 
...
568     if (strlen(sub_info->path) == 0)
569         goto out;
...
597 out:
598     call_usermodehelper_freeinfo(sub_info);
599 unlock:
600     helper_unlock();
601     return retval;
602 }

retval is initially set to 0. sub_info->path is a valid pointer, since it points
to the '\0' character, the check if (!sub_info->path) fails and we continue on
to the strlen check. This passes, and we goto out, which returns the retval of 
0.

Back to do_coredump():

688         kfree(helper_argv);
689         if (retval) {
690             printk(KERN_INFO "Core dump to |%s pipe failed\n",
691                    cn.corename);
692             goto close_fail;
693         }

We check to see if retval is nonzero. Since it is zero, we can continue on, and
get stuck at the null pointer dereference at the call to file_start_write()
pointed out earlier.

What should happen, is that we keep the same behaviour as kernels before
commit 315c69261dd3fa12dbc830d4fa00d1fad98d3b03, and enter the "if (retval)"
statement to print the "Core dump to |%s pipe failed\n" message and goto
close_fail.

We can add a simple string length check to fix the issue:

689         if (retval || strlen(cn.corename) == 0) {
690             printk(KERN_INFO "Core dump to |%s pipe failed\n",
691                    cn.corename);
692             goto close_fail;
693         }

Attached is a patch. It keeps the semantics the same as before
315c69261dd3fa12dbc830d4fa00d1fad98d3b03. Note, cn.corename will never be a
null pointer, and will always be null terminated.

If you can think of a better fix, please let me know.

Thanks,
Matthew Ruffell
Sustaining Engineer @ Canonical

Matthew Ruffell (1):
  coredump: Fix null pointer dereference when kernel.core_pattern is "|"

 fs/coredump.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.20.1