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Date:   Mon, 30 Mar 2020 10:31:58 +0200
From:   Thibaut Sautereau <thibaut.sautereau@clip-os.org>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: NULL pointer dereference in coredump code
Message-ID: <20200330083158.GA21845@clip-os.org>
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I hit a kernel NULL pointer dereference caused by the following call chain:

do_coredump()
  file_start_write(cprm.file) # cprm.file is NULL
    file_inode(file) # NULL ptr deref

The `ispipe` path is followed in do_coredump(), and:
    # cat /proc/sys/kernel/core_pattern
    |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h

It seems that cprm.file can be NULL after the call to the usermode
helper, especially when setting CONFIG_STATIC_USERMODEHELPER=y and
CONFIG_STATIC_USERMODEHELPER_PATH="", which is the case for me.

One may say it's a strange combination of configuration options but I
think it should not crash the kernel anyway. As I don't know much about
coredumps in general and this code, I don't know what's the best way to
fix this issue in a clean and comprehensive way.

I attached the patch I used to temporarily work around this issue, if
that can clarify anything.

Thanks,

-- 
Thibaut Sautereau
CLIP OS developer

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Content-Disposition: attachment; filename="0001-coredump-FIXUP.patch"

From 613dfc60429c1fc5fc19e1c8662648620dc103af Mon Sep 17 00:00:00 2001
From: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Date: Fri, 27 Mar 2020 16:34:59 +0100
Subject: [PATCH] coredump: FIXUP

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

diff --git a/fs/coredump.c b/fs/coredump.c
index b1ea7dfbd149..d0177b81345f 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -686,7 +686,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 							  UMH_WAIT_EXEC);
 
 		kfree(helper_argv);
-		if (retval) {
+		if (retval || !cprm.file) {
 			printk(KERN_INFO "Core dump to |%s pipe failed\n",
 			       cn.corename);
 			goto close_fail;
-- 
2.26.0


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