linux-hardening - Re: splat in ikheaders

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Date:   Thu, 2 Mar 2023 14:35:27 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Kees Cook <kees@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org,
        bpf@...r.kernel.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>
Subject: Re: splat in ikheaders_read (bpftrace)

On Thu, 2 Mar 2023 14:12:31 -0800 Jakub Kicinski wrote:
> On Thu, 2 Mar 2023 14:08:14 -0800 Jakub Kicinski wrote:
> > Mm. Actually stopping to look at the code - I don't see it bound
> > checking against kernel_headers_data_end :| Maybe we need:
> > 
> > @@ -34,6 +35,7 @@ ikheaders_read(struct file *file,  struct kobject *kobj,
> >                struct bin_attribute *bin_attr,
> >                char *buf, loff_t off, size_t len)
> >  {
> > +       len = min_t(size_t, kernel_headers_data_end - kernel_headers_data, len);
> >         memcpy(buf, &kernel_headers_data + off, len);
> >         return len;
> >  }  
> 
> Scratch that, the size is set at init time.
> My guess was memcpy() thinks the size of kernel_headers_data
> is 1 since it's declared as char?

Like this?

---->8--------------------

From 7549153017144281c2475d9370962ad4d0ea8d4c Mon Sep 17 00:00:00 2001
From: Jakub Kicinski <kuba@...nel.org>
Date: Thu, 2 Mar 2023 11:15:53 -0800
Subject: [PATCH] kheaders: use a flex array for the symbol

Kernels with hardened copy hit:

    detected buffer overflow in memcpy
    kernel BUG at lib/string_helpers.c:1027!
    invalid opcode: 0000 [#8] SMP KASAN
    CPU: 1 PID: 1094 Comm: tar Tainted: G      D            6.2.0-12879-g040b4d2ce1ad #646
    RIP: 0010:fortify_panic+0xf/0x20
    [...]
    Call Trace:
     <TASK>
     ikheaders_read+0x45/0x50 [kheaders]
     kernfs_fop_read_iter+0x1a4/0x2f0

Repro:

 $ cat /sys/kernel/kheaders.tar.xz >> /dev/null

Fixes: 43d8ce9d65a5 ("Provide in-kernel headers to make extending kernel easier")
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
 kernel/kheaders.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/kheaders.c b/kernel/kheaders.c
index 8f69772af77b..90562eb4093a 100644
--- a/kernel/kheaders.c
+++ b/kernel/kheaders.c
@@ -26,7 +26,7 @@ asm (
 "	.popsection				\n"
 );
 
-extern char kernel_headers_data;
+extern char kernel_headers_data[];
 extern char kernel_headers_data_end;
 
 static ssize_t
@@ -34,7 +34,7 @@ ikheaders_read(struct file *file,  struct kobject *kobj,
 	       struct bin_attribute *bin_attr,
 	       char *buf, loff_t off, size_t len)
 {
-	memcpy(buf, &kernel_headers_data + off, len);
+	memcpy(buf, &kernel_headers_data[off], len);
 	return len;
 }
 
@@ -49,7 +49,7 @@ static struct bin_attribute kheaders_attr __ro_after_init = {
 static int __init ikheaders_init(void)
 {
 	kheaders_attr.size = (&kernel_headers_data_end -
-			      &kernel_headers_data);
+			      &kernel_headers_data[0]);
 	return sysfs_create_bin_file(kernel_kobj, &kheaders_attr);
 }
 
-- 
2.39.2